Moisture content arithmetic: dry basis, wet basis, green density

What the percentage is a percentage of

When someone says "the boards are at 18 %", ask: 18 % of what? The wood industry standard is the dry basis: the mass of water divided by the mass of oven-dry wood, times 100.

MC = (m_wet - m_dry) / m_dry x 100

Fuel wood is often quoted on the wet basis, relative to the original mass including the water. The dry-basis value is always the larger and has no upper limit: a light softwood can exceed 100 % on the dry basis, which is impossible on the wet basis. The Wood Handbook gives about 267 % as the most water a wood of basic specific gravity 0.30 can hold, and about 44 % at 0.90.

Worked example 1: moisture content from two weighings

A sample weighs 1.45 kg as received and 1.00 kg after oven-drying.

• Water: 1.45 - 1.00 = 0.45 kg.
• Dry basis: 0.45 / 1.00 x 100 = 45 %.
• Wet basis: 0.45 / 1.45 x 100 = about 31 %.

Always write "MC, dry basis" or "wet basis" in specifications, kiln logs and biomass sales.

Converting between bases

• MC_wet = MC_dry / (100 + MC_dry) x 100
• MC_dry = MC_wet / (100 - MC_wet) x 100

Examples: 25 % dry basis is 25 / 125 x 100 = 20 % wet basis. 100 % dry basis is 50 % wet basis. 150 % dry basis is 150 / 250 x 100 = 60 % wet basis.

Green moisture content differs by species and zone

Wood Handbook Table 4-1 (North America), dry basis:

Softwood sapwood is usually far wetter than heartwood; hardwoods are less consistent. A Swedish study (Hajian and co-authors, 2023, Europe) measured 107 % for Scots pine sapwood, 95 % for Norway spruce sapwood and 35 % for Norway spruce heartwood. A charge mixing sapwood and heartwood starts with a wide spread, so sort by sapwood share where you can.

Density: always name the moisture state

Density is mass divided by volume (kg/m³). Specific gravity is density divided by the density of water (1000 kg/m³). Both mass and volume change with moisture, so a density figure needs its moisture state. The Wood Handbook uses oven-dry mass and changes the volume basis:

• Basic specific gravity (Gb): oven-dry mass over green volume.
• Specific gravity at 12 % MC (G12): oven-dry mass over the volume at 12 % MC.

Worked example 2: density at 12 %

White ash has G12 of about 0.605 (North America). Density with its water included: 0.605 x 1000 x 1.12 = about 678 kg/m³. As a rule of thumb, density at 12 % is about 1120 x G12 kg/m³: Sitka spruce (G12 0.40) about 450, coast Douglas-fir (G12 0.48) about 540 kg/m³.

Worked example 3: green density and water

A green piece at moisture content MC weighs Gb x 1000 x (1 + MC/100) kg per cubic metre of green volume. For a spruce with Gb 0.40 and sapwood at 120 %:

• Green density: 0.40 x 1000 x 2.20 = 880 kg/m³.
• Water in it: 0.40 x 1000 x 1.20 = 480 kg per m³.

That is the figure that matters for truck loading and for the water a yard or kiln must remove.

Checklist before you compare two numbers

1. Same basis (dry or wet)?
2. Same moisture state for density (green, 12 %, oven-dry)?
3. Same species and zone (sapwood or heartwood)?
4. Average or single piece? Oven-dry density varies by about 10 % within a species (coefficient of variation, Wood Handbook).

Key points

• Industry moisture content is dry basis: (m_wet - m_dry) / m_dry x 100. It can exceed 100 %.
• Wet basis is always the smaller number: MC_wet = MC_dry / (100 + MC_dry) x 100.
• Green softwood sapwood is usually much wetter than heartwood (Sitka spruce 142 % against 41 %, North America).
• Density at 12 % is about 1120 x G12; green density is 1000 x Gb x (1 + MC/100).
• Water per cubic metre of green wood is Gb x 1000 x MC/100 kg.