Green Lumber Shrinkage Calculator

How it works

Green wood holds free water in its cell cavities and bound water in its cell walls. Drying first removes the free water without any change in size. Shrinkage starts at the fiber saturation point, about 30% moisture content for most species, and continues as the cell walls give up bound water.

The USDA Wood Handbook tabulates the total shrinkage of each species from green to ovendry (0% moisture content) as a percentage of the green dimension, separately for the radial direction (across the growth rings) and the tangential direction (along them). Assuming shrinkage is linear below the fiber saturation point, the shrinkage to a target moisture content is the total shrinkage times the fraction of the way from the fiber saturation point to zero.

A flatsawn board has the rings roughly parallel to its face, so its width shrinks tangentially and its thickness radially; a quartersawn board is the other way round. Choose the direction to get the dried size of a green board, or the green size that dries down to the size you want.

Formula

S_x = S_0 × (FSP − MC_target) ÷ FSP        (Wood Handbook FPL-GTR-282, 2021, Eq. 4-9)
FSP = 30%, or 22% for western redcedar and redwood (Table 13-5), unless entered
width: S_0 = tangential (flatsawn) or radial (quartersawn), Table 4-3
thickness: S_0 = radial (flatsawn) or tangential (quartersawn)
green → dried: loss = green × S_x ÷ 100; dried = green − loss
dried → green: loss = dried × S_x ÷ (100 − S_x); green = dried + loss

Example

A flatsawn northern red oak board sawn 8 in wide and 1 in thick is dried from green to 8% moisture content. Table 4-3 gives 8.6% tangential and 4.0% radial shrinkage from green to ovendry. To 8%: width shrinkage = 8.6 × (30 − 8) ÷ 30 = 6.31%, so the board loses 0.505 in and dries to 7.495 in wide; thickness shrinkage = 4.0 × 22 ÷ 30 = 2.93%, a loss of 0.029 in to 0.971 in.

Working the other way, a board that must be 8 in wide at 8% loses 8 × 6.307 ÷ (100 − 6.307) = 0.538 in in drying, so it has to be sawn 8.538 in wide when green.

Assumptions and limitations

• Shrinkage values are species averages from the Wood Handbook (FPL-GTR-282, 2021), Table 4-3. The Handbook reports a coefficient of variation of about 15% for individual pieces, and somewhat more for commercial lumber whose rings are not perfectly flat or vertical.
• Shrinkage is assumed to start at the fiber saturation point and to be linear below it. By default the calculator uses the value the Handbook's Table 13-5 assumes for the species: 30% for most species and 22% for western redcedar and redwood (30%, the Handbook's general approximation, for custom shrinkage values). A different value can be entered.
• In real drying, the surface of a board dries and shrinks before the core, so a board can shrink while its average moisture content is still above the fiber saturation point. The estimate applies to the board once its moisture is evenly distributed.
• Allowances for planing, warp, cup and drying checks are not included in the green size.
• This is an estimate, not a guarantee of how a particular board will dry.

Frequently asked questions