Sources and method

Primary sources

Forest Products Laboratory. 2021. Wood handbook — wood as an engineering material. General Technical Report FPL-GTR-282. USDA Forest Service, Madison, WI. 543 p. The current edition, and the source of shrinkage, specific gravity, hardness, moisture and the equilibrium moisture equation. Prepared by federal employees and carrying no copyright notice, so the government-authored content is not subject to copyright under 17 U.S.C. 105. Landing page.

Chudnoff, M. 1984. Tropical Timbers of the World. USDA Agriculture Handbook 607. Used only for species the Wood Handbook omits — padauk, zebrawood, panga panga, and the specific gravity of several dense tropicals. Rows drawn from it carry a label in the species table: separate study, separate laboratory, figures that will not bear direct comparison with the Handbook's.

NHLA Rules for the Measurement and Inspection of Hardwood and Cypress and Voluntary Product Standard PS 20, American Softwood Lumber Standard for the board-foot conventions. These are the two documents that settle the nominal-versus-actual question, and they agree.

How the moisture equation was verified

The Wood Handbook states that its published moisture-content table was calculated from a specific equation. That makes the equation checkable against the table, which is exactly what you want from the load-bearing calculation on a site like this.

The implementation here reproduces all twelve published reference values to four decimal places:

Three further checks run on every build. The Handbook publishes two independent coefficient sets, one for Fahrenheit and one for Celsius; both are implemented and must agree to a hundredth at the same physical temperature, which catches a transcription error in either. The function must stay finite and rise monotonically with humidity across the full range of temperature. And at 70°F and 100% humidity it must land at the fibre saturation point, which it does at 28.8% — the model agreeing with itself at its own boundary.

Where this site departs from the Handbook

The Handbook publishes pre-computed dimensional change coefficients in one table and raw shrinkage measurements in another. A footnote in the Handbook gives the derivation linking them, so they are obliged to agree. For several tropical species they are not:

This site uses the derived values. The derivation reproduces the published table exactly for 30 of the 32 species checked across both fibre-saturation assumptions, and it reproduces the Handbook's own worked example for white fir precisely. Where it disagrees, the published table is the stale one. A teak tabletop sized from the pre-computed figure gets a movement allowance a third too small.

What is derived rather than published

• All movement coefficients are computed from shrinkage using the Handbook's stated method, not copied from its coefficient table, for the reason above.
• The riftsawn figure is the mean of radial and tangential. No riftsawn coefficient exists in any primary source and the tools say so on every result.
• Northern catalpa shrinkage is back-derived from its published coefficients, because it appears in the coefficient table but not the shrinkage table. Marked as derived.
• The cup drawing shows direction and relative severity, not a measured deflection. Predicting cup magnitude properly needs ring curvature through the board and the species' elastic constants. The width change beside it is a real calculation.
• Acclimation times apply the diffusion thickness rule from the Dry Kiln Operator's Manual to the FPL's measured storage rates. The rates are published; the thickness scaling is published; combining them is this site's arithmetic. Two limits follow. The published rates were measured on kiln-dried stock taking moisture up in humid storage, so using them for drying is an inference — wood desorbs along a different curve than it adsorbs. And the figures are initial rates, which flatter progress as the gap narrows. The days are a floor, not a forecast.

Gaps and unconfirmed figures

• The valid temperature range for the moisture equation. Neither edition states one. The published table spans 30°F to 270°F and that is used as the bound. Outside it the equation remains numerically well behaved but is unchecked.
• The Janka ball diameter of 11.28 mm is confirmed from the Wood Handbook. The full ASTM D143 procedure is paywalled and was not read directly.
• Osage orange has no USDA shrinkage, specific gravity or hardness figures that could be located. It is omitted rather than filled from an untraceable source.
• Eastern redcedar Janka at 12% is blank in the Handbook. The widely quoted 900 pounds-force has no primary source I could find.
• Wenge. The available shrinkage figures are explicitly for panga panga, a different species in the same genus. It is listed under its own name rather than as wenge.
• The newer alternative moisture equation published in 2014 is referenced by the Handbook but its algebraic form could not be reliably extracted from the source PDF, so it is not implemented. The equation used here is the one the published table is built from, which is the one readers can check.

Accuracy, stated plainly

The Handbook's own caveats belong here rather than buried. Shrinkage varies between pieces of the same species with a coefficient of variation around 15%, and rather more in commercial lumber than in the clear samples the figures came from. The straight-line assumption underestimates real dimensional change by about 5%. And the Handbook is blunt that predicting the shrinkage of an individual piece of wood is impossible, while the average of a quantity of pieces can be predicted well.

Every result on this site is the centre of a band. It is a far better starting point than a generic figure applied to all species, which is what the alternative usually is, but it is not a promise about your particular board.

Checking

The physics runs against 309 assertions on every build — the moisture equation against published values, the coefficient derivation against the published table, fibre saturation points per species, the board-foot conventions, the weight formula against the Handbook's worked example, and the acclimation thickness rule. A failure blocks the build. Figures checked 27 August 2026.