Estimate how much a solid-wood panel or tabletop swings across the grain between dry and humid seasons — and how much allowance to leave.
Solid wood is hygroscopic: it takes on and gives off moisture as the air around it changes through the year, and as it does it shrinks and swells. The catch is that this movement happens almost entirely across the grain — a tabletop changes width with the seasons but stays the same length. If you build as though wood is dimensionally stable, panels split, breadboard ends crack, drawers jam, and mitre joints open up.
This calculator estimates the seasonal swing in width for your specific board using the dimensional-change-coefficient method, then translates that into practical allowances: how much room a floating panel needs in its groove, and roughly where to seat it at build time so it can move both directions.
Reach for this before cutting a frame-and-panel door, a solid tabletop, a wide drawer front, a blanket-chest lid, a workbench top, or a breadboard end. Anywhere a wide solid panel meets a cross-grain part, the two move differently and something has to give. Knowing the number in advance tells you how deep to plough a groove, how big to make elongated fastener slots, and how big an expansion gap to leave so the work survives its first humid summer.
The core relationship from the USDA Forest Products Laboratory Wood Handbook is:
movement = width × Cd × (MChigh − MClow)
Cd is the dimensional change coefficient: the fraction a board's width changes for every 1% change in moisture content, within roughly the 6–14% moisture range that covers normal indoor conditions. It differs by species and by direction — the tangential coefficient applies to flatsawn faces and the smaller radial coefficient to quartersawn faces.
| Species | Tangential | Radial |
|---|
Values are standard USDA Wood Handbook dimensional change coefficients. If your species isn't listed, choose Custom species and enter your own.
Wood shrinks and swells mainly perpendicular to the grain as it gains or loses moisture below fiber saturation. Lengthwise movement is so small it's normally ignored — typically under 0.2% over the full moisture range. That's why a tabletop changes width but not length with the seasons, and why panels float while breadboard ends use elongated fastener holes.
It's the fractional change in a board's width for each 1% change in moisture content, within roughly 6–14% MC. Flatsawn boards use the tangential coefficient; quartersawn boards use the smaller radial one. Multiply width × coefficient × moisture-content change to get movement. These are published per species in the USDA Wood Handbook.
Wood equalises to an equilibrium moisture content set by surrounding humidity and temperature. Many heated homes sit around 6–8% in dry winter and 9–12% in humid summer, but this varies by climate and whether spaces are climate controlled. With a moisture meter, take a winter and summer reading; otherwise enter the low and high you expect. A wider range yields a more conservative allowance.
The panel must be free to expand into its groove. Seat it near the middle of its moisture range so it can both grow and shrink, leave at least the calculated expansion amount of free space on each side inside the groove, and never glue it into the frame — otherwise it will crack or push the joints apart.