Carbon and carbon dioxide are different substances and the figures differ by a fixed ratio. Carbon has a molar mass of about 12 and carbon dioxide about 44, because the carbon arrives with two oxygen atoms attached. So every kilogram of carbon locked in wood came from about 3.67 kilograms of carbon dioxide.
CO₂ = carbon × 44 ÷ 12
That ratio is chemistry rather than a modelling choice, which makes it one of the few numbers in this area that is not open to argument. It is also the one most often applied in the wrong direction, or not applied at all, and a factor of 3.67 is large enough to change the conclusion of whatever it was supporting.
One tree, two correct answers
Take a tree 40cm across at breast height and 20m tall, with wood at 600 kg per cubic metre. The tree carbon calculator works from measured stem volume rather than a species equation: 1.257 cubic metres of stem, 950 kilograms of total dry biomass once roots are allowed for, 447 kilograms of carbon, and 1,637 kilograms of carbon dioxide.
Both the 447 and the 1,637 are correct statements about that tree. Quote the carbon figure while meaning carbon dioxide and you have understated the tree by a factor of 3.67. Do the reverse and you have overstated it by the same factor. Whenever a figure is quoted without its unit spelled out, the first question is which of the two it is.
A stock is not a rate
The second error is subtler and more consequential. That 1.637 tonnes is a stock: the carbon standing in the tree today, accumulated over decades of growth. It is not an annual sequestration rate, and treating it as one inflates the figure by however many years the tree took to get there.
The distinction decides what the number can be used for. A stock tells you what is stored and what would be released if it burned. A rate tells you what is being removed this year, and that needs either two measurements a year apart or a growth model for the species and the site.
Putting a tonne in perspective
A 1,000 kilometre drive each way, at 170 grams of carbon dioxide per vehicle-kilometre, produces 340 kilograms for the round trip. With four people aboard that is 85 kilograms each, or 42.5 grams per passenger-kilometre, which the travel emissions calculator works out in either basis.
So the entire standing carbon stock of that mature tree, the whole of its life so far, is a little under five such trips. Not five trips a year: five trips, once. That is not an argument against planting trees, which do many things a spreadsheet does not capture. It is an argument against treating a single tree as an offset for an ongoing activity, because the arithmetic does not support it.
For the same question at the scale of a whole economy, the Kaya identity decomposes national emissions into population, income, energy intensity and carbon intensity, which is a more honest place to look for leverage than a tree count.