computedeez
computedeez.com2026-10-08

Wood Movement Calculator

How much a board will shrink and swell across the year where it will live, and the gap, slot or groove depth the joint needs so it can move without splitting.

Across the year
25/32"
tangential, 10.5%
It will shrink
≈13/32"
to ≈35 19/32"
It will swell
3/8"
to 36 3/8"
Moisture
6.2–12.0%
now 9.2%
The wood
Where it is now
Where it will live
What it is for
Wood
Oak, white, flatsawn, 36" wide and 3/4" thick
Living in
30% to 65% humidity at 70 °F, so 6.2% to 12.0% moisture
Building
A top fastened to a base

Quartersawn stock in this species would move 13/32" across the year instead of 25/32".

Along the grain wood barely moves at all: 0.1% to 0.2% from green to bone dry, which is a few thousandths over the length of a table. Length is not the problem; width is.

These are averages for the species. The handbook puts the variation between individual boards at about 15% either way, which is the range given beside the figure.

A top that moves more than half an inch across the year will split if it is screwed down solid. Every fastener has to let it slide.

What to leave

rounded up to the next thirty-second, because a gap that is too small is the thing being avoided
WhatSize
Slot length in each fastenerEach fastener has to travel half the total movement, since the top is usually fixed near the middle and moves outwards both ways. A figure-eight fastener turns instead of sliding and needs no slot.15/32"
Overhang at each end, allowHow much further the top will stand past the base at the dampest time of year.3/16"

The board

end-on, with the year's movement drawn at the edges
End grain
+3/16"+3/16"≈35 19/32" at the driest36 3/8" at the dampest36" wide now, 3/4" thickDrawn to scale except the edges 8 times life size, and the thickness 4 times, so the grain can be seenThe rings run across the face, so the width moves tangentially

Across the year

most of the movement happens at the damp end, which is where the curve steepens
Width against humidity
≈35 19/32"≈35 3/4"≈35 29/32"≈36 1/16"36 7/32"36 3/8"303744515865Relative humidity (%)Widthas built

The figures

WhatValue
Shrinkage across the width10.5%tangential, green to bone dry
Moisture content now9.2%settled at 50% humidity
At the driest6.2%30% humidity
At the dampest12.0%65% humidity
Width at the driest≈35 19/32"≈13/32" narrower than now
Width at the dampest36 3/8"3/8" wider than now
If this board is not average≈21/32" to 29/32"the same movement, 15% either way
Through the thickness0.008"radial, usually ignorable

How this is worked out

Wood is never finished taking on and giving off moisture. It settles at whatever the air around it holds it to, and the figure it settles at is its equilibrium moisture content: a function of temperature and relative humidity, and almost entirely of humidity. In a heated house in winter that is around 6.2%; in the same house in summer it is around 12.0%. A finish slows the journey and flattens the daily swings. It does not change where the wood ends up across a season.

Below about 30% moisture, where the cell walls themselves start giving up water, wood shrinks in proportion to how much it has lost. How much it shrinks in total, from soaking wet to bone dry, is tabulated by species and by direction: across the growth rings, which is radial, and along them, which is tangential. Tangential is close to twice radial in most woods, and that single fact is the whole argument for quartersawn stock. A flatsawn board shows its rings across the face, so its width moves tangentially. A quartersawn board has them running through its thickness, so its width moves radially. For riftsawn and for ordinary lumber where you cannot tell, the handbook says to use the tangential figure, and so does this.

The example on this page is a flatsawn white oak top, glued up 36" across, going into a heated house that runs between 30% and 65% humidity. It moves 25/32" across the year, which is enough to split it if it is screwed down solid. The same top in quartersawn stock would move 13/32".

Along the grain, wood barely moves at all: a tenth to two tenths of a percent from green to bone dry, which is a few thousandths of an inch over the length of a table. Length is never the problem. Width always is.

How to use it

  1. Pick the species and say how wide the piece is across the grain.
  2. Say how it was cut. If you do not know, leave it on rift or mixed, which gives the larger of the two figures.
  3. Say where the wood is now, either by measuring it with a meter or by the humidity where it has been sitting. This is the size you will be cutting to.
  4. Say what the finished piece will live in. The presets are ordinary houses; a hygrometer in the room it is going to beats all of them.
  5. Pick what you are building, and read off the gap, slot or groove depth it needs.

What it assumes

  • Shrinkage is taken as linear from fiber saturation down to bone dry, with fiber saturation at 30% moisture content, which is the handbook’s own approximation.
  • The wood is assumed to reach equilibrium with the air around it. A finish slows that down and evens out the daily swings; it does not change where the wood ends up across a season.
  • Flatsawn width moves tangentially and quartersawn width moves radially. Riftsawn and mixed stock are given the tangential figure, which is what the handbook advises where ring orientation is not known, and is the larger of the two.
  • The figures are species averages. An individual board varies by about 15% either way, and a board with the pith in it, or with grain running out, will do its own thing.
  • Movement along the grain is ignored. It is 0.1% to 0.2% from green to bone dry, far below anything that matters in furniture.

Sources