WoodCalc Pro

Wood Movement Calculator

How much a board, tabletop or panel will grow and shrink across the grain between the dry and humid seasons, using USDA shrinkage coefficients.

Grain

Don't know the moisture? Estimate it from humidity

EMC: 6.2% at 30% RH → 11% at 60% RH

Seasonal movement

0.221in

As a fraction
≈ 7/32"
In millimeters
5.6 mm
Coefficient
0.00369 /%MC
Other grain
0.095"

Leave at least this much room: slotted screw holes or tabletop fasteners, panel grooves deep enough to hide it.

Watch a 12" red oak panel breathe

Wood trades moisture with the air until it balances. Slide the room humidity and see the moisture content the wood settles at, and how wide a flat-sawn panel cut at 8% becomes.

dashed = width when built at 8% MC (movement drawn 40× actual)

EMC 8.5% · panel grows 0.02" (0.5 mm)

The relationship isn't linear: moisture content climbs slowly through the middle humidities and fast above about 70%, which is why a damp August does more damage than a whole mild spring. The curve here is the Hailwood–Joy fit published by the USDA Forest Products Laboratory.

Ask the shop

Stuck on a detail? Ask a question about this job. Your current inputs and results are sent along so the answer fits your numbers. AI answers can be wrong, so double-check anything structural with your building department.

Questions people ask

How much does wood expand and shrink?
Across the grain, wood changes size roughly in proportion to its moisture content between about 6% and 28%. Flat-sawn red oak moves about 0.37% of its width per 1% change in moisture content; a 12" wide red oak panel going from 6% in a dry winter to 11% in a humid summer grows about 0.22". Along the grain, movement is negligible (about 0.1–0.2%).
What is equilibrium moisture content?
The moisture content wood settles at when left long enough in air of a given temperature and humidity. At 70°F, 30% relative humidity gives about 6% EMC, 50% gives about 9%, and 80% gives about 16%. Indoor furniture in most of the US swings between about 6% and 11% over a year.
What moisture content should wood be for woodworking?
For indoor furniture, 6–8% in most of the US (the USDA Wood Handbook recommends 6–8% for most areas, 9–11% for the damp southern coasts and 4–9% for the dry Southwest). Framing lumber is stamped S-DRY or KD-19 at 19% or less. Measure with a pin or pinless moisture meter before building.
Is quartersawn wood more stable?
Yes. Quartersawn (radial) boards move about half as much in width as flat-sawn (tangential) boards, because wood shrinks roughly twice as much along the growth rings as across them. That is why quartersawn stock is used for wide door frames, drawer sides and tabletops that must stay flat.
How much gap do I leave for wood movement?
Use the calculator for your species and width, then allow at least that much. Tabletops attach with slotted holes, figure-8 fasteners or buttons; frame-and-panel doors leave the panel about 1/8" to 3/16" short of the groove bottom per side on a 12" panel, more if built in dry winter air.

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