How this calculator works
A tree absorbs carbon dioxide as it grows, stores the carbon in wood and roots, and releases oxygen. The United States Forest Service and the Arbor Day Foundation use a mature tree as the standard unit: about 21.8 kilograms of carbon dioxide absorbed a year, 118 kilograms of oxygen released, and roughly 5,678 litres of rainfall intercepted before it reaches storm drains. A tree also shades buildings and cools the air through transpiration, which reduces air-conditioning use by an amount that depends on the building and the planting position. The calculator scales those benefits by the square of the trunk diameter relative to a 40 cm tree, a rough proxy for the crown volume. A young 10 cm tree therefore provides only a small fraction of the mature figure, and a 60 cm tree exceeds it. The carbon stored over the period is the total absorbed multiplied by the carbon fraction of carbon dioxide, 12/44, because the rest of the mass is oxygen.
Formula
size factor = (diameter in cm / 40)^2, limited to between 0.05 and 2.5. CO2 per year (kg) = 21.8 x size factor x trees. oxygen per year = 118 x size factor x trees. stormwater per year (L) = 5,678 x the smaller of the size factor and 1.5 x trees. cooling energy saved = 150 kWh x size factor x trees. carbon stored = CO2 absorbed x 12/44.
Worked example
One 30 cm tree over 10 years. The size factor is (30/40)^2 = 0.5625, so it absorbs 21.8 x 0.5625 = 12.26 kg CO2 a year, or 122.6 kg over 10 years. The carbon stored is 122.6 x 12/44 = 33.4 kg. It releases 66.4 kg of oxygen a year, intercepts 3,194 L of stormwater and saves about 84 kWh of cooling energy a year.
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Frequently asked questions
Does the tree type matter?
Yes. Fast-growing species such as poplar and willow absorb more carbon per year, while dense hardwoods store more over a long life. Conifers intercept more rain in winter. The calculator uses a general average; a species-specific i-Tree model gives a better figure for a particular tree.
Does planting a tree offset my flights?
Only slowly. A mature tree absorbs about 22 kg of CO2 a year, so offsetting a 1,000 kg flight takes about 45 tree-years. A newly planted tree takes decades to reach that rate, and the carbon is released again if the tree dies or burns.
How accurate is the diameter scaling?
It is a rough approximation. Real carbon storage follows species-specific allometric equations that relate diameter to biomass, and crown size depends on pruning and competition. Use the result to compare trees of different sizes, not as a measured carbon credit.
What happens when the tree dies?
The stored carbon returns to the atmosphere as the wood decays or burns, unless it is harvested into long-lived products. The annual absorption stops when growth stops, so a mature tree is closer to a store than a continuing offset.