4: Calculate your carbon footprint

The better your footprint calculation is the more effectively you can shrink.

Calculating your carbon footprint can help you to prioritise the steps you can take to shrink it, by identifying what the biggest opportunities for reductions are.

This step explains the methodology we use to calculate a person’s carbon footprint for each emissions sector: housing, travel, food, products and services.  You can use this method to calculate your own footprint or try our online carbon calculator.

The Method

The calculation method used by Shrink tries to capture the full life cycle of all aspects of our personal consumption.  We focus on consumption in each of our five major categories: housing, travel, food, products and services.  In addition to these we also estimate the share of national emissions over which we have little control, government purchases and capital investment.

For simplicity and clarity all our calculations follows one basic method.  We multiply a use input by an emissions factor to calculate each footprint.  All use inputs are per individual and include things like fuel use, distance, calorie consumption and expenditure.  Working out your inputs is a matter of estimating them from your home, travel, diet and spending behaviour.

Although working out you inputs can take some investigation on your part the much more challenging aspect of carbon calculations is estimating the appropriate emissions factor to use in your calculation.  Where possible you want this emissions factor to account for as much of the relevant life cycle as possible.

Housing

To calculate your housing footprint  you need to work out your personal share of home energy use, water use and waste disposal.  This means collecting figures for your home’s annual energy, water and waste use and dividing it by the number of people in your home, to get your individual share.  Having gathered this information, you then multiply your personal usage by an emissions factor (EF)  to calculate your home footprint.

The calculations look like this:

Electricity : use (kWh/yr) * EF (kg CO2e/kWh) = emissions (kg CO2e/yr)
Natural Gas : use (therms/yr) * EF (kg CO2e/therms) = emissions (kg CO2e/yr)
Fuel Oil: use (litres/yr) * EF (kg CO2e/litre) = emissions (kg CO2e/yr)
LPG : use (litres/yr) * EF (kg CO2e/litre) = emissions (kg CO2e/yr)
Waste : use (kg/week) * 52 * EF (kg CO2e/kg) = emissions (kg CO2e/yr)
Water : use (litres/day) * 365 * EF (kg CO2e/kWh) = emissions (kg CO2e/yr)

Each of the usages are per person, per year.  For example if your home uses 3000 kWh of electricity a year and you share it with two other people your share of electricity use will be 1000 kWh.

In all calculations you need to make sure your input is in the correct units, whether that be in terms of functional units or in terms of the time period.  The input and intensity also need to match up, for instance if you are using therms for natural gas then the intensity should be per therm, not per kWh or per litre.

Electricity

Unlike with other energy sources the carbon intensity of electricity varies greatly depending on how it is produced and transmitted.  For most of us, the electricity we use comes from the grid and is produced from a wide variety of sources.  Although working out the carbon intensity of this mix is difficult, most of the work is generally done for us.

Direct emissions factors are widely published and show the amount of emissions produced by power stations in order to produce an average kilowatt-hour within that grid region.  In many carbon footprint calculations organisations simply use this direct emissions factor, which provides a good indication of how carbon intensive the electricity we use is.  In addition to these direct emissions it is desirable to account for indirect emissions, from things like mining and plant construction as well as transmission losses.

Accounting for direct emissions, indirect emissions and grid losses gives a more complete picture of the full footprint of the electricity we consume.  Using an example is useful to explain.  In the UK the average carbon intensity of electricity use is about 0.6 kg CO2e/kWh, around 82% of this figure comes directly from combustion, 11% from indirect emissions while 7% is from losses.  In the US the average has been around 0.7 kg CO2e/kWh over the last decade, but this varies across different grid sections within the country and is dropping with the increased use of shale gas.

A more complete explanation of the different intensities for electricity consumption is contained in the next section, shrink that housing footprint.  This section also has a comparison of the intensities of different generation technologies like coal, natural gas, wind and solar.

