Wood never stops moving. It takes on and gives off moisture with the surrounding air, and it changes width as it does — always across the grain, essentially never along it. Every failed panel, split tabletop and stuck drawer is this fact being ignored.
Where the numbers come from
The USDA Forest Products Laboratory measured shrinkage from green to ovendry for every commercial species, in two directions: radial, across the growth rings, and tangential, along them.
Those totals are what this calculator uses. Red oak shrinks 4.0 percent radially and 8.6 percent tangentially. White oak is 5.6 and 10.5. Mahogany is only 3.0 and 4.1, which is exactly why it was the wood of choice for wide panels for two centuries.
Wood moves only below about 30 percent moisture content, and over that range the movement is close enough to linear that you can divide the total by 30 and get a coefficient per one percent of moisture change. That is the step shown in the working above.
Flatsawn moves about twice as much
The single biggest lever is not the species, it is how the board was sawn. Tangential movement runs roughly double radial movement in most species.
A flatsawn board — the common cheap cut, with the cathedral grain pattern on the face — moves along the tangential figure. A quartersawn board, with tight straight grain and ray fleck in oak, moves along the radial figure.
So a quartersawn white oak tabletop moves about half as much as a flatsawn one of the same width, for the same cost in wood but a higher cost per board foot. On a wide top that trade is usually worth it.
Quartersawn also cups far less, because the movement is symmetrical about the board's centre rather than greater on one face.
What moisture numbers to enter
Kiln-dried lumber arrives around 6 to 8 percent. A heated house in winter drives furniture down to 6 or even lower. The same house in a humid summer comes up to 10 or 12.
So the swing a piece of furniture actually sees is commonly 6 percent of moisture content, not 30. Enter the two ends of the range your piece will live in, not the range from the sawmill.
Wood in an unheated garage or a screen porch sees a much wider swing, and a piece built in a dry shop and moved outdoors will move more than these numbers suggest in its first season.
Grain direction is the whole game
Movement along the grain is so small it can be ignored — well under a tenth of the cross-grain figure. This is why the classic failures all involve two pieces whose grain runs at ninety degrees to each other.
A solid panel glued into a frame. A breadboard end glued across the full width. A batten screwed tight across the back of a door. A wide top screwed solid to an apron.
In every case one piece wants to change width and the other does not. The wood does not compromise, so something has to give: the glue line, the screw, or the board.
The three ways to let it move
Float it. A panel sits in a groove with clearance and no glue, so it can change width inside the frame. Frame-and-panel construction exists for exactly this reason and has for five hundred years.
Pin it at one point. A breadboard end is glued only at the centre and pinned in elongated holes toward the outside, so the top can expand either way from the middle.
Fasten it loosely. Tabletop buttons, figure-eight fasteners, or screws in slotted holes hold a top down firmly while letting it slide. The slot runs across the grain of the top, not along it.