Use the free Wood Movement Calculator below. It runs entirely in your browser, with no sign-up and no download.
Wood never stops moving
A wooden board is not a fixed object. It is a bundle of fibres that takes in and gives out moisture with the air around it, and as it does, it grows and shrinks across the grain. A tabletop that fits its frame perfectly in a humid summer can crack its joints in a dry winter, and a drawer that slides smoothly in January can stick solid in July. None of this is a defect in the wood. It is the material behaving exactly as wood behaves.
This calculator turns that behaviour into a number you can plan around. Tell it the species, the board width, the grain direction and the moisture change you expect, and it returns how much the board will move. That number decides the gap you leave, the slot you cut for a screw, and whether a glued panel needs room to breathe.
How the maths works
Movement across the grain is roughly proportional to three things: how wide the board is, how much the moisture content changes, and how strongly the species reacts. Each species has a dimensional change coefficient, the fraction of its width it gains or loses for each one percent change in moisture content. Multiply width by coefficient by moisture change and you have the movement.
Grain direction matters as much as species. Flat-sawn boards, cut tangentially through the log, move about twice as much as quarter-sawn boards, cut radially. That is why two boards of the same species and width can behave very differently in the same room. If you are buying stock for a wide panel, quarter-sawn boards cost more, but they buy you stability that no finish can provide.
A worked example
Take a 36 inch wide red oak tabletop, flat-sawn, installed at 12 percent moisture content in summer and drying to 6 percent in a heated winter house. The tangential coefficient for red oak is 0.00369 per percent. Multiply 36 by 0.00369 by the 6 point drop, and the top shrinks by about 0.80 inches. That is more than three quarters of an inch of movement across one tabletop, and any frame that grips it rigidly on both sides will either split the top or break the joint.
The fix is not to fight the movement but to allow for it. Tabletop fasteners, elongated screw holes and floating panels all let the top move while staying flat and attached. The calculator tells you how much room to allow. A gap of an eighth of an inch might look generous on a narrow board, but on a wide top in a dry climate it can be far too small.
Movement along the grain
Wood barely moves along the grain, typically a tenth or less of its across-grain movement, so length changes can usually be ignored in furniture work. That asymmetry is why frame-and-panel construction exists: the frame is stable along its length, and the panel floats inside it, free to expand and contract across its width. When you see a panel glued rigidly into a frame, you are looking at a future crack, even if it looks perfect on the day it leaves the shop.
Finishes slow moisture exchange but do not stop it. A well finished board still follows the seasons, just more slowly and with smaller swings. The only honest strategy is to design for movement from the start, using the numbers for your species and your climate, and to check the moisture content of your stock with a meter before you build.
Frequently asked questions
What moisture change should I plan for? Measure your shop and your customer’s home if you can. As a rough guide, indoor moisture content often swings between about 6 percent in a heated winter interior and 11 or 12 percent in a humid summer.
Does plywood move the same way? No. Plywood’s crossed layers restrain each other, so it moves far less than solid wood and is much more stable across its face. That stability is exactly why it is used for cabinet carcasses.
Can I stop movement by sealing the end grain? End grain sealer slows moisture entry through the ends, which are the fastest path in and out, but it cannot stop movement through the faces. Use it as a helper, not a cure.
Spanish version
Calculadora de Movimiento de la Madera – Expansión Estacional a Través de la Veta