Net heat per kilogram of wood or peat falls with moisture by a single formula, net = dry value x (1 - w) - 2.44 x w in MJ/kg, where w is the water fraction on a wet basis. With assumed oven-dry values of 18.5 MJ/kg for wood and 19.5 for peat, air-dried wood at 20% gives about 14.3 MJ/kg, peat at 30% about 12.9, and bituminous coal 24-30; moisture, not species, usually decides the fuel.
Part of Peat Harvesting
The shared heating-value and moisture arithmetic for wood, peat and coal: the formula, its assumptions, and what it implies for weight, volume and a household’s supply.
One Basis For Every Fuel
To compare fuels, quote the net calorific value (NCV): the heat released when the fuel burns and its water leaves as vapour, which is what a stove can use. Units: 1 MJ/kg equals 1 GJ per tonne, and 1 MJ/kg is about 430 Btu/lb (1 Btu/lb = 0.002326 MJ/kg).
State the moisture basis every time. Wet-basis moisture w is water ÷ total mass. Dry-basis moisture u is water ÷ dry mass. They convert by u = w ÷ (1 − w) and w = u ÷ (1 + u). So 20% wet basis is 25% dry basis, and fresh wood at 50% wet basis is 100% dry basis. Every moisture figure in this article is wet basis unless it says otherwise.
The Formula
NCV as burnt = NCV(dry) × (1 − w) − 2.44 × w, in MJ/kg.
The first term is the fuel that remains once the water share is taken out. The second charges 2.44 MJ for each kilogram of water, the heat needed to evaporate it (the standard figure at 25 °C; in a real flue some more is lost). Assumptions, not measurements. Oven-dry NCV is taken as 18.5 MJ/kg for hardwood, which a later check found consistent with the 18-19 standard references give. Peat is taken as 19.5 MJ/kg, and that figure could not be confirmed by any authority — so do not lean on the “peat beats wood per kilogram” framing it implies; treat the two as comparable until you can measure your own.
Two further caveats on the formula itself. It is the common textbook wet-basis shortcut and it omits the hydrogen term: burning the hydrogen in the fuel makes more water, which also leaves as vapour, so a fuller form subtracts roughly 2.44 × 9 × (hydrogen fraction) as well. That costs about another 1-1.3 MJ/kg for wood, so the figures here run slightly optimistic. And in the coal table below, anthracite at 30-35 GJ/t is high at the top — reported averages sit nearer 29.
| Moisture (wet basis) | Wood, MJ/kg | Peat, MJ/kg |
|---|---|---|
| 15% | 15.4 | 16.2 |
| 20% | 14.3 | 15.1 |
| 30% | 12.2 | 12.9 |
| 40% | 10.1 | 10.7 |
| 50% | 8.0 | 8.5 |
Wood at 50% wet basis (100% dry basis) is the freshly cut state. A tonne of it gives 8.0 GJ. Seasoned to 20% it gives 14.3 GJ, which matches the parent coppice article’s “roughly 15 GJ” for air-dried hardwood.
Worked Example: Weight, Volume And Coal
Inputs: the parent’s densities (peat 300-500 kg/m³, hardwood 500-800, bituminous coal 1,100-1,500, all labelled “dry” there) and bituminous 24-30 GJ/t.
- Peat at 30%: 12.9 ÷ 14.3 = 0.90, so you need 14.3 ÷ 12.9 = 1.11 t of peat for each tonne of air-dried wood.
- Per volume, peat: 12.9 × 300-500 = 3,870-6,450 MJ/m³, so 3.9-6.5 GJ/m³.
- Per volume, hardwood: 14.3 × 500-800 = 7,150-11,440 MJ/m³, so 7.2-11.4 GJ/m³. Mid-range, 9.3 against 5.2, peat needs about 1.8 times the volume.
- Bituminous: 24 × 1,100 = 26.4 GJ/m³ to 30 × 1,500 = 45 GJ/m³ of solid coal.
- Coal against wood by weight: 24-30 against 14.3 is 1.7-2.1 times, close to the parent coal article’s “50-100% more”.
Where The Net Value Reaches Zero
NCV = 0 when dry × (1 − w) = 2.44 × w, so w = dry ÷ (dry + 2.44). For wood 18.5 ÷ 20.94 = 88%; for peat 19.5 ÷ 21.94 = 89%. Fresh peat at 80-90% water has between about 1.9 MJ/kg (at 80%) and none (at 89%), so it cannot sustain a fire, which is why drying is the main work.
Cross-Check: Does The Parent’s Block Count Heat A House?
Assumption (flagged, and wide): peat bulk density depends almost entirely on how decomposed it is, and measured values across sites span roughly 20 to over 1,000 kg of dry matter per m³ — around 20-50 for virgin, barely decomposed Sphagnum at the surface, rising steeply with humification. The well-humified peat you would actually cut for fuel sits in the middle; this worked example uses 100-150 kg/m³, which is a guess inside a fiftyfold range, so weigh a measured volume of your own peat before trusting any tonnage derived from it. The 3 L block (30 × 10 × 10 cm) then holds 0.30-0.45 kg dry matter. At 30% it weighs 0.30 ÷ 0.7 = 0.43 kg to 0.45 ÷ 0.7 = 0.64 kg and gives 0.43 × 12.9 = 5.5 MJ to 0.64 × 12.9 = 8.3 MJ.
- 5,000 blocks × 5.5 MJ = 27.6 GJ; 8,000 × 8.3 MJ = 66.3 GJ.
- The parent wood budget (cooking 1-2 t, heating 3-6, hot water 0.5-1 t = 4.5-9 t) × 14.3 = 64-129 GJ.
So the parent’s block count supplies from about a fifth to about all of the wood-equivalent budget. If your bog is on the dense side, 5,000 blocks is not a year’s fuel; weigh your own dried blocks before planning.
Inconsistencies To Note
The parent peat table labels its energy “GJ/tonne, dry” and gives well-dried peat 14-16, but oven-dry peat would be near 19-20 net. Read those figures as for air-dried peat, around 15-25% moisture.
Common Mistakes
- Quoting moisture without its basis. 20% wet basis is 25% dry basis, and 20% dry basis is 16.7% wet basis; two people quoting “20%” on different bases are describing different fuels.
- Comparing fuels at different moisture. Compare at the moisture you will actually burn.
- Carrying a coal rank’s value to another rank. The 24-30 GJ/t is bituminous only.
Safety
Wet fuel burns dirty
Burning wet wood or peat in a closed stove yields much more smoke, carbon monoxide and flue deposits than dry fuel (Carbon Monoxide Risks). Use the numbers to dry fuel, never to justify burning it green.
What’s Next
- Learn next: Carbon Implications — what each GJ costs the atmosphere
- Combine with: Coal Types & Quality — coal ranks and their ranges