What equilibrium moisture content is
Wood trades moisture with the air around it until the two balance. The moisture content it settles at is its equilibrium moisture content, or EMC. Humidity drives it: the same board is about 6% in a heated house in January and 12% or more in a damp garage in August. Temperature matters less; warmer air lowers it a little.
This calculator uses the Hailwood-Horrobin equation with the coefficients in the USDA Wood Handbook, the source of the EMC tables woodworkers use. It's the same math as Wood Calc's Moisture screen.
EMC chart
The moisture content wood settles at for each temperature and humidity, worked out with the same equation as the calculator above. Find your shop's temperature down the side and its humidity across the top.
| Relative humidity | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
| 30°F (-1°C) | 2.6 | 4.6 | 6.3 | 7.9 | 9.5 | 11.3 | 13.6 | 16.5 | 21.0 |
| 40°F (4°C) | 2.6 | 4.6 | 6.3 | 7.9 | 9.5 | 11.3 | 13.5 | 16.5 | 21.0 |
| 50°F (10°C) | 2.6 | 4.6 | 6.3 | 7.9 | 9.5 | 11.2 | 13.4 | 16.4 | 20.9 |
| 60°F (16°C) | 2.5 | 4.6 | 6.3 | 7.8 | 9.4 | 11.1 | 13.3 | 16.2 | 20.7 |
| 70°F (21°C) | 2.5 | 4.5 | 6.2 | 7.7 | 9.2 | 11.0 | 13.1 | 16.0 | 20.5 |
| 80°F (27°C) | 2.4 | 4.4 | 6.1 | 7.6 | 9.1 | 10.8 | 12.9 | 15.7 | 20.2 |
| 90°F (32°C) | 2.4 | 4.3 | 5.9 | 7.4 | 8.9 | 10.6 | 12.6 | 15.4 | 19.9 |
| 100°F (38°C) | 2.3 | 4.2 | 5.8 | 7.2 | 8.7 | 10.3 | 12.3 | 15.1 | 19.5 |
- Under 6%: drier than furniture
- 6–12%: furniture and interior woodwork
- Over 12%: damp
What it means for a tabletop or a panel
Wood only moves when its moisture content changes, so what matters is the swing between the driest and the dampest EMC where the piece will live, not the number on your moisture meter today. A tabletop has to be free to move across its width: fastened with buttons or figure-eight clips, not glued or screwed tight to the apron. A panel in a frame floats in its grooves, with room to grow in summer and enough depth that it doesn't pull out of them in winter.
Build with wood at about the middle of that range. Lumber straight from a damp shed will shrink once it comes indoors; let it sit in the room it's going to, or in a shop kept like it, until a meter reads close to the EMC here.
A worked example
A house heated to 70°F with 30% humidity in winter puts wood at 6.2%. The same house at 75°F and 70% humidity in a humid summer puts it at 13.0%. Furniture there goes through a swing of about 7% moisture content every year. Enter 6.2% and 13% as the initial and final moisture in the Wood Movement Calculator to see what that does to a board.