Wood Movement Calculator

Wood expands when it absorbs moisture and contracts when it dries. This seasonal movement is one of the most important ideas in woodworking, and ignoring it is the number one cause of cracked panels, split tabletops and failed joints.

Movement = Width × MC change × Coefficient

Where:

• Width is the board dimension across the grain. Movement along the grain is so small you can ignore it, which is why a table apron never splits but a wide top does.
• MC change is the change in moisture content in percentage points, not a fraction. A swing from 6% to 12% is 6, not 0.06.
• Coefficient is the dimensional change coefficient for that species and cut, published by the Forest Products Laboratory. It runs from about 0.0007 to 0.0037 per percentage point.

Movement happens in two directions relative to the growth rings:

• Tangential (flat-sawn lumber, growth rings running roughly parallel to the face): the larger figure, roughly 0.0020 to 0.0037 per 1% MC depending on species
• Radial (quarter-sawn lumber, rings running roughly perpendicular to the face): about half as much, roughly 0.0007 to 0.0019

That gap is the whole argument for quarter-sawn stock in a tabletop or a floor. It costs more per board foot because you get less of it out of a log, and it moves half as much.

Dimensional change coefficients used here

Worked example. A flat-sawn oak panel 18 inches wide, going from 6% MC in a heated house in January to 12% in a humid August:

Movement = 18 × 6 × 0.00365 = 0.394 inches, which is a shade under 13/32, so call it 3/8.

Three eighths of an inch across an 18-inch panel is not a rounding error. Glue that panel into a rigid frame and it will either split itself or push the frame apart, and it will do it in the first summer. Float it in a groove with room at the edges, or attach the top with slotted holes or buttons, and it moves quietly and nothing happens.

Common indoor MC range: 6 to 8% in winter in a heated home, 10 to 14% in a humid summer. Design for the full swing in your own climate rather than the average, because the failure happens at the extreme. And acclimate lumber in the shop for a week or two before glue-up: most cracked tabletops come from skipping that, not from bad joinery.