Wood Movement Calculator

Wood is a bundle of hollow cellulose tubes. When a tree is felled those tubes are full of liquid water and their walls are saturated. Drying removes the free water in the cavities first, and that costs the board nothing dimensionally - it gets lighter and not one bit smaller. Only when the cavities are empty and the cell walls themselves start giving up water does the wood begin to shrink. That crossover is the fibre saturation point, around 30% moisture content for most species.

Below it, the wood is hygroscopic: it exchanges moisture with the air around it until it reaches equilibrium, and then it keeps doing so forever, because the air keeps changing. A board is not a thing that dried once. It is a thing that is always partway between the last two seasons.

This is why the question "how do I stop my tabletop moving?" has no answer. Finish slows the exchange - a well-finished top might lag the room by weeks rather than days - but nothing available to a furniture maker stops it. The craft response, worked out over centuries and visible in every piece of antique furniture that survived, is not to resist the movement but to give it somewhere to go.

Wood shrinks about twice as much around the growth rings as it does across them. The reason is structural: the dense latewood bands formed at the end of each growing season are stiffer than the earlywood, and as they shrink they drag the softer wood between them along, which compounds movement in the tangential direction while restraining it radially.

What this means at the bench is that how a board was cut out of the log matters more than which tree it came from. Flatsawn red oak moves 8.6% green to ovendry across its width; quartersawn red oak moves 4.0%. That is a bigger difference than between red oak and cherry, or between oak and pine.

It also explains cupping. A flatsawn board has rings that run roughly parallel to one face and curve towards the edges, so the face nearer the bark shrinks more than the face nearer the pith, and the board curls away from the heart. Quartersawn boards have the rings running edge to edge, so both faces shrink the same and the board stays flat - which is why quartersawn stock is worth its premium for wide panels, workbench tops and anything that has to stay true.

Every traditional joint that looks fussy is usually solving this problem. A floating panel in a grooved frame is not decoration; it is a panel free to change width inside a frame whose stiles are oriented so they barely change at all. Tabletop fasteners in slots, buttons in a groove, figure-eight fasteners - all the same idea, holding the top down firmly while letting it slide sideways.

The failures are equally consistent. A wide top screwed hard to an apron at both edges will split, because the wood is stronger than the screws are patient. A solid panel glued into its groove will either split itself or push the frame joints open. A breadboard end glued across its full width will crack somewhere in its first two winters. In each case the wood is not misbehaving; the joint simply forbade something that was always going to happen.

The practical sequence is: get the wood to the moisture content of the room it will live in before you cut joinery, calculate the range that room will actually swing through, and then size every cross-grain connection to swallow half the total on each side. Two or three weeks stickered in the destination room does more for a piece than any amount of care at the bench afterwards.

The shrinkage percentages are from the USDA Forest Products Laboratory Wood Handbook, which has published green-to-ovendry values by species since 1935 and remains the reference the whole trade works from. The conversion into a per-percent coefficient is the equation given in FPL research paper RP-711, published in 2022, which is how the Laboratory computed the coefficients in the Handbook flooring chapter.

That paper matters because it settles a discrepancy woodworkers run into constantly. Extension leaflets and most online calculators use a simpler formula that references the green dimension, and it gives a smaller answer than the published coefficients do. Both are in print, both cite the Handbook, and they disagree. The reason is the reference point: shrinkage measured as a fraction of a green log understates the fraction of a dry board, and the Laboratory indexes at 10% moisture content because that is where furniture actually lives.

The moisture content itself comes from the Hailwood-Horrobin sorption model. It is the same equation the Handbook uses to generate its equilibrium moisture content table, so entering 70 degrees and 65% humidity returns 12.0% - the number in the book.