Wood Moisture Content Calculator

How it works

Wood moisture content is the weight of water in a piece divided by the weight of the wood with no water at all, its oven-dry weight. Because the divisor is the dry wood, not the wet piece, values above 100% are possible in green wood.

The oven-drying method weighs a sample, dries it in an oven at 101 to 105 °C (214 to 221 °F) until its weight no longer changes between weighings 3 hours apart, and weighs it again. The difference is the water. The Wood Handbook (ch. 13) describes this as the most universally accepted method for determining moisture content, covered by ASTM D4442 Method A.

Once the oven-dry weight is known, the weight at any other moisture content follows directly: oven-dry weight × (1 + MC ÷ 100). That gives the weight a board will have once it has dried, or how much water must leave it to get there.

Below about 30% moisture content (the fiber saturation point) wood shrinks as it dries and swells as it takes up moisture. Above it, the extra water sits in the cell cavities and drying does not change the size of the piece.

Formula

MC (%)            = (wet weight − oven-dry weight) ÷ oven-dry weight × 100
water             = wet weight − oven-dry weight
weight at target  = oven-dry weight × (1 + target MC ÷ 100)
change to target  = oven-dry weight × target MC ÷ 100 − water

Example

A sample that weighs 112 g and 100 g after oven-drying holds 12 g of water, so its moisture content is 12 ÷ 100 × 100 = 12.0%. At a target of 7% it would weigh 100 × 1.07 = 107 g, a loss of 5 g.

A green board sample weighing 450 g that dries to 250 g has a moisture content of 200 ÷ 250 × 100 = 80%, above the fiber saturation point.

Assumptions and limitations

• Moisture content is on the oven-dry basis used in the US wood industry and the Wood Handbook (FPL-GTR-282, 2021, Eq. 4-2). Moisture quoted on the wet basis (water ÷ wet weight) is a different, lower number for the same piece.
• The sample is assumed to lose only water in the oven. With woods containing volatile extractives, oven-drying can give values slightly greater than the true moisture content (Wood Handbook ch. 13).
• A small sample stands in for the whole board. A board can be wetter in the middle than near the surface, so where the sample is cut matters.
• The fiber saturation point is taken as 30%, the Wood Handbook's average; individual species and pieces vary by several points.

Frequently asked questions