Wood Shrinkage Calculator
Predict the dimensional change of a wood board as moisture content drops from initial to final equilibrium, using the species-specific shrinkage rate. Useful for milling green lumber, planning joinery clearances, and avoiding warping or splitting in finished work.
Last updated: May 2026
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About this calculator
Wood shrinks as it loses moisture below the fiber saturation point (FSP, approximately 28–30% moisture content for most species). Above FSP, water lives in the cell cavities and exits without changing dimensions; below FSP, water leaves the cell walls and the wood contracts. The formula is finalSize = initialSize × (1 − ((initialMC − finalMC) × shrinkageRate / 100)), where initialMC and finalMC are moisture content percentages (e.g., 30 for green lumber, 8 for kiln-dried interior wood), and shrinkageRate is the species-specific percent dimensional change per percent MC change (expressed as %/% MC). Variables and edge cases: shrinkage rates differ by grain orientation — tangential (across growth rings, the typical 'flat-sawn face') is roughly 1.5–2× greater than radial (through the rings, quarter-sawn face); longitudinal shrinkage (along the grain) is so small (~0.1–0.3% total from green to oven-dry) that it's usually ignored. Total tangential shrinkage values from the USDA Forest Products Laboratory range from about 5% (cedar, some softwoods) to 12% (some species like hickory). For typical kiln-dried interior service (going from 30% green to 8% indoors), expect: cherry ~6%, white oak ~7.4%, hard maple ~8%, beech ~9.7% tangential shrinkage. The formula assumes uniform moisture loss; real lumber dries first at the surface, creating a moisture gradient that can cause checking, casehardening, and uneven shrinkage between flat-sawn and quarter-sawn parts of the same board. Above FSP, the formula gives misleading negative shrinkage; clamp inputs so initialMC and finalMC are both at or below FSP. For wood-movement design in finished pieces (panels in frames, drawer slides, edge bands), use seasonal MC range (typically 6–14% in temperate climates) rather than initial-to-equilibrium values.
How to use
Example 1 — drying green white oak. Initial dimension 8 inches (tangential face), initial MC 30%, target final MC 8%, white oak tangential shrinkage rate 0.37%/% MC. Step 1: ΔMC = 30 − 8 = 22%. Step 2: % shrinkage = 22 × 0.37 = 8.14% total. Step 3: finalSize = 8 × (1 − 0.0814) = 8 × 0.9186 = 7.35 inches. The board shrinks 0.65 inches in width over the full drying range. Verify: the published total tangential shrinkage for white oak (green to oven-dry) is 10.5%; this calculation goes from 30% MC to 8% MC, which is most but not all the FSP range, so 8.14% (~78% of the full range) is consistent. Example 2 — seasonal cabinet door panel. A 12-inch-wide flat-sawn maple panel installed at 8% MC in winter; summer humidity raises panel MC to 14%. Hard maple tangential shrinkage ~0.35%/% MC; but here we're going the other direction (8 → 14% = swelling). Step 1: ΔMC = 14 − 8 = 6% (positive, swelling). Step 2: dimensional change = 12 × 6 × 0.35/100 = 12 × 0.021 = 0.252 inches expansion. Step 3: panel widens from 12.00 to 12.25 inches. Design implication: the panel must float in the frame with at least 1/8 inch clearance on each side to accommodate this movement, or the door will warp or crack. Verify by comparing to common woodworking design tables: hard maple is high-movement; allowing 1/8″ per foot of width across seasons is the standard rule of thumb.
Frequently asked questions
What shrinkage rate should I use for different wood species and grain orientations?
Shrinkage rates are published by the USDA Forest Products Laboratory in the Wood Handbook (FPL-GTR-282). Common values for tangential / radial shrinkage as percent dimensional change per percent MC change (%/% MC): cherry 0.31 / 0.16; red oak 0.37 / 0.18; white oak 0.37 / 0.18; hard maple 0.35 / 0.17; soft maple 0.34 / 0.16; walnut 0.30 / 0.19; ash 0.34 / 0.17; mahogany 0.21 / 0.13; teak 0.18 / 0.11. Softwoods are generally lower: pine 0.18–0.22 tangential; cedar 0.15–0.18 tangential. Tangential rate is the higher number — flat-sawn boards show this movement across their width. Radial rate is the lower number — quarter-sawn boards show this across their width. Longitudinal shrinkage (along the grain) is 0.005–0.015%/% MC, essentially negligible for design purposes. For mixed-grain wood (riftsawn, half-tangential half-radial), use a value midway between tangential and radial — typically the average. Always look up the specific species you're working with; using a generic rate can produce 20–40% error in shrinkage prediction.
