THE CARPENTRY TOOLBOX

Wood Movement Calculator

Estimate seasonal solid-wood width change from species, grain and 6–14% wood moisture content. Compare dry and wet widths in inches or millimeters.

MAKE ROOM FOR THE SEASONS.

Start with the actual board.

Length units

Wood species

Use wood moisture content (MC), not air relative humidity. This method supports 6–14% MC. See grain and moisture notes ↓

Measured board width

Across the grain, measured at the current MC below.

Current wood MC

Moisture content when you measure this width.

Low wood MC

Your dry-end estimate.

High wood MC

Your wet-end estimate.

YOUR BOARD, ACROSS THE RANGEExample · edit to try

Total width change across your MC range0.5414 in6% to 12% wood MC · White oak

Flatsawn · tangential estimate. Widths share one scale. Bars align at one edge for comparison, not as fixing instructions.

Expansion from current width: 0.361 in
Contraction from current width: 0.1805 in
Estimated width range: 23.8195 in – 24.361 in
Movement coefficient per MC percentage point: 0.00376

The range comes from your MC inputs, not a statistical confidence interval. Real boards vary. The required clearance at each edge depends on how the board is attached.

CarpentryCalcs · carpentrycalcs.com/wood-movement-calculator · Example measurements

Approximate solid-wood movement across the grain. Excludes plywood, MDF, along-grain movement, cupping, cracking and joint strength. Coefficients: USDA Wood Handbook, 2021.

Estimate how much a solid-wood board changes width

Wood exchanges moisture with its surroundings. A board can become wider as its moisture content rises and narrower as it falls. This wood movement calculator estimates that across-grain width change using the selected species, growth-ring orientation and wood moisture content (MC).

The result shows the estimated width at your dry and wet endpoints, the total difference between them, and expansion or contraction from the width you measure now. All three bars use the same scale. They align at one edge to make comparison easier; that alignment is not an instruction to fasten the board at that edge.

Use wood MC, not air relative humidity

Enter the actual board width and the wood MC when that width was measured. Then enter your expected low and high wood MC. The current MC must sit within that range. A moisture reading such as 8% MC is a different quantity from 50% relative humidity in the room.

This version uses the dimensional-change coefficient method in Chapter 13 of the 2021 USDA Wood Handbook. That method is intended for 6–14% wood MC. The calculator therefore checks all three moisture inputs against those limits. It does not extrapolate to green lumber or convert weather, room humidity or location into an expected MC range.

Use measurements appropriate to the board and its intended environment. A wider guessed moisture range produces a wider result range, but does not establish the actual conditions the piece will experience. If your expected MC falls outside 6–14%, use a method suited to those conditions.

Flatsawn and quartersawn boards

For a flatsawn board's width, the tool uses the species' tangential coefficient. For a quartersawn board's width, it uses the radial coefficient. These directions describe movement relative to the tree's growth rings. Commercial boards rarely follow one direction perfectly, and a glued panel may contain several ring orientations.

Choose the species actually used. The available set includes white oak, northern red oak, black walnut, black cherry, sugar maple, yellow-poplar, American beech, yellow birch, coast Douglas-fir, eastern white pine, western redcedar and white fir. Their radial and tangential coefficients come from Table 13–5 in the handbook. The active coefficient appears beside the result and in copied results.

Worked example: a 24-inch white-oak panel

For a flatsawn white-oak panel measuring 24 inches at 8% MC, with low and high estimates of 6% and 12% MC, the tangential coefficient is 0.00376 per MC percentage point:

Contraction from current width: 24 × 0.00376 × (8 − 6) = 0.18048 inches.
Expansion from current width: 24 × 0.00376 × (12 − 8) = 0.36096 inches.
Estimated low-MC width: 23.81952 inches.
Estimated high-MC width: 24.36096 inches.
Total change across the range: 0.54144 inches.

These values describe an example board and chosen moisture range. They are not a universal allowance for white oak. Selecting quartersawn uses a different coefficient and changes the result.

Formula, precision and allowance

Handbook equation 13–3 estimates dimension change as ΔD = D × C × ΔMC, where D is the measured dimension, C the dimensional-change coefficient and ΔMC the change in moisture percentage points. An increase from 8% to 11% is 3 percentage points, so use 3 in the formula, not 0.03.

As a source check, the handbook's flatsawn white-fir example starts at 232 mm and 8% MC. Its coefficient is 0.00245. At 11% MC, the predicted increase is 232 × 0.00245 × 3 = 1.7052 mm, giving a width of 233.7052 mm. You can reproduce it with current and low MC set to 8%, and high MC set to 11%.

The calculator keeps unrounded values internally and displays inches to four decimals or millimeters to two. Real material variability is larger than this display precision suggests. The low-to-high range reflects your MC inputs; it is not a confidence interval or a prediction of cupping and splitting.

Do not automatically use the total range as a gap at each edge. Consider how the board is located and attached, and use the expansion from its current width when assessing available travel. Joint design, fastener travel and structural adequacy require separate decisions. Our methods page records the source and scope; contact and corrections accepts reproducible examples.

Good questions. Straight answers.

Should I enter air humidity or wood moisture content?

Enter wood moisture content, or MC. Air relative humidity is a different measurement. This calculator does not convert air humidity into wood MC, and its coefficient method is limited to 6–14% MC.

Which grain option should I choose?

Flatsawn uses the tangential coefficient for an approximate width change; quartersawn uses the radial coefficient. Real boards can have mixed or intermediate growth-ring orientation, so either result is an estimate.

Is total movement the gap I need at each side of a tabletop?

No. Total movement is the estimated width difference between your low and high MC inputs. Clearance at any particular edge depends on the current width, attachment points and joint design. The tool also shows expansion and contraction from the current width.

Can I use this for plywood or a board's length?

No. The model estimates solid-wood movement across the grain. It does not model plywood, MDF, along-grain movement, warping or the strength of a joint.