Wet vs Dry Basis Moisture Converter
This tool was built by Serge (MSc) using established science and government sources. My academic background covers Biogeochemistry, Forest Science, Environmental Biology, and Plant Biology, and my field research directly measured soil moisture and tree growth in open-air experiments. My tools are for educational purposes and give practical guidance, not professional advice.
Firewood moisture can be written two ways, and the same log gives two different numbers.
Wet basis divides the water by the total weight of the wood. Dry basis divides the water by the weight of the dry wood alone. A log at 20% wet basis is 25% dry basis.
I built this tool so you can convert a number from a label, a guide or a moisture meter before you compare it with the 20% target. You can also work out both figures from your own oven test weights.
How This Works
Both figures describe the same amount of water. They only divide it by a different weight.
To go from wet to dry basis, the tool uses dry = wet ÷ (100 − wet) × 100.
To go from dry to wet basis, it uses wet = dry ÷ (100 + dry) × 100.
With oven test weights, the water is the weight lost in the oven. Dividing it by the oven-dry weight gives dry basis, and dividing it by the starting weight gives wet basis.
The tool then compares the dry basis figure with the 20% firewood target.
| Wet basis | Dry basis |
|---|---|
| 10% | 11.1% |
| 15% | 17.6% |
| 16.7% | 20% |
| 20% | 25% |
| 25% | 33.3% |
| 33.3% | 50% |
| 50% | 100% |
Understanding Your Result
Under 20% dry basis: ready to burn. This is the scale most moisture meters use.
20% to 25%: close to the line. Give it more time, or re-test a freshly split piece from the middle of the stack.
Above 25%: still too wet. It will smoke and waste heat.
The gap between the two bases grows as wood gets wetter. At low moisture the numbers sit close together, but at 50% wet basis the dry basis figure is 100%. Mixing them up can make wet wood look far drier than it is.
Important Limitations
The conversion is exact arithmetic, but it is only as good as the number you put in.
Check which basis your source uses before you convert. Most pin meters read dry basis, while some fuel and biomass figures are given on wet basis.
A meter reading also depends on wood temperature and species. A single reading can land on a knot or a crack, so average several readings from a fresh split.
For oven tests, the weight must stop dropping before you record the dry weight, or the result will read low.
Scientific Basis
The definitions come from government sources. The USDA Forest Products Laboratory Wood Handbook expresses wood moisture content as the weight of water as a percentage of the oven-dry weight, which is the dry basis. Wet basis divides the same water by the total weight instead.
The handbook also covers how moisture content is measured, by oven drying and with moisture meters. The 20% target comes from the EPA Burn Wise programme, which advises testing wood with a moisture meter before burning.
USDA Forest Products Laboratory, Wood Handbook Chapter 4: Moisture Relations and Physical Properties of Wood (moisture content as a percentage of oven-dry weight)
USDA Forest Products Laboratory, Wood Handbook Chapter 13: Drying and Control of Moisture Content (measuring moisture content, oven-dry and meter methods)
EPA Burn Wise, Best Wood-Burning Practices (wood burns best under 20% moisture)
Related Reading
For the full explanation with worked examples, see my guide to wet vs dry basis for firewood moisture.
If you test outdoors in winter, read why cold firewood reads drier than it is.
You can also check your reading with my Firewood Moisture Target Checker, or see how much longer wet wood needs with the Firewood Seasoning Time Estimator.
This calculator is for general educational use. It converts moisture figures and gives a guide to firewood readiness, and is not a substitute for professional or laboratory analysis. Results depend on your own wood, meter and conditions.