How does fiber saturation point work, and why does wood not shrink above it?
Fiber saturation point (FSP) is the moisture content at which all free water (water inside the cell cavities) has been removed, but the bound water (water absorbed into the cell walls) is still fully present. FSP varies slightly by species but is generally 25–30% MC. Above FSP, water lives in the cell cavities — the lumens between cell walls — and when this water evaporates, the cell walls remain saturated and the wood does not shrink dimensionally. Once MC drops below FSP, water is removed from the cell walls themselves; the walls contract, and the wood shrinks. This is why green lumber (often 60–100% MC at felling) can dry from 80% MC down to 30% MC without any measurable size change, but then shrinks measurably as MC continues to drop to interior service levels of 6–14%. Practical implications: when drying lumber, the dimensional change concentrates in the second half of the drying curve, not the first half; checking and casehardening defects also form during this lower-MC phase, which is why kiln drying schedules slow down significantly below 30% MC. For most calculations, set initialMC at 30% (FSP) and finalMC at your target equilibrium MC, ignoring any drying above FSP.
How much movement should I allow in joinery for seasonal humidity changes?
Interior wood reaches equilibrium with indoor humidity; in temperate climates, this typically ranges from about 6% MC in heated winter conditions to 12–14% MC in humid summer conditions. That seasonal swing of about 6–8% MC causes substantial dimensional change in wide tangential panels. Rule of thumb: allow 1/8″ per foot of width per season for tangential movement of typical hardwood. So a 24″ wide flat-sawn tabletop should be expected to move 1/4″ across the year, requiring elongated screw holes for breadboard ends or expansion gaps under buttons. Specific examples for an 8% MC change at typical species: 12″ wide hard maple flat-sawn panel: 12 × 8 × 0.35/100 = 0.336″ ≈ 11/32″ movement. 12″ wide cherry flat-sawn: 12 × 8 × 0.31/100 = 0.30″ ≈ 5/16″. Quarter-sawn versions of these moves are roughly half. Drawer slides, frame-and-panel doors, and case sides must all account for this — use floating tenons, slotted screw holes, expansion gaps in panel grooves, and avoid hard-glue connections perpendicular to grain over long lengths. The Forest Products Laboratory's Wood Handbook publishes detailed climate-zone MC equilibrium maps for the US.
What are common mistakes when calculating and accounting for wood shrinkage?
The most common mistake is using a generic 'wood is 8% shrinkage' value when species-specific rates differ from 5% to 12% — produces 50%+ error. Another error is applying tangential shrinkage rates to a quarter-sawn board; quarter-sawn moves about half as much, so using the higher rate over-builds clearances. Calculating shrinkage above FSP (e.g., from 80% MC to 60% MC) gives non-zero results from the formula but is physically wrong — wood doesn't shrink above FSP. Failing to allow for seasonal movement in finished pieces: a tabletop installed perfectly flush in summer can split in winter as it shrinks across the grain while constrained by the apron. Using oven-dry shrinkage (the published total from 30% to 0% MC) for service conditions overstates real movement — interior wood rarely drops below 6% MC, so use the proportional range. Forgetting that mixed-grain stock (rift-sawn, half-tangential) behaves between the tangential and radial extremes — use an average rate. Designing breadboard ends with rigid attachment instead of elongated slots is a classic failure that splits the main panel. Finally, not measuring actual MC of incoming lumber with a moisture meter assumes 'kiln-dried 8%' when actual MC may be 12–18% — install at the wrong MC and the piece moves dramatically once it acclimates.
When should I NOT use this calculator?
Skip this calculator for green lumber above FSP — wood doesn't shrink above FSP, so a calculation from 80% MC to 50% MC will report a substantial size change that doesn't actually happen. Do not use it for engineered wood (plywood, MDF, particleboard, OSB, LVL) — these are designed to minimize dimensional movement via cross-grain construction and use different design rules. Avoid it for wood used in steady-humidity conditions where moisture content stays constant — humidors, conditioned wine cellars, museum storage — those don't experience cyclic dimensional change. The formula does not capture casehardening, internal stress, surface checking, and end-grain checking that occur during drying — those are quality defects, not dimensional changes. For wood being used outdoors where MC swings from saturated wet (raining) to ambient dry, the formula's assumptions about uniform moisture distribution don't hold; outdoor wood movement is more violent and unpredictable. For ancient or historic wood (very old furniture, structural beams in old buildings), MC may already be in equilibrium and additional shrinkage is minimal. Finally, for any structural calculation involving long-term wood deformation under load (creep, mechano-sorptive creep), shrinkage alone is incomplete — those calculations need full stress-strain analysis from wood-mechanics references.