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Wood movement calculator
Solid wood gets wider in humid months and narrower in dry ones. Enter the width across the grain, the species, how it was sawn and the moisture swing to see how much it will move.
How to use this calculator
Wood shrinks and swells as its moisture content changes below the fiber saturation point, and it moves far more across the grain than along it. Along the length of a board the change is small enough to ignore for furniture; across the width it is what cracks tabletops and splits glued-on cleats.
The calculator uses Equation 13-3 from the USDA Forest Products Laboratory Wood Handbook (FPL-GTR-282, Chapter 13): ΔW = W × C × (MC_final − MC_initial), where W is the width across the grain, MC is moisture content in percent and C is the dimensional change coefficient from Table 13-5. The handbook gives two coefficients per species: tangential (C_T), which applies to the width of flat-sawn boards, and radial (C_R), which applies to quartersawn boards. Tangential movement is larger, which is why quartersawn stock is prized for stability.
As an example, a 36-inch flat-sawn northern red oak tabletop (C_T = 0.00304) going from 8% to 12% moisture content changes by 36 × 0.00304 × 4 ≈ 0.44 inch. A quartersawn top of the same oak (C_R = 0.00137) would move about 0.20 inch.
The coefficients are published for moisture contents between 6% and 14%, which covers most indoor furniture; outside that range the handbook uses a different method, so treat results there as rough. Real panels mix grain orientations, so actual movement usually falls between the radial and tangential figures. The moisture content of wood follows the relative humidity around it, so the swing for a piece depends on your home and climate.
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Questions about this calculator
Why do tabletops crack?
Usually because the top was fixed so it couldn't move. If screws, glue or a rigid frame hold a wide solid top at both edges, the wood tears itself apart when it shrinks in dry months. Tabletop fasteners, elongated screw holes and breadboard ends with slotted pegs let it move.
How do I find the moisture content of my wood?
Use a pin or pinless moisture meter on the boards. Your maker should know the moisture content of their stock; ask about it in your brief, especially for wide tabletops.