Understanding wood movement and moisture content
Why solid wood changes size with the seasons, how the fiber saturation point works, and how to build so panels expand without splitting.
Wood is hygroscopic
Solid wood constantly trades moisture with the air around it, taking on water when the room is humid and giving it up when the air is dry. As the bound water in the cell walls rises and falls, the board swells and shrinks across its width. This is why a tabletop can feel tight against its breadboard end in July and open a hairline gap in January. The exchange never really stops, so a finished piece keeps moving gently for its whole life.
The fiber saturation point
Below roughly 28 percent moisture content, the wood begins to change size as bound water enters or leaves the cell walls. Above that point the extra water is free water sitting in the cell cavities, and it has no effect on dimensions. That is why this calculator clamps both moisture readings to 28 percent before doing any math. Green lumber straight off the saw sits well above fiber saturation, which is why it must dry before it is worked.
Tangential versus radial movement
Wood does not move the same amount in every direction. Tangential movement, along the growth rings, is the largest, radial movement across the rings is smaller, and movement along the grain is so small it is usually ignored. A flatsawn board shows mostly tangential movement and a quartersawn board shows mostly radial movement, which is the whole reason quartersawn stock is more stable. The species coefficients in this tool encode exactly that difference.
Building for movement
Good joinery expects the wood to move rather than fighting it. Frame and panel doors let a solid panel float in a groove so it can grow and shrink without stressing the joints. Tabletops are attached with slotted screws, figure eight fasteners, or buttons that slide as the top expands. Sizing that allowance to the movement this calculator predicts is the difference between a panel that lasts decades and one that cracks in its first dry winter.