Humidity movementPanel allowanceSafer joinery
Wood Expansion Calculator
Predict seasonal movement from moisture content changes so panels float, frames behave, and your work stays tight without splitting.
Fast workflow
- Select species (movement varies a lot).
- Set your humidity or moisture-content range.
- Enter panel width across the grain.
- Build joinery that allows that movement.
Tip: Width moves. Length barely does. Most “mystery cracks” are trapped cross-grain movement.
Wood Expansion Calculator
Across-the-grain dimensional change using Hailwood–Horrobin EMC + species shrinkage (radial/tangential).
Board
Fiber saturation point
Wood
Environment
Temperature
Converted
70°F
Results
Initial EMC
6.94%
at 35% RH
Final EMC
11.96%
at 65% RH
Δ EMC
5.02%
more moisture
Estimated movement across the grain
0.2259 in
Allowance per side (split): 0.1129 in
Species: White Oak
Shrinkage used: 10.5% (tangential)
FSP: 28%
Wood Movement Report Species: White Oak Grain: Flat-sawn / Tangential Shrinkage Used: 10.5% Board Width: 12 in Temperature: 70°F (70°F) Initial RH: 35% -> EMC: 6.94% Final RH: 65% -> EMC: 11.96% Δ EMC: 5.02% FSP (assumed): 28% Result: Expansion (humidifying) Estimated Movement: 0.2259 in Allowance per side (split): 0.1129 in
Formula
Common simplified form
ΔW = W × C × ΔMCΔW movement in width, W panel width, C species coefficient, ΔMC change in moisture content.
How to use
- Choose species and your shop-to-home humidity swing.
- Enter panel width across the grain direction.
- Use the movement result to size slots, clips, or floating panel gaps.
Pro tips
- • Float panels. Do not trap them in grooves with glue.
- • Avoid cross-grain glue-ups (breadboard ends need slots/pegs).
- • Wide tabletops: use Z-clips or elongated screw holes.
FAQs
Does finish stop movement?
No. Finish slows moisture exchange, but wood still equalizes and moves. Design for movement anyway.
Why do panels cup more than they widen?
Uneven moisture across thickness creates uneven shrink/swell. Symmetry helps (finish both sides, balanced construction, stable core choices).
Where should I measure width for movement?
Measure across the grain direction (tangential/radial). Lengthwise movement is usually negligible for design allowances.