What this calculator does
A tree 40 cm across at breast height and 20 m tall holds about 1.257 cubic metres of stem. At a wood density of 600 kg per cubic metre and a root allowance of 26 per cent, that is 950 kg of dry biomass, 447 kg of carbon, and 1,637 kg of carbon dioxide taken out of the air, which is 1.637 tonnes.
Two things go wrong with figures like this. The first is confusing carbon with carbon dioxide, which differ by a factor of 3.67 because the two oxygen atoms come along with the carbon. The second is reading a stock as a rate: this is what the tree is holding today after decades of growth, not what it takes up in a year. A single mature tree is not an annual offset for much of anything, and the honest version of the number is more useful than the flattering one.
The formula
The chain is deliberately short and every link is an input you can change. Basal area is π × (DBH / 200)² in square metres, stem volume is basal area × height × form factor, biomass is volume × wood density × (1 + root ratio), carbon is biomass times the carbon fraction, and carbon dioxide is carbon times 44/12. There is no species allometric equation here on purpose: those are local, numerous and easy to apply to the wrong region, so this page asks for the density and the form factor instead.
| Term | Meaning |
|---|---|
| DBH | Diameter at breast height, measured 1.3 m up. The standard forestry measurement. |
| Form factor | How much of a cylinder the trunk actually fills. Around 0.5 for a typical stem. |
| Root to shoot ratio | Below-ground biomass as a fraction of above-ground. Often 0.2 to 0.3. |
| Carbon fraction | Share of dry biomass that is carbon, close to 0.47 for wood. |
The inputs explained
| Field | What to enter |
|---|---|
| Diameter at breast height (cm) | Diameter at breast height in centimetres, from a girth tape or a diameter tape at 1.3 m. |
| Height (m) | Total height in metres. A clinometer or a phone app beats an estimate by a long way. |
| Form factor (stem taper) | Form factor for the taper of the stem. Half is a reasonable default for a single-stemmed tree. |
| Wood density, oven dry (kg/m³) | Oven-dry wood density for the species, which is the input most worth looking up properly. |
| Root to shoot ratio | Root to shoot ratio for the species and site. Drier sites generally push it higher. |
| Carbon fraction of dry biomass | Carbon fraction of dry biomass. The IPCC default of 0.47 is the usual choice. |
When to use it
Measuring a tree in a garden or paddock
Tape the trunk at 1.3 m, estimate the height, and use a species density from a wood database. The result is the carbon standing there now.
Scaling to a stand
Work out a representative tree, then multiply by stems. For anything more careful than that, measure a size distribution rather than an average, because diameter enters squared.
Sanity-checking a published claim
If a figure for one tree looks high, put the dimensions in here. Numbers in the tonnes of carbon dioxide are plausible for a large tree; numbers in the tonnes of carbon a year are not.
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 carbon does tree size hold?
Only the diameter at breast height changes.
| Diameter at breast height | Carbon dioxide stored | Stem volume | Total dry biomass |
|---|---|---|---|
| 20 cm | 409 kg | 0.314 m³ | 238 kg |
| 30 cm | 921 kg | 0.707 m³ | 534 kg |
| 40 cm | 1,637 kg | 1.257 m³ | 950 kg |
| 50 cm | 2,558 kg | 1.963 m³ | 1,484 kg |
| 60 cm | 3,684 kg | 2.827 m³ | 2,138 kg |
How much does wood density matter?
The same tree with the density of different species.
| Wood density | Carbon dioxide stored | In tonnes of CO₂ | Total dry biomass |
|---|---|---|---|
| 350 kg/m³ | 955 kg | 0.955 t | 554 kg |
| 450 kg/m³ | 1,228 kg | 1.228 t | 713 kg |
| 600 kg/m³ | 1,637 kg | 1.637 t | 950 kg |
| 750 kg/m³ | 2,046 kg | 2.046 t | 1,188 kg |
| 900 kg/m³ | 2,456 kg | 2.456 t | 1,425 kg |
Questions
Is this an annual sequestration figure?
No. It is a stock: the carbon standing in the tree today, accumulated over its whole life. An annual rate would need two measurements a year apart, or a growth model for the species and site.
Why 44/12?
Because carbon dioxide has a molar mass of about 44 and carbon about 12, so every kilogram of carbon in the wood came from about 3.67 kg of carbon dioxide. That ratio is chemistry and is the only fixed constant on the page.
Why not use a species equation?
Allometric equations are fitted to particular species in particular regions and give poor answers outside them. Taking density, form factor and root ratio as inputs makes the assumptions visible instead of hiding them in a fitted constant.
Are branches and leaves counted?
Not separately. The calculation runs on stem volume, so a tree with a heavy crown is understated unless you raise the form factor to cover it. The result is conservative, which is the better direction to err in.
Where do I find wood density?
A wood density database or a local forestry service, and make sure the figure is oven-dry density rather than green or air-dry, since those are substantially higher and would inflate the result.
For the trunk measurement on its own there is tree basal area, which also handles stand density per hectare. Travel emissions puts a tonne of carbon dioxide in context, and the Shannon diversity index covers the species side of a stand.