Quick answer: Wood moisture content (MC) is the weight of water in wood as a percentage of its oven-dry weight. The reference is the oven-dry method (weigh, dry at about 103 °C to constant weight, weigh again). For daily use, pin meters measure electrical resistance between two pins driven into the wood and can read at depth; pinless meters measure dielectric properties through the surface without damage. Both need corrections for wood species, and pin meters for temperature. Wood settles to an equilibrium moisture content set by the surrounding air: about 9 % at 50 % RH and 12 % at 65 % RH at room temperature, which is why storage humidity matters.
Quick answer
- Reference: oven-dry method (dry basis).
- Pin meters: read at depth, small holes, need species and temperature correction.
- Pinless: fast, no damage, reads a fixed depth, needs species (density) setting.
Instrument types
| Method | How it works | Pros | Cons |
|---|---|---|---|
| Oven-dry (reference) | Weigh sample, dry to constant weight, reweigh | Most accurate; defines MC | Destroys sample; takes hours to a day |
| Pin (resistance) meter | Resistance between pins falls as MC rises | Reads at chosen depth with insulated pins; shows moisture gradient | Holes; temperature and species corrections; less accurate above fibre saturation (about 25–30 %) |
| Pinless (dielectric) meter | Electromagnetic field through the surface | Fast; no damage; good for scanning | Fixed depth; affected by density, surface water, metal; species setting needed |
| In-kiln and inline sensors | Resistance probes or capacitance | Continuous drying control | Calibration to species and process |
The USDA Forest Service has published research on electrical moisture meters, their corrections and accuracy (USDA Forest Service; USDA Forest Service).
Formula
MC (%) = (wet weight − oven-dry weight) ÷ oven-dry weight × 100
Example: a sample weighing 112 g wet and 100 g oven-dry has MC = 12 %. Note this is a dry basis, unlike most food and grain measurements.
Equilibrium moisture content of wood
Wood in service takes up or loses water until it matches the air around it. Approximate EMC at about 20 °C (from the USDA Wood Handbook):
| Air RH | Approximate wood EMC |
|---|---|
| 30 % | about 6 % |
| 50 % | about 9 % |
| 65 % | about 12 % |
| 80 % | about 16 % |
| 90 % | about 20 % |
Wood stored in humid air will swell and gain moisture after it is dried. Controlling the storage room's humidity keeps it at the target; see warehouse humidity by stored goods.
Typical target moisture content
| Use | Typical target MC |
|---|---|
| Interior furniture, cabinetry, flooring (heated buildings) | about 6–9 % |
| Exterior joinery | about 10–14 % |
| Structural framing | Below about 19 % at installation (common building-code limit for "dry" lumber) |
| Firewood | Below about 20 % |
Match wood to the expected service conditions; flooring makers usually specify the allowed difference between subfloor and flooring MC. See laminate on a wood subfloor.
Getting reliable readings
- Set the species (or specific gravity for pinless meters).
- Correct for temperature on pin meters (cold wood reads low, warm wood reads high).
- Measure at depth: for thick lumber, take readings at about a quarter to a fifth of the thickness, and compare with the surface to see the gradient.
- Avoid surface water, knots and metal.
- Take several readings per board and several boards per batch.
- Check the meter against oven-dry samples periodically.
The USDA Forest Service provides a guide to drying and measuring hardwood lumber moisture (USDA Forest Service).
Drying wood to target
Kilns and drying rooms remove water to reach target MC; dehumidification drying works at lower temperatures and is common for small and medium operations and high-value species. See drying room setup and our industrial dehumidifiers.
FAQ
Frequently asked questions
How do you measure moisture in wood?
With a pin or pinless moisture meter for daily checks, and the oven-dry method as the reference.
Are pin or pinless moisture meters more accurate?
Both can be accurate when corrected for species (and temperature for pin meters). Pin meters read at depth; pinless meters are faster and non-damaging.
What moisture content should wood be?
About 6–9 % for interior furniture and flooring in heated buildings; below about 19 % for framing.
What is equilibrium moisture content?
The moisture content wood reaches when it stops exchanging water with the surrounding air, set by the air's RH and temperature.
Sources
- USDA Forest Service, Wood Handbook; research on electrical moisture meters; Dry Kiln Operator's Manual / hardwood drying guides.
