This tree carbon calculator estimates how much carbon dioxide a tree absorbs each year and stores across its life, from its trunk diameter — the estimate behind “plant a tree” carbon claims, made transparent.
Estimates use the widely-cited allometric rule that a tree in active growth sequesters on the order of 0.6–0.8 kg CO₂ per year per centimeter of DBH, and that standing biomass ≈ (DBH in cm)^2.2 × 0.14 kg dry mass, ~47% of which is carbon (×44/12 for CO₂). Honest ±40% territory — species, site and climate move real numbers widely; forestry-grade answers need species-specific equations.
From Diameter to Carbon — the Allometric Chain
Forest science estimates tree mass from trunk diameter with allometric equations (mass grows faster than diameter — roughly DBH^2.2 here), takes ~47% of dry mass as elemental carbon, and converts carbon to CO₂ by the molar ratio 44/12. Annual absorption scales with the growth the tree is currently doing — the “one mature tree ≈ 21 kg CO₂/year” figure you’ll see everywhere corresponds to a ~30 cm DBH tree on this calculator. Age that tree first: our Tree Age Calculator. Part of our Ecology Calculators collection.
Worked Example (Verified)
- 30 cm DBH, average density (defaults): absorption ≈ 21 kg CO₂/year — the canonical “mature tree” statistic, demystified; stored stock ≈ 430 kg CO₂ in the standing tree
- Scale check — a young 10 cm tree: ~7 kg/yr and ~40 kg stored; a veteran 60 cm oak-class tree (density 1.15): ~48 kg/yr with over 2 tonnes stored. Carbon is a big-tree game: protecting veterans beats planting saplings on any near-term ledger
- Perspective: the average US resident emits ~14 tonnes CO₂/year — roughly 650 such 30 cm trees’ annual absorption per person. Trees help; arithmetic tempers slogans
- Oxygen mirror: the same 21 kg CO₂ uptake releases ~15 kg O₂/year via photosynthesis stoichiometry (32/44)
Honest Caveats
- Sequestration is storage, not destruction: a tree that burns or rots returns its carbon. Permanence is the weakness of every tree-offset claim.
- Growth rates vary 5× by climate and species; desert and boreal trees play a slower game than tropical plantings.
- Soil carbon under forests rivals the wood itself — this calculator counts the tree, not its ecosystem.
- Certified offset projects use species-specific equations, plot sampling and buffer pools; treat web numbers (this one included) as orientation, not audit.
Related Tools
- Tree Age Calculator — diameter to age by species
- Carbon Footprint Calculator — the emissions side of the ledger
- Ecology Calculators — the full collection
Frequently Asked Questions
How much CO₂ does one tree absorb per year?
The famous figure is ≈ 21 kg (48 lb) per year for a mature tree — corresponding to roughly a 30 cm DBH trunk in this calculator’s model (0.7 kg per cm DBH annually). Young trees absorb just a few kg; large veterans can exceed 50 kg. Species, climate and site quality swing real values widely.
How is tree carbon calculated from diameter?
Via allometry: dry biomass ≈ DBH^2.2 × a species-density factor; carbon ≈ 47% of dry biomass; CO₂-equivalent = carbon × 44/12 (the molar mass ratio). This calculator shows the standing stock and an annual sequestration estimate side by side — they’re different quantities and offset claims often blur them.
How many trees offset a car?
A typical car emits ~4.6 tonnes CO₂/year. At 21 kg per tree-year, that’s ~215 mature trees’ annual absorption — or, framed as planting: you’d need to grow roughly that many trees to maturity and keep them standing. It’s why credible climate plans pair trees with emission cuts rather than substituting one for the other.
Does a tree release its carbon when it dies?
Yes, eventually — decomposition and fire return stored carbon to the atmosphere (some fraction becomes long-lived soil carbon and wood products extend storage). This is the “permanence” problem in carbon accounting, and why forest protection (keeping existing stock stored) scores so highly.
Which trees absorb the most CO₂?
Fast-growing, long-lived, dense-wooded species in good growing conditions — many oaks, Douglas-fir, and tropical hardwoods rank high on lifetime storage; pure speed (poplar, eucalyptus) stores fast but often dies or is harvested young. Total lifetime storage favors the long game: big, old, still-standing trees.
Is this tree carbon calculator free?
Yes — free, instant, no sign-up. An estimator for education and planning, not a carbon-credit audit.