Wood Movement Calculator
How much will that panel expand or shrink with the seasons? Pick a species and grain orientation, set the moisture swing, and design the joint before the wood decides for you.
Verified against independent hand calculation · sources cited below
0.516″ ≈ 17/32″ · 13.1 mm
Panel expands across the grain (tangential 8.6%)
Design for it: leave 17/32″ of total clearance (half per side in a frame), use elongated screw holes or buttons on tabletops, and never glue a panel of this width into a rigid frame.
FPL method: ΔW = W × S/100 × ΔMC/30. Species values are averages — boards vary, and finish slows moisture exchange but never stops it. Length change along the grain is ~0.01% and ignorable.
How much does wood expand across the grain?
A 12″ wide flatsawn red oak board grows about 0.21″ (roughly 7/32″) when its moisture content rises from 6% to 12%. Cut quartersawn, the same board grows only 0.10″ (about 3/32″). The rule behind the numbers: for each 1% change in moisture content, flatsawn red oak moves about 0.0029″ for every inch of width, and quartersawn about 0.0013″. Scale it to your board:
| Board width | Flatsawn | Quartersawn |
|---|---|---|
| 4″ | 0.069″ (≈ 1/16″) | 0.032″ (≈ 1/32″) |
| 6″ | 0.103″ (≈ 3/32″) | 0.048″ (≈ 1/16″) |
| 8″ | 0.138″ (≈ 1/8″) | 0.064″ (≈ 1/16″) |
| 12″ | 0.206″ (≈ 7/32″) | 0.096″ (≈ 3/32″) |
| 24″ | 0.413″ (≈ 13/32″) | 0.192″ (≈ 3/16″) |
| 36″ | 0.619″ (≈ 5/8″) | 0.288″ (≈ 9/32″) |
| 48″ | 0.826″ (≈ 13/16″) | 0.384″ (≈ 3/8″) |
The USDA Wood Handbook has a second method for wood that stays between 6% and 14% moisture content: change in width = width × a species coefficient × the change in MC. Its coefficient for northern red oak, tangential, is 0.00304 per percentage point, which puts the 12″ board at 0.22″, about 6% more than the 0.21″ above. The two methods start from different baselines (green wood versus 10% MC), and the Handbook puts board-to-board variation near 15%, so read any figure as an estimate. Along the grain the change is small enough to ignore: 0.1–0.2% from green all the way to ovendry.
The formula, shown
ΔW = W × (S ÷ 100) × (ΔMC ÷ 30), where S is the species’ green-to-ovendry shrinkage percentage (tangential for flatsawn width, radial for quartersawn) and 30% is the fiber saturation point — below it, movement is close to linear. Worked example: 30″ flatsawn red oak (T = 8.6%), 6% → 12% MC: 30 × 0.086 × 6/30 = 0.516″.
Flatsawn vs quartersawn: how much less does quartersawn move?
About half as much. The Wood Handbook says wood shrinks and swells most around the growth rings (tangentially, the width of a flatsawn board), about half as much across them (radially, the width of a quartersawn board), and only slightly along the grain. In the species data behind this calculator the tangential figure runs 1.2× the radial one for Ipe up to 2.9× for eastern white pine. Here is a 12″ board of common species between 6% and 12% MC:
| Species | Tangential / radial | Flatsawn | Quartersawn |
|---|---|---|---|
| Oak, white | 10.5% / 5.6% (1.9×) | 0.252″ | 0.134″ |
| Maple, hard (sugar) | 9.9% / 4.8% (2.1×) | 0.238″ | 0.115″ |
| Oak, red (northern) | 8.6% / 4.0% (2.1×) | 0.206″ | 0.096″ |
| Ash, white | 7.8% / 4.9% (1.6×) | 0.187″ | 0.118″ |
| Walnut, black | 7.8% / 5.5% (1.4×) | 0.187″ | 0.132″ |
| Cherry, black | 7.1% / 3.7% (1.9×) | 0.170″ | 0.089″ |
| Pine, eastern white | 6.1% / 2.1% (2.9×) | 0.146″ | 0.050″ |
| Mahogany, Honduran | 4.1% / 3.0% (1.4×) | 0.098″ | 0.072″ |
Species matters as much as the cut: Honduran mahogany flatsawn moves 0.098″, about the same as red oak quartersawn at 0.096″. Commercial boards are rarely perfectly flatsawn or quartersawn. The Handbook notes that treating such lumber as true flatsawn stock will probably overestimate its width change, and suggests tangential values when the ring orientation is mixed or unknown, which makes the tangential column the cautious choice. Look at the end grain to decide which column your board belongs to.
