Wood never stops moving. A tabletop glued tight in a humid summer can split wide open by dry winter — or buckle if you build it in the dry season and summer arrives. Enter your panel width, species and the moisture swing below to find out exactly how much it will move, and how big a gap to leave.
Your panel
How the math works
Wood swells as it gains moisture and shrinks as it loses it, almost entirely across the grain. The formula, using the USDA Wood Handbook shrinkage coefficient for your species and cut, is:
Movement = panel width × coefficient × |damp-season MC − dry-season MC|
If the panel dries (damp MC > dry MC) it shrinks: new width = width − movement. If it gains moisture it swells: new width = width + movement. The gap to leave is the movement rounded up to the nearest 1/16″.
Worked example: a 36″ wide flatsawn white oak panel (coefficient 0.00365) going from 12% moisture content in the damp season to 6% in the dry season moves 36 × 0.00365 × 6 = 0.79″. It shrinks to a new width of 35.21″, so leave at least a 13/16″ gap for it to move into.
Tips before you build
- Let the wood acclimatise. Bring boards into the room they will live in for at least a week (two in a heated winter house) and check moisture with a meter before you cut.
- Design for movement, don’t fight it. Breadboard ends, elongated screw slots and floating panels let a top move without splitting. Gluing a wide solid top rigidly to its apron is how cracks happen.
- Quartersawn costs more, moves less. A quartersawn board moves roughly half as much as the same species flatsawn — worth it for wide tabletops and doors.
- Pick stable species for wide panels. Our beginner’s wood guide compares common species, and cherry and pine are among the calmest choices for a first wide panel.
- Check the load too. A wide, thin top also sags — run it through the shelf sag calculator before you settle on a thickness.
Frequently asked questions
Why does wood only move across the grain?
Wood fibres are long hollow tubes running along the grain. When moisture enters or leaves the cell walls, the tubes swell or shrink in diameter — across the grain — but barely change in length. Lengthwise movement is roughly 0.01–0.02% over a whole season, so small it can be ignored in almost every project; width is where all the action is.
What is the difference between flatsawn and quartersawn wood?
Flatsawn boards are cut tangent to the growth rings, showing wide cathedral grain, and they move the most across their width — and can cup. Quartersawn boards are cut radially, showing straight grain, and move about half as much and stay flatter. Quartersawn costs more because the mill gets fewer boards from a log, but for wide panels and doors it is the safer choice.
How much gap should I leave for a tabletop?
Leave at least the full seasonal movement for your panel — the calculator rounds it up to the nearest 1/16″ for you. For a typical 36″ oak top in a home that swings from about 12% to 6% moisture, that is roughly 13/16″ total across the width. Split the gap between the two long edges (or hide it under an overhang) and use slotted fixings so the top can slide as it moves.
Does plywood move like solid wood?
Barely. Plywood’s layers are glued with the grain running in alternating directions, so each layer restrains the next — seasonal movement is typically under 0.05% across a sheet, about a tenth of solid wood. That is why plywood is safe for cabinet carcasses and drawer bottoms. It still expands slightly with humidity, so leave a hairline gap at tight joints, but you do not need the full solid-wood allowance.