Single board movement
Panel glue-up mode — wood panel expansion calculator
Add every board in the glue-up. The calculator totals the panel width and its movement, and tells you the expansion gap to leave per side.
Breadboard end gap calculator
A table top expansion calculator for breadboard ends: the top moves cross-grain while the breadboard barely moves along its length.
Breadboard ends are pinned at the center; the top expands toward each side. Leave half the total movement per side, with elongated screw slots.
Equilibrium moisture content (EMC) calculator
An EMC wood calculator (EMC calculator): wood drifts toward EMC for the surrounding temperature & humidity. Computed with the Hailwood–Horrobin moisture content formula (Simpson coefficients, USDA Wood Handbook).
What the numbers mean
| Condition | Typical EMC |
|---|---|
| Heated home, winter (70°F / 30% RH) | ~6% |
| Heated home, spring/fall (70°F / 45% RH) | ~8.5% |
| Air-conditioned summer (75°F / 50% RH) | ~9% |
| Humid summer, no AC (82°F / 70% RH) | ~13% |
| Unheated shop (55°F / 60% RH) | ~11% |
| Outdoor, covered | ~14–16% |
| Kiln-dried lumber at the yard | ~7–8% |
Design for the full annual swing (e.g. 6% winter → 12% summer = 6 points), not the moisture on build day.
Wood movement chart — by species (click headers to sort)
This wood movement by species chart lists dimensional change per inch of width per 1% MC change. M = published laboratory coefficient (USDA Wood Handbook). E = estimated from USDA green-to-ovendry shrinkage (within ~7% where both exist).
| Species ▲ | Type | Quartersawn (radial) | Flatsawn (tangential) | Stability | Source |
|---|
How wood movement works
Wood movement over time follows the seasons: boards swell in humid summers and shrink in dry heated winters. The calculator above turns that annual cycle into exact numbers for your species and board.
Quartersawn vs flatsawn: which moves more?
The quarter sawn vs flat sawn question (also written quartersawn vs flatsawn) has a simple answer: flatsawn boards move roughly twice as much across their width as quartersawn boards of the same species.
The formula
Change in width = width × change in moisture content × species coefficient. Moisture change is measured in percentage points (e.g. 14% → 6% = 8 points). The coefficient depends on species and grain direction: flatsawn (tangential) coefficients are roughly double quartersawn (radial) ones. Movement along the grain (length) is negligible — about 0.1% — and can be ignored for furniture.
Design rules that never fail
- Never trap a wide solid-wood panel without expansion room — split the total movement into a gap per side.
- Breadboard ends: pin the center only; gap each side = half the top's movement.
- Attach tabletops with figure-8 fasteners, Z-clips, or slotted holes — never rigid cross-grain glue.
- Build (or buy) lumber at the moisture content it will live at; let it acclimate in the shop first.
- Finish slows moisture exchange; it never stops it. Design for the full swing anyway.
Frequently asked questions
What is the wood movement formula?
The standard formula from the USDA Forest Products Laboratory Wood Handbook is: change in width = board width x change in moisture content (in percentage points) x the species' dimensional change coefficient. For example, a 20-inch-wide flatsawn red oak board (coefficient 0.00369) going through a 10-point moisture swing moves 20 x 10 x 0.00369 = 0.738 inches. This calculator applies that exact formula with published coefficients for 55 species.
How much will my wood expand or shrink?
Enter your board width, species, grain orientation (flatsawn moves roughly twice as much as quartersawn), and the moisture content change above — the calculator answers exactly how much will wood expand in humid months and how much does wood shrink in dry months. As a rule of thumb, indoor furniture in a heated home sees a 4-8 point seasonal moisture swing; a 24-inch flatsawn oak tabletop can move nearly half an inch across that range. For critical joinery, always measure your boards' actual moisture content with a meter rather than guessing.
What is the difference between flatsawn, quartersawn, and rift-sawn movement?
Flatsawn (plain-sawn) boards move across their width in the tangential direction - the most movement. Quartersawn boards move in the radial direction, typically about half as much, and are more stable. Rift-sawn falls in between; this calculator approximates it as the average of the radial and tangential coefficients. The species chart tab shows both coefficients for every species so you can compare directly.
What is equilibrium moisture content (EMC) and how is it calculated?
EMC is the moisture content wood eventually reaches when left in air at a given temperature and relative humidity. This calculator computes it with the Hailwood-Horrobin two-hydrate sorption model using Simpson's coefficients - the same method published in the USDA Wood Handbook. For example, 68 degF at 65% relative humidity gives 12.0% EMC, and a heated winter home at 70 degF and 30% humidity gives about 6.2% EMC.
How do I measure the moisture content of wood?