How much gap should you leave for a solid wood tabletop?
Leave room for the full calculated movement, shared by the two edges. Take a 36″ wide flatsawn red oak top that swings from 6% to 12% moisture content: it changes width by 0.62″ (about 5/8″). If the top is held at its centerline and free to slide everywhere else, each edge moves half of that, 0.31″ (about 5/16″). In quartersawn red oak the same top moves 0.29″ in all, 0.14″ per edge. The table scales it to common widths:
| Top width | Flatsawn, total | Flatsawn, per edge | Quartersawn, total | Quartersawn, per edge |
|---|---|---|---|---|
| 24″ | 0.41″ | 0.21″ | 0.19″ | 0.10″ |
| 30″ | 0.52″ | 0.26″ | 0.24″ | 0.12″ |
| 36″ | 0.62″ | 0.31″ | 0.29″ | 0.14″ |
| 42″ | 0.72″ | 0.36″ | 0.34″ | 0.17″ |
| 48″ | 0.83″ | 0.41″ | 0.38″ | 0.19″ |
Build the top near the moisture content it will live at. The Wood Handbook’s advice is to install wood at the moisture content it will experience in service, and it recommends about 8% for interior wood in most of the United States. A 36″ flatsawn red oak top installed at 8% then loses 0.21″ if the house dries to 6% and gains 0.41″ if it humidifies to 12%. A top built at 6% in a dry winter will only expand, so it needs the full gap.
Designing for the number
- Frame-and-panel: the panel floats in grooves — give it the calculated clearance, half per side, and never glue it in.
- Tabletops: fasten through elongated holes, figure-8s, or buttons so the top slides on the base.
- Breadboard ends: pin the center tenon tight, elongate the outer pins — the end stays flush at the middle and the top moves at the edges.
- Drawer fronts and doors: size winter gaps knowing summer will close them by the calculated amount.
The full reasoning, with finishes, plywood and acclimation, is in the wood movement guide.
Humidity and moisture content: which numbers to use
Wood settles at a moisture content set by the air around it, so the inputs for this calculator are really humidity questions. The Wood Handbook’s table of equilibrium moisture content at 70°F translates between the two, and the last column shows what each step does to a 12″ flatsawn red oak board measured from the driest row:
| Relative humidity | Equilibrium MC | Width change vs 30% RH |
|---|---|---|
| 30% | 6.2% | baseline |
| 40% | 7.7% | +0.052″ |
| 50% | 9.2% | +0.103″ |
| 60% | 11.0% | +0.165″ |
| 65% | 12.0% | +0.200″ |
| 70% | 13.1% | +0.237″ |
For where wood should start, the Handbook’s recommended installation moisture content for interior woodwork, flooring and furniture is about 8% (6–10% for individual pieces) in most areas of the United States, about 6% (4–9% for individual pieces) in dry southwestern areas, and about 11% (8–13% for individual pieces) in damp, warm coastal areas. This calculator’s default of 6% → 12% corresponds to about 30% → 65% relative humidity at 70°F, which is a wide swing; if you know your space stays between narrower limits, enter those instead and the allowance shrinks with them.
Frequently asked questions
How much does wood expand across the grain?
For each 1% change in moisture content a board changes by about width × (shrinkage % ÷ 100) ÷ 30. Flatsawn red oak moves 0.0029″ per inch of width for each 1% MC (quartersawn 0.0013″), so a 12″ board going from 6% to 12% MC grows 0.21″ (about 7/32″) flatsawn and 0.10″ quartersawn. Along the grain the change is tiny: the USDA Wood Handbook puts it at 0.1–0.2% from green all the way to ovendry.
How much will a 12-inch wide board move?
