Wood Shrinkage Calculator
Predict how much a wood board will shrink or swell as its moisture content changes between initial and final conditions. Essential for furniture makers and floor installers planning for wood movement.
About this calculator
Wood is hygroscopic: it gains and loses moisture in response to ambient humidity, changing dimensions in the process. Shrinkage is greatest perpendicular to the grain (tangential and radial) and nearly zero along the length. The final dimension is calculated as: finalDimension = initialDimension × (1 − ((initialMoisture − finalMoisture) / 30) × shrinkageCoefficient × (orientation / 100)). The divisor 30 approximates the fiber saturation point (FSP) in percent — above the FSP, wood does not shrink further with moisture loss. The shrinkageCoefficient varies by species; tangential shrinkage (flat-sawn) is typically larger than radial (quarter-sawn). A negative moisture change (drying) causes shrinkage; a positive change (wetting) causes swelling. Grain orientation determines whether you are modeling tangential, radial, or longitudinal movement.
How to use
Example: a flat-sawn red oak board, 12 inches wide, drying from 19% to 8% moisture content. Red oak tangential shrinkage coefficient ≈ 8.9, orientation = 100 (tangential). Step 1 — Moisture change: 19 − 8 = 11%. Step 2 — Shrinkage fraction: (11 / 30) × 8.9 × (100 / 100) = 0.3667 × 8.9 = 3.263%. Step 3 — Final dimension: 12 × (1 − 0.03263) = 12 × 0.9674 ≈ 11.61 inches. The board will shrink approximately 0.39 inches across its width. Account for this movement when sizing panels or planning expansion gaps in flooring.
Frequently asked questions
How much does wood shrink when it dries from green to air-dry moisture content?
Green lumber typically has a moisture content of 30–100%, while air-dried lumber stabilizes around 12–19% depending on climate. Using red oak as an example, drying from 30% to 12% across a 6-inch wide flat-sawn board produces roughly 6 × ((18/30) × 8.9 / 100) ≈ 0.32 inches of shrinkage. Species with higher tangential shrinkage coefficients (like beech or hickory) move more than stable species like teak or cedar. Always dry lumber to its expected in-service moisture content before final dimensioning.
What is the difference between tangential and radial wood shrinkage?
Tangential shrinkage occurs around the growth rings and is the dominant movement in flat-sawn (plain-sawn) boards. Radial shrinkage occurs across the growth rings and is the primary movement in quarter-sawn boards. Tangential shrinkage is typically 1.5–2× greater than radial shrinkage for most species, which is why quarter-sawn lumber is prized for stability. The grain orientation input in this calculator lets you choose which direction of movement you are predicting.
Why do woodworkers need to calculate wood shrinkage before building furniture?
Ignoring wood movement is one of the most common causes of furniture failure — panels crack, drawers stick, and joints open as moisture content shifts with the seasons. By calculating expected movement, a woodworker can leave appropriate expansion gaps in panels, use elongated screw holes for tabletops, and choose the most dimensionally stable species for critical components. In climates with large seasonal humidity swings, a 12-inch oak panel can move nearly half an inch between summer and winter, easily enough to split a glued-up frame.