Wood Movement Calculator

How this works

The calculator uses the standard shop formula for dimensional change: change = starting width x change in moisture content (percentage points) x (species shrinkage% / 30 / 100). The 30 is the fibre saturation point — the moisture content, by weight, above which wood has stopped changing dimension because the cell walls are already saturated and any further water just fills the empty cell cavities. It's a nominal figure (real fibre saturation points range roughly 26–34% by species), but it's the standard shop approximation and accurate well within the tolerance of the wood itself. Pick a species and the calculator reads that species' real radial or tangential shrinkage percentage — whichever grain orientation you select — from this site's data, rather than a single generic "wood moves X%" number.

Radial vs. tangential — why the orientation field matters

Wood shrinks roughly twice as much across the growth rings (tangential) as it does along a radius from the pith outward (radial), which is why a flatsawn board (where the growth rings run roughly parallel to the wide face) moves noticeably more across its width than a quartersawn board of the same species and width. If you don't know which you're working with, flatsawn/tangential is the safer assumption for standard lumberyard stock — it's the larger of the two numbers, so assuming it gives you the bigger, safer gap allowance.

Reading the result

A positive result means the board is expected to grow (gaining moisture — set your ending moisture content higher than your starting one); a negative result means it will shrink. The millimetre figure is the same physical change, just in metric, for readers working from metric plans or a metric moisture meter. Moisture content, not relative humidity, drives the calculation — if you only know the room's relative humidity, /reference/moisture-content-targets-by-region/ gives typical seasonal wood moisture-content ranges by region as a starting point, though a moisture meter reading on your actual stock is always more reliable than a regional average.

What this sizes for

This number is the gap allowance to leave in a frame-and-panel construction, a breadboard end, or any joint where solid wood crosses grain against a fixed point — leave at least this much clearance (usually rounded up) so the panel can move without splitting itself or the frame around it. It does not account for finish (a good film finish slows moisture exchange but does not stop it over a season), acclimation time, or the wood's own internal stress from milling — those all interact with, but don't replace, the base movement figure. For the deeper derivation and worked examples across several species, see flatsawn vs. quartersawn and movement and the full species reference for every shrinkage figure this calculator draws from.