How much a board will shrink or swell across its width as the seasons change — using the actual Forest Products Laboratory shrinkage numbers for your species, flat-sawn or quarter-sawn, and either a measured moisture content or the room’s humidity.
Green-to-ovendry shrinkage from the USDA Forest Products Laboratory Wood Handbook. Not listed? Choose “Custom” and enter your own.
How was it cut?
The dimension you care about — usually the width of a board or panel. The same method applies to thickness using the other grain coefficient.
Moisture change basis
Measure with a moisture meter if you can. Kiln-dried lumber is often 6–9%; freshly milled or air-dried stock is higher.
Or pick a seasonal scenario
Humidity is converted to the moisture content the wood settles at (its equilibrium moisture content) using the Hailwood-Horrobin equation at your temperature.
The moisture content below which wood starts to move. Average is 28%; leave it unless you have a species-specific figure.
An estimate based on average laboratory shrinkage values. Individual boards vary with growth, drying history and finish, and shrinkage is not perfectly linear. Build in a margin and, for critical fits, measure your own stock.
Below about 28% moisture content — the fibre saturation point — wood gives up or takes on water in its cell walls, and the cells change size as it does. The board gets narrower as it dries and wider as it takes on humidity, season after season, for as long as it exists. You cannot stop it; you can only design around it. Length barely changes (about 0.1%), so the problem is almost always across the width.
The catch is that wood doesn’t move equally in all directions. It shrinks roughly twice as much tangentially (along the growth rings) as radially (across them). That single fact is why grain orientation matters so much:
Flat-sawn boards show the growth rings running across the face in cathedral arches. Their width runs tangentially, so they move the most. Most lumber is flat-sawn.
Quarter-sawn boards show straight, vertical grain lines. Their width runs radially, so they move much less — often only about half as much — which is exactly why quarter-sawn stock is chosen for tabletops, doors and anything that has to stay flat and tight.
Rift-sawn sits between the two.
The math (for the curious)
The calculator uses the dimensional-change equation from the USDA Forest Products Laboratory Wood Handbook. With initial dimension D, starting and ending moisture content MCi and MCf, the species shrinkage S (tangential for flat-sawn, radial for quarter-sawn), and fibre saturation point FSP:
When you work in relative humidity instead, each humidity is first converted to the moisture content the wood will settle at — its equilibrium moisture content — using the Hailwood-Horrobin sorption equation at your temperature. That is the same model behind the Forest Products Laboratory EMC tables, so 40% relative humidity at 70°F comes out at about 7.7% moisture content, and 65% at about 12%, just as the published tables show.
What counts as “a lot” of movement?
There is no single safe number — it depends on the joint. What matters is leaving room for the wood to return to its widest expected size:
Frame-and-panel doors: the panel floats in a groove with a gap on each side. Size the gap for the full seasonal swing so the panel can grow without bursting the frame.
Tabletops & breadboard ends: fasten cross-grain joints with slotted holes, figure-8 fasteners or buttons so the top can move while staying put. A 30" top can move the better part of a quarter-inch.
Flooring & decking: leave a deliberate gap between boards and an expansion gap at the walls. Exterior decking that arrives wet will shrink; interior flooring that arrives dry may swell.
Tight glue-ups: orient boards so movement is consistent, and never trap a wide solid panel between two fixed cross-grain members.
A typical seasonal swing
Relative humidity (70°F)
Equilibrium moisture content
30% (dry, heated winter)
~6.2%
40% (comfortable interior)
~7.7%
50%
~9.2%
65% (humid summer / exterior target)
~12.0%
80% (damp)
~16.0%
Most conditioned interiors cycle between roughly 6% and 11% moisture content over the year, which is why furniture is usually built to an intermediate target near 8%.
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Frequently asked questions
How much does wood move across its width?
It depends on the species, the grain, and how far the moisture content changes. As a rough guide a flat-sawn board moves about 1% of its width for every 4–5 points of moisture-content change — so a 12" flat-sawn oak board going from 12% to 7% shrinks on the order of a quarter inch. Quarter-sawn of the same species moves noticeably less. This tool uses each species’ actual radial and tangential numbers instead of one rule of thumb.
What’s the difference between flat-sawn and quarter-sawn for movement?
Wood shrinks about twice as much tangentially (along the rings) as radially (across them). A flat-sawn board’s width runs tangentially, so it moves most; a quarter-sawn board’s width runs radially, so it moves about half as much — which is why quarter-sawn is chosen for tabletops and doors. Rift-sawn is in between. Pick your orientation and the right coefficient is applied automatically.
Should I enter moisture content or relative humidity?
If you have a moisture meter, enter the measured moisture content — it’s the most direct input. If not, switch to relative humidity and the tool converts your shop or room humidity to the moisture content the wood will reach (its equilibrium moisture content) at the temperature you set. The seasonal scenarios fill in typical winter-to-summer humidity swings.
What is equilibrium moisture content?
It’s the moisture content wood eventually settles at for a given temperature and humidity — where it stops gaining or losing moisture. Most heated, conditioned interiors sit at 6–9%, which is why furniture is built near 8%. Wood only changes size below the fibre saturation point (about 28%); above that it takes on water without moving.
How much gap should I leave for movement?
Leave room for the wood to grow back to its widest expected size. Float frame-and-panel doors with a gap each side; gap tongue-and-groove flooring and decking; slot the screw holes on breadboard ends and cross-grain tabletop fasteners. Calculate the movement across your full seasonal humidity range, then leave at least that much clearance plus a small margin.