Shrinkage PredictorWood Dimensional Movement Calculator
Predict exactly how much wood will shrink or swell when its moisture content changes. Calculate radial, tangential and volumetric movement for precise allowances in furniture, flooring and joinery.
Why this calculation matters
Movement in millimetres decides board width, gap size and fixing method. It is the number that determines whether a panel stays flat.
Shrinkage Predictor
Wood Dimensional Movement Calculator
Sets shrinkage coefficients automatically.
Current moisture content.
Target / service moisture content.
Green to oven-dry. Auto-set by species.
Green to oven-dry. Auto-set by species.
No shrinkage above this MC.
| Dimension | Original | Final | Change | % Change |
|---|
About Shrinkage Predictor
Wood shrinks as it loses moisture below the Fiber Saturation Point (about 28% MC) and swells as it gains moisture. Shrinkage is not equal in all directions: tangential movement (along growth rings) is typically about twice the radial movement (across rings). This difference causes cupping, warping and checking. This tool predicts the exact dimensional change so you can build in proper allowances. Woodworkers use it as a tangential shrinkage calculator before glue-ups, and importers use it to predict wood shrinkage percentage between origin and destination climates.
Where Is This Used?
Formulas Used
MC change below FSP: dMC = (MC_start - MC_final), capped at FSPShrinkage % = S_total x (dMC_below_fsp / FSP)where S_total = radial or tangential total shrinkage coefficientDimension change = Original x (Shrinkage% / 100)Volumetric ~ Radial% + Tangential% (small-strain approximation)T/R ratio = Tangential / Radial (warp risk: under 1.5 stable, over 2.0 high)How the calculation works
The moisture the wood will lose is expressed as a fraction of the range over which it actually moves. That fraction is applied to the species’ radial and tangential shrinkage rates. Which of the two applies to width depends on how the board was sawn: flatsawn boards move at the tangential rate across their width, quartersawn at the radial rate.
Show the formula
Dry fraction = (MC1 - MC2) / FSP, bounded to the range below fibre saturation. Radial change (%) = radial shrinkage coefficient x dry fraction; tangential change (%) = tangential coefficient x dry fraction. Flatsawn: width uses tangential, thickness uses radial. Quartersawn: reversed. Riftsawn: both use the mean. Dimension change (mm) = dimension x percentage / 100.Longitudinal movement is under 0.2% in most species and is not modelled. T/R ratio is reported because it predicts cupping better than either coefficient alone.
References and what each one provides
- USDA Wood Handbook (FPL GTR-190)USDA Forest Products LaboratoryRadial and tangential shrinkage coefficients and fibre saturation values.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Assumes linear shrinkage below fibre saturation using the coefficients entered. It does not account for longitudinal movement, collapse, case hardening, coatings that slow exchange, or variation between boards from the same log.
Do not use this for Tolerance specification in a contract, or warranty claims about movement.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- USDA FPL Wood Handbook GTR-190 — radial, tangential and volumetric shrinkage
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter your wood and climate details below, then press Calculate.
- Pick your cut orientation from the dropdown.
- Fill in board width, board thickness, starting mc, final mc.
- Press Calculate.
Worked example
Common mistakes to avoid
- Applying tangential shrinkage numbers to quarter-sawn boards. Quarter-sawn moves by the radial (smaller) number.
- Predicting shrinkage from green to oven-dry for indoor use. Real service range is much narrower — use realistic MC endpoints.
- Forgetting that shrinkage is per direction. Width, thickness and length all behave differently; length barely moves.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers