Wood Shrinkage Calculator
Tangential is twice radial. That is why boards cup.
Work out Wood Shrinkage. Tangential is twice radial. That is why boards cup. Free, with no account and nothing to install.
Shrinkage
-5.58 mm
Red oak, flatsawn (tangential) · 12% to 7% moisture content
Wood moves across the grain and effectively not at all along it. That asymmetry is what breaks furniture — a panel glued rigidly into a frame has nowhere to go, so it splits itself or the frame. Tangential movement is roughly twice radial in almost every species, which is why flatsawn boards cup and quartersawn ones stay flat. It is the single most useful fact in the subject. Equilibrium moisture content comes from relative humidity, not from how long the wood has sat. A board at 12% in a damp workshop will keep drying to 6% in a centrally heated room however well seasoned it was — which is why furniture should acclimatise where it will live before it is built.
How the Wood Shrinkage Calculator works
Seasonal movement across the width of a board from species, cut and the humidity swing of the room it will live in. Equilibrium moisture content is derived from humidity rather than assumed.
Also known as: how much will this board move · wood expansion gap allowance · quartersawn versus flatsawn movement · equilibrium moisture content wood
Frequently asked questions
How much does wood move?
Across the grain, roughly 0.2 to 0.4% per percentage point of moisture content depending on species and cut. A 300 mm oak board can move 6 mm between a damp summer and a heated winter.
Why do flatsawn boards cup?
Because tangential movement is about twice radial in nearly every species. The face nearer the bark shrinks more than the face nearer the heart, and the board curls away from the heart.
What is equilibrium moisture content?
The moisture level wood settles at for a given humidity and temperature — about 6% at 30% RH and 12% at 65% RH. It depends on the air, not on how long the wood has sat.
Is quartersawn worth the extra cost?
For wide panels and tabletops, usually. It moves roughly half as much across the width and does not cup, which is exactly where movement causes failures.
Do I need to allow for movement along the length?
Effectively no. Longitudinal movement is about 0.1% of the cross-grain figure — negligible in furniture, which is why the two directions have to be detailed differently.
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