Fuels

In addition to electricity many homes use fuels for their energy needs, like natural gas, heating oil, liquid petroleum gas (LPG) or coal.  For each of these fuels we can estimate a carbon intensity that tells us the amount of emissions our fuel use creates.  Again we want this  factor to capture both the direct emissions from combustion of fuels and indirect emissions from the mining, processing and transportation of the fuel.

The direct emissions from the combustion of fuels are the same throughout the world as they are determined by the physical properties of the fuel.  The indirect emissions from the rest of the fuel’s life cycle vary depending on the technology used to prepare and transport the fuel, as well as the distances it must travel.  The indirect factor is often around 10-20%  of the total life cycle and is estimated from life-cycle analysis literature.

Combining the direct and indirect factors gives an emissions factor for the full life-cycle of the fuel.  Natural gas is roughly 6.6 kg CO2e/therm or 0.22 kg CO2e/kWh, more than 85% of which arises directly from combustion.  Heating oil is around 11.6 kg CO2e/US gallon or 3.1 kg CO2e/litre.  LPG is 6.8 kg CO2e/US gallon or 1.8 kg CO2e/litre.

Waste and Water

Beyond energy sources the two further components of our housing footprint we calculate are waste disposal and water use.

Emissions from waste disposal are mainly the result of methane produced at landfill sites as well as transport.  By calculating how much waste you produce each week and multiplying by 52 you can get your annual waste production.  This is then multiplied by a carbon intensity to get your footprint.  The intensity will differ greatly from country to country depending on how much waste goes to landfill, how much is incinerated and how much is recycled.  Intensities capturing these differences can be drawn from life cycle literature or estimated from national inventories of greenhouse gasses.

Water use can also be surprisingly carbon intensive in some places.  Emissions come from two main sources, electricity used in pumping water during its supply and the methane and nitrous oxide that arises from waste water and sewage treatment.  Again, intensities differ greatly from country to country but once you work out your daily usage you can multiply by 365 for your yearly usage and apply an intensity to calculate your footprint.

Anything else?

One important area missing from these calculations is the emissions involved in housing construction.  Construction emissions are omitted from these calculations because they are an area over which most of us have no control and are generally counted as part of capital emissions.  A calculation for these emissions can be done as a share of national construction emissions or by floor space per person given average build emissions and lifespan, but neither of these methods is very reliable.

Despite not including construction emissions in our calculation it is worth noting that construction emissions are generally quite large, ranging from 30 to 80 t CO2e for a typical family home depending on building size, materials and location.  Due to these large ‘embedded’ emissions the construction footprint should be considered in tandem with use emissions when designing low carbon homes.

Travel

To calculate your travel footprint you need to work out how much travel you have done in the last year using various forms of transport.   Taking these distances you can multiply by a carbon intensity for each form of transport.

Vehicle : distance (km/yr) /*EF (kg CO2e/km) = emissions (kg CO2e/yr)
Bus : distance (km/yr) * EF (kg CO2e/km) = emissions (kg CO2e/yr)
Metro: distance (km/yr) * EF (kg CO2e/km) = emissions (kg CO2e/yr)
Taxi: distance (km/yr) * EF (kg CO2e/km) = emissions (kg CO2e/yr)
Rail: distance (km/yr) * EF (kg CO2e/km) = emissions (kg CO2e/yr)
Flying : distance (km/yr)* 1.09 * EF (kg CO2e/km) = emissions (kg CO2e/yr)

The emissions factor we use try to account for the direct emissions from fuel combustion, indirect emissions from the fuel production and a share of construction emissions for the vehicle.

Vehicle

To work out your vehicle footprint you need to multiply the distance you have driven in the year by an emissions factor for your vehicle. For the distance we choose to focus on distance driven rather than passenger miles traveled.  This avoids the complication of estimating all the distance traveled as a driver and passenger in different vehicles with their fuel economy and number of passengers.  If you would prefer to use total passenger distance then you will need to adjust your emissions factor appropriately.