Across the 26 species in the calculator, a 12″ flatsawn board moves from 0.10″ (Honduran mahogany) to 0.29″ (American beech) between 6% and 12% MC; red oak is 0.21″. Quartersawn, the range is 0.05″ (eastern white pine) to 0.18″ (yellow birch). Type your own species, width and moisture swing into the calculator above.
How much gap should I leave for a solid wood tabletop?
Leave room for the full calculated movement, split across the two edges. A 36″ flatsawn red oak top moving 6% → 12% MC changes width by 0.62″ (about 5/8″); pinned at its centerline, each edge moves half, 0.31″ (about 5/16″). Quartersawn, the same top moves 0.29″ in total, 0.14″ per edge. Keep that clearance between each edge and any rigid surface, and make the outer slots or button pockets long enough to let the top slide.
How much does wood shrink per percent moisture change?
For red oak, 0.00287″ per inch of width per 1% MC flatsawn and 0.00133″ quartersawn — the tangential or radial shrinkage % divided by 30, since shrinkage runs from the 30% fiber saturation point. The Wood Handbook’s in-service coefficient for northern red oak is about 6% higher: 0.00304″ per inch of width per 1% MC (tangential, valid from 6% to 14% MC). The Handbook also puts board-to-board variation near 15%, so read any figure as an estimate.
Does quartersawn wood move less than flatsawn?
Yes, roughly half as much across the width. In this calculator’s data tangential shrinkage runs from 1.2× (Ipe) to 2.9× (eastern white pine) the radial figure; for red oak it is 2.1×. A 12″ red oak board moves 0.21″ flatsawn but 0.10″ quartersawn between 6% and 12% MC. Lumber is seldom perfectly either; the Wood Handbook suggests tangential values when ring orientation is mixed or unknown.
What moisture contents should I enter for indoor furniture?
The USDA Wood Handbook recommends installing interior wood at about 8% moisture content in most of the United States (individual pieces 6–10%), about 6% in the dry Southwest and 11% in damp coastal areas, so wood starts near the average it will see in service. At 70°F, 6.2% MC corresponds to about 30% relative humidity and 12% to about 65% (Handbook Table 4-2). The calculator’s 6 → 12 default is therefore a wide design swing; narrow it if your space is climate-controlled.
How much does a tabletop actually move?
A 30″-wide flatsawn red-oak top swinging from 6% MC (winter, heated house) to 12% (humid summer) moves about 0.52″, roughly half an inch. Quartersawn stock of the same width moves 0.24″, under a quarter inch — less than half. That difference is the whole argument for quartersawn tabletops.
Does finish stop wood movement?
No — it only slows the exchange. Film finishes stretch the cycle over weeks instead of days, which evens out short humidity spikes, but seasonal change gets through everything. Design joints for movement; don’t ask varnish to fight physics.
Why does wood move across the grain but not along it?
Water lives in the cell walls, and cells are long tubes aligned with the grain. Losing water shrinks the tube walls — the diameter changes, the length barely does. Longitudinal shrinkage is about 0.1–0.2% total, which is why a 6-foot rail effectively never changes length.
Flatsawn vs. quartersawn — which number applies to my board?
Look at the end grain. Growth rings parallel to the face (arching across the end) = flatsawn, use tangential. Rings perpendicular to the face (vertical stripes) = quartersawn, use radial. Riftsawn falls between; the calculator’s two cases bracket it. If the orientation is mixed or unknown, use the flatsawn (tangential) figure, as the Wood Handbook suggests.
Sources & further reading
- USDA Forest Products Laboratory — Wood Handbook (FPL-GTR-282, 2021), Ch. 4 “Moisture Relations and Physical Properties of Wood” (Glass and Zelinka): shrinkage values (Table 4-3), the dimensional-change formula (Eq. 4-9), tangential vs radial vs longitudinal shrinkage, equilibrium moisture content by humidity (Table 4-2)
- USDA Forest Products Laboratory — Wood Handbook (FPL-GTR-282, 2021), Ch. 13 “Drying and Control of Moisture Content and Dimensional Changes” (Bergman): recommended installation moisture content (Table 13-2), in-service dimensional change coefficients (Eq. 13-3, Table 13-5), coatings retard but do not prevent change
- Method: ΔW = W × S/100 × ΔMC/30, fiber saturation point taken as 30% MC
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