Below you will find an explanation of how the calculator works and how we arrive at our figures.
In order to calculate carbon emissions you need to know how much CO₂ any particular activity produces. The UK Department for Environment, Food & Rural Affairs (Defra) produces an annual list of emissions factors. We use these factors as well as country/region specific factors in all of our calculations. We always use the most up-to-date figures for our calculations - currently those for 2020 for Defra's figures.
We use factors for CO₂e in all our calculations - the "e" stands for "equivalent" and takes into account the effect of other greenhouse gases (for example, Methane (CH4) and Nitrous Oxide (N₂O)), rendering them as the equivalent CO₂ - this means we only need to do one calculation for any particular activity, rather than three or four.
- For flights, the emissions factors are multiplied by the distance and the number of passengers to give an emission value.
- For ground based journeys, the factors are based on either the given fuel economy figure or the type and size of vehicle used with an assumed average occupancy - the factor is simply multiplied by the distance to calculate the emissions. It should be noted that when doing calculations based on claimed fuel economy, real-world results may vary depending on factors such as vehicle condition, vehicle modifications, driving style, traffic conditions, etc.
- For energy usage at home, the factors are based on the unit of the energy type used - kWh, kg, etc. The factor is multiplied by the quantity of energy used to produce the emissions figure.
We calculate emissions to the nearest kilogram of CO₂, convert the resultant figure to tonnes and then multiply that figure by the offset cost per tonne (£30) to give a suggested offset cost. The cost per tonne reflects the total costs to us for sequestrating 1 tonne of CO₂ through our partners.
In the air
Our airport database contains 5,581 airports from all around the world. It is based on the open source OpenFlights database with modifications and corrections. For each airport we know its latitude and longitude - using that information we can calculate the distance between any two airports using the Great Circle method.
The emissions factors that we use for flights include two "extras":
- Uplift - this is an 8% increase in all of the factors to reflect the fact that airplanes cannot always fly an exact Great Circle route and often have to wait before landing - unlike a car you can't just switch the engines off whilst waiting so airplanes "stack" near the airport which, inevitably, uses more fuel.
- Radiative Forcing - this is a 90% increase in all factors to take account of the fact that emissions high up in the atmosphere have a greater environmental impact than ground-level emissions. Defra recommends (from 2014 onwards) that everyone use factors in their calculations that include RF and we have chosen to do so to reflect the real environmental cost of flying.
- Well To Tank - the emissions associated with the extraction, refining and transportation of the raw fuel prior to combustion. Defra supplies WTT data to accompany the base+RF emissions factors. We use WTT figures as they help to reflect the true carbon cost of an activity. Including WTT figures adds approximately 10% to the emissions figures.
One question that often comes up in relation to flying is the effect that class has on emissions. It comes down, simply, to the number of people on the flight. For example, flights that only carry Economy class passengers tend to pack them in like sardines and thus the overall environmental cost of the flight is shared between more people - hence a lower emissions factor. The Defra factors are based on typical aircraft passenger configurations for the given classes.
On the ground
For car journeys we offer multiple ways of choosing the type of vehicle. In certain countries people are more familiar with fuel economy figures - MPG or L/100km - for those countries car journeys can be calculated that way. Elsewhere, choose by engine size as follows.
|Small||< 1.4 litres|
|Medium||1.4 - 2.0 litres|
|Large||> 2.0 litres|
|Small||< 1.7 litres|
|Medium||1.7 - 2.0 litres|
|Large||> 2.0 litres|
For motorcycles the engine sizes are:
|Medium||125cc to 500cc|
According to Defra "The conversion factors [for cars] are based on information from the DfT (Department for Transport) who regularly analyse the mix of cars on the road in Britain through DVLA records and automatic number plate recognition (ANPR) data. The conversion factors are updated each year to reflect changes in the spectrum of cars of different types and ages being driven."
The emissions factors for cars assume an average occupancy of 1.6 people.
You can download Defra's methodology paper which gives details of how all of their factors are derived.
Hints, Tips & Info
- The calculator remembers as many of your choices as possible during a session. If you do a calculation but don't add it to your footprint and then move to another tab (or page on the site) the calculator will remember the calculation for that tab. It will also remember settings such as class, number of passengers, vehicle type, fuel units, etc. rather than resetting the form each time - but it only remembers those details for the current session.
- Clicking the "reset" button on any tab will clear all information on that form and set it back to default values. It will also clear any calculation that you have made that has not been added to your footprint.
In the air
- The two fields for departure and arrival airports use an auto-complete system to find the right airport based on what you type. You need to type a minimum of three letters before the search will start and the calculator will first try to find the airport by its IATA code. You can type something, not wait for the auto-complete and then press "calculate" - the calculator will do its best to find the right airport but it's best to allow the auto-complete a few moments to do its work.
On the ground
- Depending on your choice of car type (segment or size) the available options for fuel type will change. This is because Defra does not give emission factors for every combination of vehicle and fuel, for example, there are no factors small cars using LPG or CNG because no manufacturer currently makes a small car that uses these types of fuel.
- If some of your electricty or gas come from renewable sources you can enter the percentage and the appropriate proportion of emission will be deducted from your footprint. We subsequently record both figures - total energy usage and amount of renewables so that you will still be able to track your overall energy usage - we don't throw any data away!