Estimating the distance you have driven in a year is simple enough but finding an accurate emissions factor is more complicated.  Although you could use the advertised emissions value for your vehicle this fails to capture your car’s actual fuel economy, indirect fuel emissions and vehicle construction emissions.  To account for these we need to use an intensity that includes direct emissions from actual fuel use, indirect emissions from fuel production and an estimate of vehicle construction emissions.

Using well understood information about fuel life cycle we know that the direct emissions from petrol (gasoline) combustion are about 2.32 kg CO2e/litre or 8.78 kg CO2e/US gallon.  Indirect emissions are an additional 0.41 kg CO2e/litre or 1.55 kg CO2e/US gallon, although this figure varies a little depending on how the oil is extracted and transported.  If you know the fuel economy your vehicle achieves you can work out both the direct and indirect emissions factors for your own driving.  You can calculate this fuel economy by keeping track of your mileage and fuel purchases over a period of time.

In America fuel economy is typically given in miles per gallon (MPG).  If for example you know your car averages 25 MPG then you can divide the direct emissions intensity of gasoline, 8.78 kg CO2e/US gal, by 25 MPG to give 351 g CO2e/mile for direct emissions.  The same method will give 62 g CO2e/mile for the indirect emissions.

In Europe fuel consumption is typically measured in L/100 km.  In this case we multiply the emissions factor, 2.32 kg CO2e/litre,  by the actual  consumption and divide by 100.  If your car average 8 L/100 km this would be 186 g CO2e/km for direct emissions and 33 g CO2e/km for indirect emissions based on the emission factor of 0.41 kg CO2e/litre.

Having calculated both direct and indirect fuel emissions based on the real life fuel economy we also need to add an estimate for vehicle construction emissions.  This is done by dividing the total construction footprint of the vehicle by its expected lifetime miles.

The manufacturing emissions for an automobile are typically between 5 to 12 t CO2e, but can vary depending on materials, size, production techniques and vehicle technology.  For a 25 MPG sedan the construction footprint might be 9 tonnes while expected lifetime mileage could be around 150,000 miles.  Dividing 9,000 by 150,000 gives us a construction footprint of 60 g CO2e/mile or 37 g CO2e/km.

Using our 25 MPG example we can bring together our direct, indirect and construction emissions.  They are 351 g CO2e/mile direct emissions, 62 g CO2e/mile indirect emissions and 60 g CO2e/mile construction emissions.  When combined these make up the total emissions factor of 473 g CO2e/mile.  This is multiplied by total miles driven per year to calculate the driving footprint.

Public Transport

Once you know how much we have traveled each year using various forms of public transport you can make a reasonably accurate calculation of your emissions.  Using average vehicle efficiency and average loading the emissions factors per passenger kilometre (g CO2e/km) or per passenger mile traveled (g CO2e/mi) for each form of transport can be calculated.

Such emissions factors are often published by governments, some of which include both direct emissions and indirect emissions.  To this you can add an estimate for construction emissions per passenger distance traveled from life cycle literature.  Then you have an emissions factor that includes direct fuel emissions, indirect fuel emissions and vehicle construction emissions in your unit of choice.  Due to the far greater utilization on public transport vehicles construction emissions are typically much lower than for private vehicles.

Flying

To calculate your flying emissions you need to work out the distance you have flown in the year you are calculating.  Distances should be calculated in great circle paths and can be worked out using any number of web-based calculators.  A 9% uplift factor is also used in the calculation to account for the non-direct nature of flights, though this is likely to be an underestimate for short flights and an overestimate for long ones.

Once you have those distances you can follow a similar method using emissions factors published by government agencies.  These emission factors vary for short, medium and long haul flights.  They also depend on whether you fly economy, business and first class.  To these values you once again can add an estimate of construction emissions.  These are calculated per passenger mile or per passenger kilometre.

