Wood Movement Calculator

Wood moves across its width and effectively not at all along its length, and how much it moves depends on how it was cut out of the log. A flatsawn board moves about twice as much as a quartersawn one of the same species and width, because it presents a different face to the growth rings. A twelve inch flatsawn oak top swings about a quarter of an inch between a dry winter and a humid summer — which is why it cannot be screwed down rigidly across the grain, and why every traditional joint for wide panels exists.

Seasonal movement across the width: −7/32" (shrinks) 12" of red oak, flatsawn, over a 5.0 point change in moisture content. The board finishes at 11.779". Coefficient 0.00369 per percent — from the Wood Handbook, and it varies within a species as well as between them.
The same board quartersawn: −3/32" Red oak moves 2.3 times as much tangentially as radially. Quartersawn stock presents the radial face across its width, so it moves roughly 57% less — which is why it costs more and why it is worth it for a wide panel or a table top.
Movement along the length: Negligible 6 ft of board moves essentially nothing lengthwise — about a hundredth of a percent per moisture point. This asymmetry is the whole problem: a frame member that stays put next to a panel that does not.
Moisture change assumed: 11% → 6% Wood equilibrates with the humidity around it. Indoors in a heated house that means roughly 6 to 8 percent in winter and 10 to 12 in summer, so a four to six point annual swing is normal in most of the country. A shop that is unheated in winter sees more.
Never fix a wide board rigidly across the grain: Slotted holes or buttons A 12" board that wants to move 7/32" and cannot will do one of two things: cup, or split. Table tops are attached with slotted holes, figure-eight fasteners or wooden buttons that let the top slide across the frame while holding it down. Screwed through a fixed hole across the grain, it splits — usually in the first winter.
Breadboard ends and panel frames: Same principle A breadboard end runs across the grain of the panel it caps, so it is glued at the centre only and pinned through elongated holes toward the edges. A raised panel floats in its frame groove with no glue at all, sized so it can grow without pushing the frame apart. Both are solutions to this one calculation.
Acclimatise before machining: Not after Wood delivered from a heated warehouse into an unheated shop, or the reverse, moves for days or weeks. Milling it flat and square before it has equilibrated produces a board that is neither by the time it is assembled. Stack and sticker it in the space where it will live first.
Wide panels move more than narrow ones: 1.85% of width Movement is proportional to width, so gluing up a wide panel from narrow boards does not reduce it — the total is the same. What alternating the growth ring orientation does is spread the cupping into smaller, opposing amounts instead of one large one.

Three directions, three different numbers

Wood shrinks and swells as its moisture content changes, and it does so by very different amounts in three directions.

Tangentially — around the growth rings — it moves most. Radially, from the pith outward, it moves roughly half as much. Longitudinally, along the grain, it moves so little that it is ignored in almost all work.

Which of the first two applies depends on how the board was sawn. A flatsawn board has growth rings running roughly parallel to its face, so its width is the tangential direction and it gets the large coefficient. A quartersawn board has rings running perpendicular to the face, so its width is the radial direction and it gets the small one.

This is most of the reason quartersawn stock costs more. It is also more stable in thickness, it takes finish more evenly, and in oak it shows the ray fleck that people pay for separately.

How much moisture change to expect

Wood equilibrates with the relative humidity around it, and the equilibrium moisture content indoors follows the seasons. In a heated house that is roughly six to eight percent in winter and ten to twelve in summer.

A four to six point swing is normal across most of the country. Arid regions swing less and stay drier; humid coastal regions sit higher year round. An unheated workshop follows the outdoor humidity and swings considerably more than a house does.

The number that matters for a piece of furniture is the range in the room it will live in, not the range in the shop. A table built in an unheated garage at fourteen percent and moved into a heated house will lose several points and shrink, whatever the joinery.

Acclimatising is the practical answer: stack and sticker the stock in the space where the finished piece will live, and leave it until it stops moving before milling it flat and square. Milling first and acclimatising afterwards produces a board that is neither flat nor square by assembly time.

Designing for movement rather than against it

A board that wants to move and is prevented from moving does not stay still. It cups, or it splits, and which one depends on where the restraint is.

Every traditional solution for wide panels is a way of allowing movement while still holding the piece together. A table top is fastened with slotted holes, figure-eight fasteners or wooden buttons that let it slide across the frame. A raised panel floats in its groove with no glue, sized with room to grow. A breadboard end is glued at the centre only and pinned through elongated holes toward the edges.

The failures are all the same failure. A top screwed through fixed holes into an apron. A wide panel glued into a frame. Trim fastened tight across the grain of something that moves. Each one holds for a season and then splits in the first dry winter.

Gluing up a wide panel from narrow boards does not reduce the total movement — the width is the width. What alternating the ring orientation does is break one large cup into several small opposing ones, which is a flatness benefit rather than a movement one.

What this is based on

USDA Wood Handbook, Chapter 4 — dimensional change coefficients by species and direction
Equilibrium moisture content for interior conditions, typically 6–12% through the year

An estimate from published average coefficients. Within-species variation is substantial, and reaction wood, interlocked grain and figured stock move differently again. For critical work, measure a sample of the actual boards through a seasonal cycle rather than relying on table values.

Frequently asked questions

How much does wood move seasonally?

Across the width, typically a quarter of an inch on a twelve inch flatsawn oak board over a normal four to six point moisture swing. Along the length, effectively nothing. The asymmetry is what causes almost every wood movement failure.

Does quartersawn wood move less?

Across its width, about half as much as flatsawn in most species, because its width is the radial direction rather than the tangential one. It is the main practical reason quartersawn stock costs more.

Why did my table top split?

Almost certainly because it was fastened rigidly across the grain. A top that wants to shrink a quarter of an inch and is screwed through fixed holes into an apron has nowhere to go, so it splits — usually in the first dry winter.

How do I attach a table top properly?

With fasteners that let it slide across the frame: slotted holes, figure-eight fasteners, or wooden buttons in a groove. Fix it firmly at one point, usually the centre or one end, and let everything else move.

Does gluing up narrow boards reduce movement?

No — a twenty-four inch panel moves like a twenty-four inch board whatever it is made from. What alternating growth ring orientation does is break one large cup into several small opposing ones, which helps flatness rather than movement.