Equilibrium moisture content calculator
Wood takes on or gives up water until it balances with the air around it. Enter the temperature and relative humidity of the room, shop or porch, and this gives the moisture content wood will settle at there, by the USDA Wood Handbook's equation.
Units
Equilibrium moisture content
8.5%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Simpson's equation from the Wood Handbook, which matches its EMC table closely.
Why these numbers?
The situation
Air at 70 °F and 45% relative humidity.
From your input: temperature and humidity
What that means
Wood gains or loses moisture until it balances with the air around it: its equilibrium moisture content (Wood Handbook, Chapter 4).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
So
The Handbook gives Simpson's equation for it (Eq. 4-5); from 30 to 130 °F it reproduces the Handbook's Table 4-2 within 0.07% MC (checked cell by cell).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
The number
Equilibrium moisture content 8.5%.
Arithmetic: exact by definition
Sources
EMC chart
The Wood Handbook's Table 4-2, as printed, from 30 to 130 °F. Find the row for the temperature and the column for the humidity. The Handbook notes that the 0.1% steps only show the trend; real wood can differ, especially at high humidity.
| Temp | 5% | 10% | 15% | 20% | 25% | 30% | 35% | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 30 °F (-1.1 °C) | 1.4 | 2.6 | 3.7 | 4.6 | 5.5 | 6.3 | 7.1 | 7.9 | 8.7 | 9.5 | 10.4 | 11.3 | 12.4 | 13.5 | 14.9 | 16.5 | 18.5 | 21.0 | 24.3 |
| 40 °F (4.4 °C) | 1.4 | 2.6 | 3.7 | 4.6 | 5.5 | 6.3 | 7.1 | 7.9 | 8.7 | 9.5 | 10.4 | 11.3 | 12.3 | 13.5 | 14.9 | 16.5 | 18.5 | 21.0 | 24.3 |
| 50 °F (10 °C) | 1.4 | 2.6 | 3.6 | 4.6 | 5.5 | 6.3 | 7.1 | 7.9 | 8.7 | 9.5 | 10.3 | 11.2 | 12.3 | 13.4 | 14.8 | 16.4 | 18.4 | 20.9 | 24.3 |
| 60 °F (15.6 °C) | 1.3 | 2.5 | 3.6 | 4.6 | 5.4 | 6.2 | 7.0 | 7.8 | 8.6 | 9.4 | 10.2 | 11.1 | 12.1 | 13.3 | 14.6 | 16.2 | 18.2 | 20.7 | 24.1 |
| 70 °F (21.1 °C) | 1.3 | 2.5 | 3.5 | 4.5 | 5.4 | 6.2 | 6.9 | 7.7 | 8.5 | 9.2 | 10.1 | 11.0 | 12.0 | 13.1 | 14.4 | 16.0 | 17.9 | 20.5 | 23.9 |
| 80 °F (26.7 °C) | 1.3 | 2.4 | 3.5 | 4.4 | 5.3 | 6.1 | 6.8 | 7.6 | 8.3 | 9.1 | 9.9 | 10.8 | 11.7 | 12.9 | 14.2 | 15.7 | 17.7 | 20.2 | 23.6 |
| 90 °F (32.2 °C) | 1.2 | 2.3 | 3.4 | 4.3 | 5.1 | 5.9 | 6.7 | 7.4 | 8.1 | 8.9 | 9.7 | 10.5 | 11.5 | 12.6 | 13.9 | 15.4 | 17.3 | 19.8 | 23.3 |
| 100 °F (37.8 °C) | 1.2 | 2.3 | 3.3 | 4.2 | 5.0 | 5.8 | 6.5 | 7.2 | 7.9 | 8.7 | 9.5 | 10.3 | 11.2 | 12.3 | 13.6 | 15.1 | 17.0 | 19.5 | 22.9 |
| 110 °F (43.3 °C) | 1.1 | 2.2 | 3.2 | 4.0 | 4.9 | 5.6 | 6.3 | 7.0 | 7.7 | 8.4 | 9.2 | 10.0 | 11.0 | 12.0 | 13.2 | 14.7 | 16.6 | 19.1 | 22.4 |
| 120 °F (48.9 °C) | 1.1 | 2.1 | 3.0 | 3.9 | 4.7 | 5.4 | 6.1 | 6.8 | 7.5 | 8.2 | 8.9 | 9.7 | 10.6 | 11.7 | 12.9 | 14.4 | 16.2 | 18.6 | 22.0 |
| 130 °F (54.4 °C) | 1.0 | 2.0 | 2.9 | 3.7 | 4.5 | 5.2 | 5.9 | 6.6 | 7.2 | 7.9 | 8.7 | 9.4 | 10.3 | 11.3 | 12.5 | 14.0 | 15.8 | 18.2 | 21.5 |
How it is worked out
The calculator uses Simpson's equation, which the Handbook gives as Eq. 4-5 and says fits its tabulated data. Checked cell by cell, it reproduces Table 4-2 within 0.07% moisture content across 30 to 130 °F, which is why the calculator stops there. Relative humidity stops at 98%, because the equation climbs without limit toward 100%. To see what a change in moisture does to a board, use the wood movement calculator.