Wood Shrinkage Calculator
Predict the final dimension of a wood board after drying from an initial moisture content to a target moisture content, given the species shrinkage rate. Essential for woodworkers milling green lumber or planning joinery tolerances.
About this calculator
Wood shrinks as it loses moisture below the fiber saturation point (roughly 28–30% MC). The amount it shrinks depends on the species and the grain orientation (radial, tangential, or volumetric). The formula used here is: Final Size = Initial Size × (1 − ((Initial MC − Final MC) × Shrinkage Rate / 100)), where moisture content (MC) is expressed as a percentage and shrinkage rate is expressed as percent dimensional change per percent MC change (%/% MC). For example, white oak has a tangential shrinkage rate of about 0.37%/% MC. This formula gives you the expected final dimension after drying, allowing you to mill green lumber slightly oversized and account for movement in joinery design. Species shrinkage rates are published by the USDA Forest Products Laboratory and differ significantly — tangential shrinkage is typically 1.5–2× radial shrinkage.
How to use
You have a green white oak board with an initial width of 8 inches at 30% MC and want to dry it to 8% MC. White oak's tangential shrinkage rate is approximately 0.37%/% MC. Apply the formula: Final Size = 8 × (1 − ((30 − 8) × 0.37 / 100)). Calculate inside the brackets first: 30 − 8 = 22, then 22 × 0.37 = 8.14, then 8.14 / 100 = 0.0814. So Final Size = 8 × (1 − 0.0814) = 8 × 0.9186 = 7.35 inches. The board will shrink roughly 0.65 inches in width — important information when sizing rough stock.
Frequently asked questions
What is wood shrinkage rate and where can I find values for different species?
Shrinkage rate describes how much a board's dimension changes for every 1% change in moisture content, expressed as %/% MC. It varies by grain orientation: tangential (flat-sawn) shrinkage is highest, radial (quarter-sawn) is lower, and longitudinal shrinkage along the grain is negligible. The authoritative source for species-specific shrinkage coefficients is the USDA Forest Products Laboratory's Wood Handbook, available free online. Common values range from about 0.18%/% MC (radial, white oak) to 0.40%/% MC (tangential, hard maple). Using the wrong rate can lead to significant errors when planning joinery clearances.
Why does wood shrink more tangentially than radially?
The difference arises from the structure of wood cells and the arrangement of cellulose microfibrils in the cell wall. Tangential shrinkage (across the growth rings) is roughly 1.5 to 2 times greater than radial shrinkage (through the rings) for most species because the rays — radial tissue bundles — act as structural reinforcement that resists radial movement. This is why flat-sawn boards (tangential face exposed) cup and move more than quarter-sawn boards across the same moisture change. Understanding this helps woodworkers choose the right board orientation for tabletops, panel doors, and other wide assemblies.
How much should I oversize green lumber to account for shrinkage before milling to final dimension?
A common rule of thumb is to add the calculated shrinkage amount plus an extra 10–15% buffer for any uneven drying or surface checking. For example, if the calculator predicts 0.65 inches of shrinkage on an 8-inch-wide board, start at roughly 9 to 9.5 inches rough width. Also add thickness — plan for at least 1/4 inch extra in thickness to allow for flattening after drying, since boards often cup, bow, or twist during the process. Drying slowly and stacking lumber with stickers (spacers) between each layer promotes even moisture loss and reduces the amount of material lost to defects.