akar.earthCarbon fraction: Definition
What is the carbon fraction of biomass? The IPCC 0.47 default, how it turns dry biomass into carbon and CO2, and when to use other values.
Carbon fraction is the share of a plant’s dry biomass that is elemental carbon, expressed in tonnes of carbon per tonne of dry matter. For tropical wood the IPCC 2006 default is 0.47, meaning a tonne of oven-dry wood holds about 470 kg of carbon.
Why it matters
Carbon fraction is the bridge between biomass and carbon. Every forest carbon calculation multiplies dry biomass by this value before converting to CO2. Because it applies to every tonne in the inventory, a small change, say 0.47 versus 0.50, shifts total credits by about 6%.
How it is used
The standard conversion is:
tCO2 = dry biomass (t) × carbon fraction × 44/12
The 44/12 factor (about 3.67) converts the mass of carbon to the mass of CO2, based on the atomic weights of carbon (12) and oxygen (16). Our CO2 from biomass guide works through examples.
The IPCC AFOLU Guidelines give default values by climate domain and tissue type. Methodologies such as VM0047 typically allow the IPCC default or a species-specific value from peer-reviewed sources. Measured values usually range a few percentage points around 0.47 depending on species and tissue.
Indonesia angle
For Indonesian ARR, REDD+ and agroforestry projects, 0.47 is the usual default for woody biomass. Mangrove and peat projects apply it to tree biomass, but soil and peat carbon are measured directly as carbon content, not through this factor. Leaves, litter and dead wood can have different fractions, and some methodologies give separate defaults for them.
Common mistakes
- Using 0.5 out of habit, which is not conservative.
- Applying the carbon fraction to fresh (wet) weight instead of dry weight.
- Forgetting the 44/12 step and reporting tonnes of carbon as tonnes of CO2.
- Applying the factor twice when a tool already outputs carbon.
Related terms
Carbon fraction sits next to above-ground biomass, wood density and tCO2e. Try it in our tree CO2 calculator or see the ARR hub.
Frequently asked questions
What carbon fraction should I use for tropical trees?
The IPCC 2006 default for tropical and subtropical wood is 0.47 tonnes of carbon per tonne of dry matter. Use it unless your methodology names another value or you have robust species data.
Why do some calculators use 0.5?
0.5 is an older rounded value. It slightly overstates carbon compared with the IPCC tropical default, so most current methodologies avoid it.