Where the species data comes from
The eight preset radial and tangential shrinkage values are green-to-oven-dry percentages from the USDA Forest Products Laboratory Wood Handbook, FPL-GTR-282, Table 4-3. Those percentages use the green dimension as their denominator. This tool converts them to a relative width factor at each entered moisture content, then scales from your board's actual current width. The preset numbers are averages, not measurements of your board.
Formulas and limits
effective shrinkage = radial × cos²(angle) + tangential × sin²(angle)relative width factor at MC = 1 − S × (1 − min(MC, 28) ÷ 28)target width = current width × target factor ÷ current factorseasonal swing = width at high MC − width at low MCcenter-fixed outer travel = seasonal swing ÷ 2slot length starting point = shank diameter + outer travel × extra factorHere S is the selected green-to-oven-dry shrinkage expressed as a decimal. The linear model assumes a practical 28% fiber-saturation point; moisture above it is clamped for movement arithmetic. The Forest Products Laboratory's dimensional-change explanation describes this estimate and notes that radial and tangential behavior differ. Below-fiber-saturation movement is approximate, and wet-to-dry extremes are less reliable than moderate seasonal estimates.
Making room for a panel
In the center-fixed sketch, the panel is restrained at its center and outer fasteners slide across the grain. Each outer location moves half as much as the full panel width changes. The slot number includes the entered fastener shank diameter and an extra factor for travel; it is not a preapproved slot design. Grain variation, cupping, screw head/washer clearance, joinery loads, and the direction of force all need separate review. Avoid rigidly gluing or trapping a solid-wood panel across its full width.
Frequently asked questions
Why does flat-sawn lumber move more than quarter-sawn lumber?
For the same species, tangential shrinkage along the growth rings is usually larger than radial shrinkage across them. A flat-sawn board's width is closer to the tangential direction; a quarter-sawn board's width is closer to radial.
Can I enter room humidity instead of wood moisture content?
No. Relative humidity is not the same number as wood moisture content. Use a moisture meter or a separately validated equilibrium-moisture-content estimate for the expected room conditions.
Does this predict cupping or cracking?
No. It estimates uniform across-grain width movement. It does not model stress gradients, drying defects, warped boards, coatings, fastener forces, or exact growth-ring curvature.