Many organisations also apply a radiative forcing multiplier to their calculations, typically 1.9, to account for flying’s additional climate effects resulting from water vapour, contrails and NOx gasses.  The use of such multiplier explains the vastly different results you get for flying the same distance on many different online calculators.

We do not use such a multiplier in our calculations because doing so would be inconsistent the bounds of the carbon footprint, which omits other forcings agents like black carbon, tropospheric ozone and aerosols.  This is not to say such other forcings are unimportant, they most certainly are, but their effects are much harder to quantify reliably and can’t be equated easily with the global warming potentials used for carbon footprint calculations.

Food

To calculate your food footprint you need to estimate the amount of food you consume and the emissions that result from the supply of that food.  To simplify this process you can estimate the typical food energy you consume each day in different food groups, and base your calculation on this.

The calculations look like this:

Red meat: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
White meat: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Dairy: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Cereals: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Vegetables: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Fruit: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Oils: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Snacks: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)
Drinks: consumption (kCal/day)*365*EF (kg CO2e/kCal) = emissions (kg CO2e/yr)

For measuring food consumption you can potentially use expenditure, weight or food energy.  Using expenditure implies large potential errors from prices and measuring food in terms of weight is not very practical.  Instead you can choose to use daily food energy consumption (kCal/day) because it is an intuitive way of understanding both your intake and its footprint.

You start by dividing our diets into major food groups which have broadly similar emissions factors.  These are: red meat, white meat, dairy, cereals, vegetables, fruit, oils, snacks and drinks.  Using these groups you can estimate how your daily energy consumption is typically divided.  This can take a bit of effort at first but by referring to the total you can help to see if your daily intake makes sense.  Typical energy consumption is in the range between 2000 and 3000 kCal/day (often written Calories/day).

To get food emissions factors we use a combination of IO-LCA and LCA literature.  We first estimate an average emissions factor for the cradle to gate emissions associated with producing a kilo of food for each group based on a weighted average of individual foods within the group (kg CO2e/kg).  This is then divided by average energy content (kCal/g) for each food group, which is taken from government statistics, to give an emissions factor per Calorie produced (g CO2e/kCal produced).

To account for the large losses in the food system we also need to adjust for all the additional food that is produced and lost at either retail or consumer level.  The adjustments for retail loss (RL) and consumer loss (CL) are necessary to account for that fact that sometimes as little as a half of food produced is consumed due to spoilage and waste.

For each food group the emissions factor per Calorie produced (g CO2e/kCal produced) is divided by one minus the retail loss percentage (1 – RL) and one minus the consumer loss percentage (1 – CL) to account for the production emissions of lost food.  The new emissions factor now reflects emissions per Calorie consumed (g CO2e/kCal consumer).  This emissions factor is made up of emissions from the calories we eat, throw away and are spoiled in stores.

The accuracy of this calculation could potentially be improved by breaking the diet down into individual foods, rather than having food groups.  While further breakdown isn’t very practical for calculating an entire diet footprint it is invaluable when trying to compare the carbon merits of different foods.

What about storage, cooking and waste emissions?

The calculations above focus on cradle to gate emissions, which are the emissions involved in the growing, processing, packaging and transporting of food to a supermarket or store.  Although the full life cycle of food also includes transporting food to homes, storage, preparation and waste, these emissions are already counted in vehicle use, electricity, fuel and waste respectively.  To avoid double counting these emissions are not included in this calculation.

Land Use Change

Land use change is typically not accounted for in the calculation of a food emissions factors, but it is hugely important.  Deforestation in tropical rain forest dominates global land use change emissions and is driven largely by agricultural production in south-east Asia and South America.  Although land use change emissions are generally not  calculated in food production footprints the global demand for palm oil, soy and beef in particular drives the conversion of forest into plantations and grazing land.

Products

The method we use for our product footprints is based on how much we spend each month on new goods.  This expenditure is divided up into six different product groups from which we estimate an emissions factor that captures the emissions arising from the materials, manufacturing and distribution associated with each product group .

