From Tree Biomass to Tonnes of CO2

Updated 2026-11-02 · 4 min read · By the akar.earth specialist team

How to convert tree biomass to CO2: allometric equations, root-to-shoot ratios, carbon fraction and the 44/12 factor, with a worked example.

To convert tree biomass to CO2, estimate dry above-ground biomass with an allometric equation, add below-ground biomass, multiply the total by the carbon fraction (the IPCC default for wood is 0.47) to get carbon, and multiply carbon by 44/12 (about 3.67) to get CO2. Project totals then scale up from plots to hectares, with uncertainty applied.

Each step adds assumptions, and errors multiply. Getting the inputs right (good diameter and height measurements, the right wood density and an appropriate equation) matters far more than the arithmetic.

The calculation chain

Worked example (illustrative)

One tree with diameter 30 cm, height 20 m and wood density 0.5 g/cm³:

1. ρD²H = 0.5 × 30² × 20 = 9,000.
2. AGB = 0.0673 × 9,000^0.976 ≈ 487 kg dry biomass.
3. Assume a root-to-shoot ratio of 0.24 for illustration: BGB ≈ 117 kg.
4. Total biomass ≈ 604 kg.
5. Carbon = 604 × 0.47 ≈ 284 kg C.
6. CO2 = 284 × 44/12 ≈ 1,040 kg, or about 1.04 tCO2.

The root-to-shoot ratio here is an assumption for the example. Use the ratio your methodology specifies or one appropriate to your forest type. Try different inputs in our tree CO2 calculator.

Choosing the allometric equation

• Species-specific equations are best where they were developed on similar sites and size ranges. Indonesia has published equations for some plantation and native species; check their sample size and range.
• Pantropical equations such as Chave et al. 2014 are robust defaults across tropical forests when height and wood density are measured or well estimated.
• Height matters. Equations that use diameter only can bias results if local height-diameter relationships differ from the training data.
• Do not extrapolate beyond the diameter range used to build an equation.

See allometric equations for a fuller comparison.

Wood density

Wood density varies by species, within species, and with tree age and site. Fast-growing species such as sengon tend to have low density; dense hardwoods such as ulin have high density. Use species values from recognised databases, give a range when uncertain, and note that density for young plantation trees may be lower than published values from mature trees.

Indonesia specifics

• Tall dipterocarp forests need careful height measurement, since a few large trees can hold much of a plot’s biomass.
• Agroforestry trees are often pruned or pollarded, changing the shape that equations assume.
• Mangrove species have their own published equations and high-density wood; see mangrove soil carbon for the sediment pool.
• Palms, bamboo and tree ferns need specific equations; generic tree equations do not apply.

From one tree to a project

A single tree calculation is only the start. To get project-level tonnes:

1. Sum trees per plot to get biomass and CO2 per plot.
2. Convert to per hectare using the plot area, corrected for slope if required.
3. Average within each stratum, such as species mix, age class or land type.
4. Multiply by stratum area from mapped polygons.
5. Subtract the baseline stock or growth that would have happened anyway.
6. Apply deductions for uncertainty, leakage and the buffer pool.

Under VM0047’s area-based approach, remote sensing and a dynamic performance benchmark from control plots also enter the calculation, while plot data calibrate and check the results. For smaller plantings, the census-based approach measures every tree instead of sampling.

Keep the whole calculation reproducible. Store raw measurements, the equation and parameters used, and the code or spreadsheet that produced the totals. Validators will re-run parts of it, and a transparent chain from tree to tonne makes verification faster.

Common mistakes

• Applying the carbon fraction to fresh, not dry, biomass.
• Forgetting the 44/12 conversion, or applying it twice.
• Using DBH in metres or wood density in kg/m³ in equations built for cm and g/cm³.
• Using a single wood density for a whole mixed-species plot.
• Ignoring uncertainty, which can significantly reduce credited tonnes. See uncertainty deductions.

Read about the five carbon pools, or start at the forest carbon hub. Our carbon modelling service builds full project models.