Wood Shrinkage Calculator

How It Works

Wood is hygroscopic — it absorbs and releases moisture from the surrounding air. As moisture content (MC) changes, wood cells shrink or swell, causing dimensional changes. This calculator uses published shrinkage coefficients from the USDA Forest Products Laboratory to predict how much a board will move.

Key principles:

• Fiber Saturation Point (FSP) — approximately 28% MC for most species. Above FSP, free water fills cell cavities without dimensional change. Shrinkage only occurs below FSP.
• Shrinkage is directional — wood moves most tangentially (across the growth rings), less radially (toward the pith), and negligibly along the grain.
• Coefficient — each species has a total shrinkage percentage from green (FSP) to oven-dry (0% MC). The calculator interpolates linearly between your initial and target MC.

How to Use

1. Select your wood species — choose from 20 common hardwoods and softwoods
2. Choose the cut type — this determines which shrinkage direction applies to width vs thickness
3. Set initial MC — the current moisture content of your lumber (use a pin or pinless meter)
4. Set target MC — the expected equilibrium MC for the finished piece's environment
5. Enter board dimensions — width and thickness in millimeters
6. Read results — see predicted dimensional changes and final sizes

Understanding Cut Types

• Flat Sawn (Plain Sawn) — growth rings run roughly parallel to the wide face. Width changes follow tangential shrinkage (higher movement). Most common and least expensive cut.
• Quarter Sawn — growth rings are perpendicular to the wide face (60-90°). Width follows radial shrinkage (less movement). More dimensionally stable but more waste in milling.
• Rift Sawn — growth rings at 30-60° to the face. The calculator averages radial and tangential coefficients for this intermediate cut.

Tips for Woodworkers

• Acclimate lumber for 1-2 weeks in your shop before milling to final dimension. The wood needs to reach equilibrium with the working environment.
• Allow expansion gaps in solid wood panels — a 600mm wide tabletop in red oak can move ±3mm seasonally.
• Quarter-sawn stock is worth the premium for drawer fronts, door panels, and instrument tops where stability matters.
• Seal all surfaces equally (including end grain) to slow moisture exchange and reduce warping.
• Indoor furniture should be built at 6-8% MC. If your lumber arrives at 12%, expect shrinkage as it acclimates.

FAQ

Why does my calculation show zero change?

If both initial and target MC are above the fiber saturation point (~28%), no shrinkage occurs — the wood is still saturated. Dimensional change only happens below FSP.

Is wood shrinkage reversible?

Mostly yes. Wood swells when it gains moisture and shrinks when it loses moisture. However, repeated cycles can cause permanent set (compression set), and extreme drying can cause checking or splitting that won't reverse.

How accurate is this calculator?

Shrinkage coefficients are species averages from laboratory testing. Individual boards may vary ±10-20% due to growth conditions, grain angle, and density variation within a species. Use results as planning estimates, not machining tolerances.

Does wood shrink along its length (grain direction)?

Longitudinal shrinkage is negligible — typically 0.1-0.2% from green to oven-dry, which is too small to matter for most woodworking applications. This is why a 2-meter board won't measurably change in length, but can change significantly in width.

How do I measure moisture content without a meter?

Without a moisture meter, you can weigh a small sample, oven-dry it at 103°C for 24 hours, then reweigh. MC% = (wet weight - dry weight) / dry weight × 100. This destructive method is the most accurate but requires sacrificing a wood sample.

Related Tools

• Box Template Creator — Design custom boxes with precise joinery dimensions
• Lye & Water Calculator — Calculate soap-making lye for woodworkers who also craft

Last reviewed: June 2026