What this calculator does
A 1,000 km trip each way at 90 grams of carbon dioxide equivalent per passenger-kilometre comes to 180 kg, which is 0.18 tonnes, and costs $4.50 to offset at $25 a tonne. The arithmetic is trivial. The reason online calculators disagree so wildly is not the maths, it is the factor, which is why the factor is yours to enter here rather than something buried in the page.
The one thing to get right is which basis your factor uses. A car at 170 grams per vehicle-kilometre with four people aboard produces 340 kg for the round trip, or 85 kg each. Treat that same factor as per passenger and you get 1,360 kg, four times too much. Flight figures are almost always per passenger and car figures are almost always per vehicle, and mixing them up is the single largest error in this kind of estimate.
The formula
The total is distance × journeys × factor, and the only subtlety is the split. A per-vehicle factor covers the whole vehicle, so the total is independent of how many people are in it and the per-person figure divides by the occupants. A per-passenger factor already describes one person, so the total multiplies by the occupants instead and the per-person figure is the factor itself. The offset cost is the tonnage at the price you enter.
| Term | Meaning |
|---|---|
| CO₂e | Carbon dioxide equivalent, which bundles other greenhouse gases in at their warming effect. |
| Emission factor | Grams of CO₂e per kilometre. The one input that decides the answer. |
| Per passenger-kilometre | One person moved one kilometre. The usual basis for flights, trains and buses. |
| Per vehicle-kilometre | The whole vehicle moved one kilometre, whoever is in it. The usual basis for cars. |
The inputs explained
| Field | What to enter |
|---|---|
| Distance one way (km) | Distance one way. For flights use the great-circle distance between airports rather than the road distance. |
| Journeys | One way or return. |
| Emission factor (g CO₂e/km) | Your emission factor in grams of CO₂e per kilometre, from your own national inventory if you have one. |
| The factor is | Whether that factor covers one passenger or the whole vehicle. Check the source before choosing. |
| People travelling | People travelling. On a per-vehicle factor this splits the total; on a per-passenger factor it multiplies it. |
| Offset price ($/tonne) | Price per tonne for the offsets you are actually considering, which varies enormously by scheme and quality. |
When to use it
Comparing driving against flying
Run the car with a per-vehicle factor and the flight with a per-passenger one, both for the same number of people. The per-kilometre per-person figure is the fair comparison.
Reporting a work trip
Use the factor your organisation or national inventory specifies rather than a general one, so the number matches the rest of the reporting.
Pricing an offset
Enter the real price of the offsets you would buy. The result is usually low enough to be worth doing and far too low to be a licence, and both of those are useful to see.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does a 2,000 km round trip emit?
Only the emission factor changes.
| Emission factor | Emissions for the journey | In tonnes | Offset cost at that price |
|---|---|---|---|
| 40 g/km | 80.0 kg CO₂e | 0.080 t | $2.00 |
| 90 g/km | 180.0 kg CO₂e | 0.180 t | $4.50 |
| 130 g/km | 260.0 kg CO₂e | 0.260 t | $6.50 |
| 170 g/km | 340.0 kg CO₂e | 0.340 t | $8.50 |
| 250 g/km | 500.0 kg CO₂e | 0.500 t | $12.50 |
Does carpooling change the total?
A per-vehicle factor, with only the number of people changing.
| People travelling | Emissions for the journey | Per person | Per kilometre, per person |
|---|---|---|---|
| 1 person | 340.0 kg CO₂e | 340.0 kg CO₂e | 170.0 g |
| 2 people | 340.0 kg CO₂e | 170.0 kg CO₂e | 85.0 g |
| 3 people | 340.0 kg CO₂e | 113.3 kg CO₂e | 56.7 g |
| 4 people | 340.0 kg CO₂e | 85.0 kg CO₂e | 42.5 g |
| 5 people | 340.0 kg CO₂e | 68.0 kg CO₂e | 34.0 g |
Questions
Where do I get an emission factor?
Your national greenhouse inventory or transport department publishes them, and they are the figures to use because they match your electricity mix and vehicle fleet. A general international figure can be out by a factor of two.
Should a flight figure include a multiplier for altitude?
That depends on the factor you chose. Some published flight factors already include the non-carbon warming effects and some do not, and the difference is large, so check which one you have rather than applying a multiplier on top.
Why is the total the same no matter how many people are in the car?
Because a per-vehicle factor describes the vehicle. Extra passengers add a little weight and almost no fuel, so the honest treatment is to keep the total fixed and divide it, which is what the per-person row does.
Does an electric car have a factor here?
Yes, but it depends on your grid rather than the car, so it has to come from a local source. Enter it per vehicle-kilometre and the rest of the page works the same way.
Are offsets worth buying?
That is a judgement rather than a calculation, and offset quality varies enormously. The page prices whatever rate you enter so you can see the figure; it does not take a view on the scheme.
For emissions at the scale of a whole economy, see the Kaya identity. Electric versus petrol running cost covers the money side of the same trip, and trip fuel cost works out what the drive costs in fuel.