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An average UK petrol car emits about 0.260 kg CO₂e per mile (0.162 kg per km) on the government's 2026 conversion factors. Drive the average of 7,100 miles a year and that's about 1.85 tonnes — which a mature tree, absorbing ~22 kg a year, would take around 84 tree-years to soak up. Our car CO₂ calculator does it for any fuel, size and mileage.
Two caveats worth being honest about. First, trees absorb slowly and over decades, so "planting to offset" is never a like-for-like swap for the emission you make today. Second, the far bigger lever is the car itself: fewer miles, or an electric car (0.048 kg a mile, charging included) on an increasingly clean grid, cuts the number dramatically. Trees are the picture; reducing first is the plan.
Source: DESNZ greenhouse gas conversion factors 2026 (average petrol car); DfT National Travel Survey 2025, table NTS0901 (all cars, England); tree absorption: Arbor Day Foundation (“more than 48 pounds” a year, as published by the US Forest Service). Figures are illustrative UK equivalents.
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Frequently asked questions
Is planting trees a real offset?
Not directly. Trees lock up carbon slowly, over decades, and results vary hugely by species, site and survival. Treating a tree count as an illustration of scale is fair; treating it as a guaranteed offset is not — reducing the emission is always the stronger move.
What about an electric car?
An average electric car charged on the UK grid is about 0.048 kg CO₂e a mile on DESNZ's 2026 figures, against 0.260 for petrol — under a fifth — and it falls further every year as the grid decarbonises.