Wood Movement Calculator
How much will that panel expand or shrink with the seasons? Pick a species and grain orientation, set the moisture swing, and design the joint before the wood decides for you.
Verified against independent hand calculation · sources cited below
0.516″ ≈ 17/32″ · 13.1 mm
Panel expands across the grain (tangential 8.6%)
Design for it: leave 17/32″ of total clearance (half per side in a frame), use elongated screw holes or buttons on tabletops, and never glue a panel of this width into a rigid frame.
FPL method: ΔW = W × S/100 × ΔMC/30. Species values are averages — boards vary, and finish slows moisture exchange but never stops it. Length change along the grain is ~0.01% and ignorable.
The formula, shown
ΔW = W × (S ÷ 100) × (ΔMC ÷ 30), where S is the species’ green-to-ovendry shrinkage percentage (tangential for flatsawn width, radial for quartersawn) and 30% is the fiber saturation point — below it, movement is close to linear. Worked example: 30″ flatsawn red oak (T = 8.6%), 6% → 12% MC: 30 × 0.086 × 6/30 = 0.516″.
Designing for the number
- Frame-and-panel: the panel floats in grooves — give it the calculated clearance, half per side, and never glue it in.
- Tabletops: fasten through elongated holes, figure-8s, or buttons so the top slides on the base.
- Breadboard ends: pin the center tenon tight, elongate the outer pins — the end stays flush at the middle and the top moves at the edges.
- Drawer fronts and doors: size winter gaps knowing summer will close them by the calculated amount.
Frequently asked questions
How much does a tabletop actually move?
A 30″-wide flatsawn red-oak top swinging from 6% MC (winter, heated house) to 12% (humid summer) moves about 1/2″. Quartersawn stock of the same width moves under 1/4″ — less than half. That difference is the whole argument for quartersawn tabletops.
What moisture contents should I enter?
Indoor furniture in most of North America cycles between roughly 6% (winter heating) and 12% (humid summer); coastal and tropical climates run higher, deserts lower. Wood acclimated in a shop usually sits at 9–12%. When in doubt, use 6 → 12 as the design case.
Does finish stop wood movement?
No — it only slows the exchange. Film finishes stretch the cycle over weeks instead of days, which evens out short humidity spikes, but seasonal change gets through everything. Design joints for movement; don’t ask varnish to fight physics.
Why does wood move across the grain but not along it?
Water lives in the cell walls, and cells are long tubes aligned with the grain. Losing water shrinks the tube walls — the diameter changes, the length barely does. Longitudinal shrinkage is about 0.1–0.2% total, which is why a 6-foot rail effectively never changes length.
Flatsawn vs. quartersawn — which number applies to my board?
Look at the end grain. Growth rings parallel to the face (arching across the end) = flatsawn, use tangential. Rings perpendicular to the face (vertical stripes) = quartersawn, use radial. Riftsawn falls between; the calculator’s two cases bracket it.
Sources & further reading
- USDA Forest Products Laboratory — Wood Handbook (2021), Ch. 4: shrinkage values and the dimensional-change formula
- Method: ΔW = W × S/100 × ΔMC/30, fiber saturation point taken as 30% MC