Wood is a sponge with a memory of the air around it. Leave a board in a room and, over days to weeks, it drifts to the equilibrium moisture content for that room's temperature and humidity — and stops there. Furniture doesn't care what MC the board was at in the lumber yard; it cares about the EMC of the house it will live in, because the gap between the two is exactly how much it will move after you build it.
| Condition | Air | EMC |
|---|---|---|
| Heated home, winter | 70°F / 30% | 6.2% |
| Heated home, humidified | 70°F / 40% | 7.7% |
| Air-conditioned home, summer | 72°F / 45% | 8.4% |
| Mild coastal interior | 68°F / 55% | 10.1% |
| Humid summer, no AC | 85°F / 70% | 12.7% |
| Gulf Coast summer | 95°F / 75% | 13.7% |
| Dry mountain winter | 65°F / 25% | 5.4% |
| Unheated garage, temperate winter | 45°F / 60% | 11.3% |
Computed with the USDA Handbook's Hailwood–Horrobin equation — run any other combination in the EMC calculator.
The craft rule is to build at the middle of the range your piece will live in. For most heated-and-cooled US homes that means lumber at 6–8% in cold-winter regions and 8–10% in humid ones — and then design the joints for the swing on either side, which is what the movement calculator quantifies per species. Kiln-dried furniture lumber at 6–8% is not an upsell; it is the difference between a top that stays flat and one that splits its breadboard in February.
Two traps: measuring surface moisture on a board whose core is still wet (pin meters read the shell; stickered lumber needs time in the shop — a week per inch of thickness is the old rule), and building in a humid summer shop for a heated winter house, which manufactures the maximum possible swing into every joint.