To learn how to measure moisture content of wood in practice, use a pin or pinless moisture meter - take several readings across the board and average them. The laboratory reference method is oven-drying: weigh a sample, dry it at 103 degC until its weight stops changing, and compute (wet weight - dry weight) / dry weight x 100. If you only know your shop's relative humidity, use the EMC tab above to estimate moisture content from temperature and humidity.
Does applying finish stop wood movement?
No. Finish only slows the rate at which moisture moves in and out - it does not stop it. A heavily finished board may take months instead of weeks to respond to a humidity change, but it will eventually reach the same equilibrium and move the same total amount. Always design joinery to accommodate the full expected movement, regardless of finish.
How much gap should I leave for breadboard ends?
Pin (or glue) the breadboard only at its center and let the tabletop expand toward each side. Leave a gap on each side equal to half of the tabletop's total expected cross-grain movement. For example, a 36-inch white oak top swinging 8 moisture points moves about 1.05 inches total, so allow roughly 1/2 inch of gap per side plus slightly elongated screw slots in the breadboard.
How much room does a solid-wood panel need to expand?
Compute the panel's total expected movement for your climate's moisture swing, then split it: leave half the movement as clearance on each side (for a panel trapped in a frame, about 1/8 inch per side is the classic minimum for typical indoor swings). The panel glue-up tab totals multi-board panels and gives you the per-side gap automatically.
Does plywood move with moisture changes?
Very little. Because of its cross-ply construction, plywood and MDF move roughly an order of magnitude less than solid wood across the face — that is why this wood movement calculator for plywood uses a small nominal coefficient for Baltic birch plywood, flagged as an estimate. You can generally treat sheet goods as stable for joinery planning, though edges can still swell if they get wet.
Which wood species move the least?
Among the 55 species in the database, the most dimensionally stable include teak and eastern redcedar - both rated 'Very stable' in the species chart - followed by redwood and Honduran mahogany ('Stable'). Choosing a stable species is the simplest way to reduce movement problems, though grain orientation matters just as much.
Which wood species move the most?
American beech, true hickory, Pacific madrone, flowering dogwood, and persimmon have the highest tangential coefficients in the database and are rated 'Unstable'. Wide flatsawn boards of these species demand generous expansion allowances - or choose quartersawn stock, which cuts movement roughly in half.
Can I use this wood movement calculator in mm?
Yes. Toggle the unit switch between inches and millimeters at any time; all inputs, results, and the species chart convert instantly. You can also type fractional inches like 24 3/4 and the calculator parses them. The underlying math is unit-agnostic: a 500 mm white oak board moving 8 moisture points changes by 14.6 mm.
Why do gaps open up in my furniture every winter?
Heated winter air is dry: a home at 70 degF and 30% relative humidity brings wood to about 6% moisture content, while the same home in a humid summer sits near 11-12%. That 5-6 point swing shrinks every wide solid-wood part each winter and swells it back each summer. Designing for the full annual swing - rather than the day you built it - is the entire point of calculating movement.
Can I glue boards with the grain running in different directions?
Avoid it wherever possible. Cross-grain glue joints fight the wood's natural movement and eventually fail - either the joint opens or the board cracks. Where cross-grain assembly is unavoidable (tabletop to apron, breadboard ends), use mechanical fasteners that allow sliding: elongated screw holes, figure-8 fasteners, Z-clips, or buttons, glued or screwed tight at only one reference point.
How does this compare to the Shrinkulator?
The Shrinkulator (woodbin.com, 2007) is the classic tool woodworkers have referenced for years, and this calculator uses the same USDA Forest Products Laboratory math. The differences: 55 species instead of a short list (each flagged as published or estimated coefficient), flatsawn/quartersawn/rift-sawn in one control, a multi-board panel glue-up mode, built-in EMC climate presets, a dedicated breadboard gap calculator, metric support, and a sortable species chart. If you liked the Shrinkulator or the Katz-Moses (Katz Moses) wood movement calculator, this is the modern replacement.
Are these numbers exact for my boards?
No - they are species averages from laboratory testing, and individual boards vary with growth conditions, exact grain angle, and extractives content. Treat results as planning estimates: add a safety margin for critical joinery, and measure the actual moisture content of your lumber with a meter before final fitting. Coefficients marked 'estimated' in the species chart are derived from green-to-ovendry shrinkage data (within about 7% of measured values where both exist) rather than direct laboratory measurement.
Sources
Dimensional change coefficients: USDA Forest Products Laboratory, Wood Handbook — Wood as an Engineering Material (Table 13-5, via the coefficient chart published by Fine Woodworking; supplementary values from the WOOD Magazine Dec/Jan 2013–14 movement chart). EMC model: Hailwood–Horrobin two-hydrate sorption equation with Simpson's temperature coefficients, as published in the Wood Handbook. Shrinkage data for estimated coefficients: USDA-derived extension tables (Purdue FNR-163; Oklahoma State NREM-5009).