The calculations look like this:

Electrical : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Household : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Clothes : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Medical : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Recreational : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Other : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)

Calculating your products footprint would ideally involve adding up all the footprints of the products you purchase.  This would be done given footprints calculated based on a life cycle analysis (LCA) of each product you buy.  However, because we make so many purchases and LCA data is not available for each of these this is not possible.  Instead we focus on expenditure in product groups and use estimates from Input-Output Life Cycle Assessment (IO-LCA) to get average emissions factors each group.

First we work out average monthly expenditure in each of the following six product groups:  electrical, household, clothes, medical, recreational and other.  These are then multiplied by 12 to get annual consumption in each group.  Each annual consumption is then multiplied by an emissions factor (kg CO2e/$) which is calculated using weighted averages from IO-LCA literature or multi-regional studies where possible.

This is completed for each of the six product groups, the sum of which give the total products footprint.  Although this method is the best way to get around the problem of the numerous products we buy, it has obvious drawbacks.  The global nature of many supply chains means input-output techniques can’t be very precise.  By using prices instead of quantities we also introduce a large source of potential error.  Also by aggregating a large number of products into each group we assume they have similar carbon intensities when in fact they may not.

Although in big samples much of this potential error will balance out, where possible it is best to focus on the individual products you purchase.  By working backwards you can see if the emissions factor you are using is appropriate or whether it needs to be adjusted.

Services

The process for calculating our services footprint is very similar to that used in products footprints.  Using monthly expenditures for seven different groups of services we can estimate the total footprint.

The calculations look like this:

Health : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Finance: spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Recreation: spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Education : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Vehicle: spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Communications : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)
Other : spend ($/month) * 12 * EF (kg CO2e/$) = emissions (kg CO2e/yr)

Using IO-LCA makes even more sense in the case of services because it is very difficult to add together LCA data for services as diverse as mechanics, bank accounts or a hair cut.  In this respect thinking about services in terms of emissions factor per unit of expenditure makes a lot of sense.

Once again we divide our spending into different groups, namely: health, financial, recreation, education, vehicle services, communications and other. Once we have our monthly expenditure for each service group this is multiplied by 12 to get yearly expenditure.  Yearly expenditure is then multiplied by an emissions factor (kg CO2e/$) which is calculated using weighted averages from IO-LCA data.

Once again the challenge with this method is whether the average emissions factor used is actually representative of the services we purchase.  This problem is less pronounced with services than it is with products because the footprints of different services are more homogenous than those of different products.

Government and Capital

To consider our total footprint beyond personal emissions we also need to consider emissions that arise from consumption that we do not control, namely that of government expenditure and capital investment in fixed assets.  Government emissions are dominated by services, while capital emissions arise predominantly from construction and equipment procurement.

We do not manually calculate these figures but instead estimate national shares for each based on input-output literature and government data.  These shares are the total emissions from government and capital expenditures divided by a country’s population.

Whether or not government and capital emissions should be attributed to individuals is open for debate, but in order to compare a footprint to national or international averages they need to be included somehow.  While each individual benefits from government and capital spending to varying degrees adding a national share to each personal footprint is the simplest way to attribute these emissions in order to make comparisons.

Summary

In this step we have focused on the methodology used to calculate your carbon footprint.  After outlining the general calculation method we have seen how you can calculate your housing, travel, food, product and services footprint.  In each case we have attempted to calculate a footprint that reflects the full life cycle of each form of consumption.

In the next step we will look at how you can shrink your housing footprint.

 

Navigation Menu

Introduction: The Shrink Guide
1: What is a carbon footprint?
2: What is climate change?
3: Carbon targets for your footprint
4: Calculate your carbon footprint
5: Shrink your housing footprint
6: Shrink your travel footprint
7: Shrink your food footprint
8: Shrink your product footprint
9: Shrink your service footprint
10: Take further climate action
Conclusion: Take action