Estimate dimensional change between two moisture contents using USDA FPL Wood Handbook (FPL-GTR-190) Table 4–3 green-to-oven-dry radial and tangential percents. Fiber saturation taken as 30% — the usual shop simplification. Not a kiln schedule.
Use it when you are building something that will move with the seasons: sizing the panel gaps in a frame-and-panel door, spacing the slots for tabletop fasteners, or checking how much a board milled at one moisture content will shrink once it settles in a heated house.
Flatsawn faces move with the tangential figure and quartersawn faces with the radial one, so choose the direction that matches the face of your board.
Below fiber saturation, shrinkage is treated as linear with moisture content. The handbook green-to-oven-dry percent is spread over a 30% MC span (FSP ≈ 30%). That gives a dimensional change coefficient C. Change in size is starting size times C times the MC drop (or rise).
C = (shrink_% / 100) / 30
ΔD = D × C × (MCstart − MCend)
Dfinal = D − ΔD (positive ΔD when drying)
Species percents come from FPL-GTR-190 Table 4–3. Only species in that table are listed. Individual boards vary (~15% coefficient of variation in the handbook). This is not a kiln schedule and not a mill moisture cert.
| Case | C | ΔD | Final |
|---|---|---|---|
| Douglas-fir Coast, tangential 7.6%, 12 in, 12%→6% | 0.0025333… | 0.1824 in | 11.8176 in |
| White oak, tangential 10.5%, 12 in, 12%→6% | 0.0035 | 0.252 in | 11.748 in |
| Generic C=0.00265 (reference only), 12 in, 12%→6% | 0.00265 | 0.1908 in | 11.8092 in |
Default example uses Douglas-fir Coast tangential from Table 4–3. The calculator reproduces each row.
FPL notes that if the true FSP for a species is unknown, 30% MC is the usual approximation. Real FSP sits roughly 25–32% depending on species. This page locks FSP at 30% and does not invent per-species FSP numbers.
Flatsawn face width is mostly tangential. Quartersawn face width is mostly radial. If the ring orientation is mixed or unknown, the 50/50 average is a rough middle. Tangential is typically about twice radial.
Same formula. The MC difference is 2 points, not 12. Green-to-oven-dry percent is only used to build C; you do not have to start from green.
No. Lengthwise change is usually 0.1–0.2% green to oven-dry for normal wood and is out of scope here.
No. Kiln schedules control wet-bulb, dry-bulb, and time. This calculator only estimates how much a dimension moves between two MC values.
USDA Forest Products Laboratory Wood Handbook, FPL-GTR-190 (2010), Table 4–3 — average radial and tangential shrinkage from green to oven-dry, expressed as a percent of the green dimension.