Wood Shrinkage Calculator
Calculates the final width of a wood board after it shrinks or swells due to a change in moisture content. Use it when designing joints, panel gaps, or floating floors that must accommodate seasonal movement.
About this calculator
Wood is hygroscopic — it absorbs and releases moisture in response to ambient humidity, causing it to shrink and swell primarily across the grain. The formula used here is: finalWidth = initialWidth × (1 − ((initialMoisture − finalMoisture) / 30) × (shrinkageCoefficient / 100) × (grainFactor / 100)). The divisor of 30 represents the approximate fiber saturation point (FSP) of wood in percent; below the FSP, shrinkage is roughly linear with moisture change. The shrinkage coefficient varies by species (tangential shrinkage for flat-sawn oak is about 9.9%, radial for quartersawn is about 5.5%). The grain direction factor distinguishes flat-sawn (tangential, ~100%) from quartersawn (radial, ~55%) movement. Understanding this movement is critical for furniture makers and floor installers to design expansion gaps that prevent buckling or cracking.
How to use
A flat-sawn white oak panel is 12 inches wide at 19% moisture content (freshly dried). It will be installed indoors where equilibrium moisture content is 8%. Species tangential shrinkage coefficient ≈ 9.9 (entered as woodSpecies), grainDirection = 100 (flat-sawn). finalWidth = 12 × (1 − ((19 − 8) / 30) × (9.9 / 100) × (100 / 100)) = 12 × (1 − (11/30) × 0.099 × 1) = 12 × (1 − 0.3667 × 0.099) = 12 × (1 − 0.0363) = 12 × 0.9637 ≈ 11.56 inches The panel shrinks about 0.44 inches — leave at least that much expansion gap in your design.
Frequently asked questions
How much does wood shrink per percent change in moisture content?
The shrinkage per percentage point of moisture change depends on species and grain orientation, but a useful rule of thumb for flat-sawn hardwoods is roughly 1% dimensional change for every 4% change in moisture content. For quartersawn boards, the figure is closer to 1% per 7–8% moisture change because radial shrinkage is roughly half of tangential. In practice, a 10-inch flat-sawn oak board moving from 19% to 8% moisture will shrink approximately 0.36–0.44 inches. Always look up the specific species coefficients for critical applications like furniture or flooring.
What is the difference between tangential and radial wood shrinkage?
Tangential shrinkage occurs parallel to the growth rings — this is the dominant movement in flat-sawn (plain-sawn) boards and is typically the largest dimension change. Radial shrinkage occurs perpendicular to the growth rings — this is the primary movement in quartersawn boards and is usually 40–60% of the tangential value. This difference is why quartersawn lumber is prized for flooring and instrument soundboards: it moves less and more predictably. Knowing the cut of your board lets you choose the correct shrinkage coefficient for an accurate prediction.
Why do woodworkers need to account for wood movement when designing furniture joints?
Wood moves seasonally as indoor humidity cycles between summer highs and winter lows, often swinging 6–10 percentage points in moisture content. A wide panel like a table top or drawer bottom can move 1/4 to 1/2 inch across its width over that range. If the panel is glued or screwed rigidly across its width, the internal stress will eventually crack the wood or split a joint. Woodworkers use techniques like slot screws, figure-8 fasteners, and breadboard ends with elongated mortises to allow the wood to move freely while keeping the piece structurally sound.