How movement is figured
Wood shrinks and swells across the grain as its cell walls lose and gain water. That happens only below the fiber saturation point, about 28% moisture content; above it, wood holds extra water in its cells without changing size. Below it, the change is close to proportional to the moisture lost or gained, so the calculator uses Hoadley's formula, ΔD = D·S·ΔMC / fsp, where S is the species' shrinkage from green to oven-dry.
S is measured against the green width, so the calculator uses Hoadley's refined form of the formula, which holds for wood that starts partly dry. The short form reads about 5% low for dry wood.
Humidity sets moisture content
Wood settles at a moisture content set by the air around it. At room temperature, 25% relative humidity gives about 5%, 50% gives about 9%, and 75% about 14%; at 100% the wood reaches fiber saturation. Warmer air lowers it by about one point for every 25 to 30 °F.
Choosing the seasonal range
Indoor humidity swings more than people expect. A house heated through a cold winter can fall below 25% RH, and without air conditioning, summer humidity can stay above 70% for weeks. Furniture in such a house averages 7.5 to 8% moisture content over the year. Use the lowest and highest humidity the piece will live with for more than a few weeks, not brief extremes.
Flatsawn, riftsawn and quartersawn
Wood shrinks about twice as much along its growth rings (tangentially) as across them (radially): on average about 8% against 4% from green to oven-dry. A flatsawn board's width runs tangentially and a quartersawn board's radially, so quartersawn wood moves about half as much. Riftsawn uses the average of the two. If the grain is mixed or unknown, choose flatsawn to stay on the safe side.
Cupping
The ratio of tangential to radial shrinkage (T/R) averages about 1.8. The higher it is, the more a flatsawn board cups: drying, it cups away from the pith, and swelling, it cups the other way. Quartersawn boards seldom cup. The drawing shows the direction, much exaggerated.
Along the grain
Normal wood shrinks only about 0.1% along the grain from green to oven-dry, so length changes are ignored. Wood from near the pith and reaction wood can shrink up to 2% along the grain, unevenly, which is what bows and crooks boards.
Wood that can't move
Wood can be squeezed across the grain by less than about 1% before it crushes permanently. A board held tight at its edges that swells more than that is crushed, then shrinks smaller than it started as it dries, so joints loosen and panels split. Glued-in panels, chair rungs and tool handles fail this way. Give solid wood room to move, or fasten it so it can slide.
Two woods in one piece
Parts joined with their widths side by side, such as a drawer front dovetailed to its sides, should move alike. A drawer 10 in deep with flatsawn beech sides and a quartersawn mahogany front changes shape every season, because the sides move about three times as much as the front. Use Compare with to check a pairing of species or cuts.
Accuracy
Shrinkage values are species averages from green to oven-dry. Individual boards vary by about 15%. Fiber saturation varies too, from about 22% in species rich in extractives, such as redwood and mahogany, to 35% in birch. Finishes slow moisture exchange but don't stop it.
Entering values
Type decimals (11.25) or fractions (11 1/4, 11-1/4). Measure the width across the grain at the moisture you enter as Now. For wet wood, enter its moisture content, or any value of 28% or more, and it's treated as green.
For a species that isn't listed, choose Other at the top of the list and enter its tangential and radial shrinkage from green to oven-dry.
Sheet goods
Plywood, MDF and particleboard hardly move across their faces, since their layers or fibers run every way. This calculator is for solid wood.