How to use the wood moisture content and shrinkage calculator
Wood gives up or takes on moisture until it matches the surrounding air, and it changes width as it does. Most flooring gaps, sticking cabinet doors and cupped panels start with that movement. Pick the species and direction, enter the current and target moisture content and the board dimension, and this tool returns how much the piece will move in inches.
The formula is dimensional change = dimension × total shrinkage ÷ 100 × moisture change ÷ 30, the dimensional change coefficient approach used in the Forest Products Laboratory Wood Handbook. The 30 is the fiber saturation point: above roughly 30 percent moisture the cell walls are already saturated and the wood barely moves, so values entered above 30 are capped. Moisture content is on the dry basis, as is standard for wood.
Direction matters enormously. Tangential movement, which follows the growth rings, is close to twice radial movement toward the pith, while movement along the grain is negligible. Quartersawn boards move mostly in the smaller radial direction, which is why they stay flatter in wide panels. The most reliable precaution is to acclimate material in the space before installation, since indoor equilibrium moisture content runs about 6 to 9 percent in most heated US interiors and higher in humid coastal regions. Verify the shrinkage value for your exact species in a wood properties table.
Frequently asked questions
A pin type moisture meter is fine for field checks. For an accurate number, oven dry a sample at 215°F and compute (wet weight minus ovendry weight) divided by ovendry weight.
Leave at least the calculated expansion for the full width of the floor at the perimeter. Wide rooms accumulate more movement, so consider intermediate transition joints as well.