Firewood Moisture Calculator
Dry-basis moisture from wet and dry sample weights, with a seasoned or not check.
What it does
Burning wet wood wastes heat: every pound of water in the fuel absorbs energy as it boils away before the wood can heat the room. Moisture also drives creosote, smoke, and chimney risk, which is why EPA Burn Wise and every stove manual push seasoned wood under 20 %.
The standard definition is dry-basis moisture content, MC = (wet weight - dry weight) / dry weight x 100, the same definition the USDA Forest Products Laboratory uses in the Wood Handbook.
You do not need a lab. Weigh a fresh split, dry it completely (household oven near 105 °C / 220 °F until the weight stops falling, or a long spell in a warm dry room), reweigh, and run the numbers. A pin meter cross-checks the result.
The wet (as-received) basis divides by the starting weight instead. Both are reported, because meters vary and EPA's 20 % guidance is on the dry basis, so that is the number to compare.
How to use
- Pick pounds or kilograms.
- Weigh a freshly split piece: the wet weight.
- Dry it fully in a warm oven or dry room until weight is stable.
- Weigh it again: the dry weight.
- Enter both weights.
- Read the dry-basis percent and the reading line.
Formula
- wet
- Weight of a fresh, as-is split sample.
- dry
- Weight after the sample is fully dried (oven-dry).
The dry-basis definition follows the Wood Handbook (formula 4-2): water mass over oven-dry wood mass. Oven-dry means no free or bound water remains, typically about 105 °C until the weight quits changing. The wet basis reports the same water against the starting weight and always reads lower, so a 15 % dry-basis sample shows about 13 % wet basis.
Worked example
Example: 9.2 lb sample dries to 8.0 lb
A fresh split weighs 9.2 lb; fully dried it weighs 8.0 lb.
- Water: 9.2 - 8.0 = 1.2 lb.
- Dry basis: 1.2 / 8.0 x 100 = 15.0 %.
- Wet basis: 1.2 / 9.2 x 100 = 13.0 %.
- 15 % sits under the 20 % line: seasoned, ready to burn.
- Tool shows 15 %, 13 %, 1.2 lb of water, seasoned.
Reference table
| Wet weight | Dry weight | Dry-basis MC | Reading |
|---|---|---|---|
| 9.2 lb | 8.0 lb | 15.0 % | Seasoned |
| 10.4 lb | 9.0 lb | 15.6 % | Seasoned |
| 10.0 lb | 8.5 lb | 17.6 % | Seasoned |
| 12.0 lb | 10.0 lb | 20.0 % | Borderline |
| 14.0 lb | 10.0 lb | 40.0 % | Unseasoned |
Limits
- The dry weight must come from a truly dry sample; a half-dried split overstates seasoning.
- Oven drying needs about 105 °C until weight stabilizes; cutting it short understates moisture.
- One split rarely represents a whole cord: sample several pieces from different logs.
- Pin meters use their own scales and species corrections; trust the oven method when they disagree.
Common mistakes
- Comparing a wet-basis number against the 20 % dry-basis rule.
- Drying the sample in a hot fire where it chars instead of fully drying.
- Weighing damp bark or a snowy split.
- Testing one piece from a stack and calling the whole cord 15 %.
- Assuming rain-soaked surface re-wets the interior; measure a fresh split face, not the outside.
Try it yourself
Quick checks. Answers sit tucked under each question.
12.0 lb dries to 10.0 lb?
20.0 %, borderline: EPA wants under 20 %.
10.4 lb dries to 9.0 lb?
15.6 %, seasoned.
14.0 lb dries to 10.0 lb?
40 %, unseasoned: still green or rain soaked.
Questions
How do I test firewood moisture without a meter?
Weigh a fresh split, dry it fully (oven near 105 °C / 220 °F until weight stops dropping), reweigh, then run (wet - dry) / dry x 100.
What moisture content should firewood be?
Under 20 % on a dry basis per EPA Burn Wise. 20 to 25 % is borderline, and over 25 % burns cool, smokes, and builds creosote.
Why is dry basis used?
It compares water to the actual wood substance, the wood science standard in the Wood Handbook, so the number does not shift with how much water the sample started with.
Can firewood be too dry?
Kiln-dried wood under 10 % burns fast and hot but is fine; outdoors, sub-10 % is rare. Very dry fuel just needs a stove you can damper down.
Does rain ruin seasoned wood?
Rain re-wets only the surface. A few dry days recover most of it; the interior stays near its seasoned moisture.
How long does firewood take to season?
Split wood typically needs 6 to 12 months: softwoods faster, dense hardwoods slower, all faster with sun, wind, and a covered top.
Sources
- USDA Forest Products Laboratory: Wood Handbook, Chapter 4, Moisture Relations (PDF)
- EPA Burn Wise: Learn Before You Burn