Wood Shrinkage: Tangential, Radial, and Why Boards Cup
How the Wood Handbook 2021 Eq. 4-9 actually behaves, why flat-sawn boards cup, and how to plan stock for the finished size you need.
Wood only shrinks below the fibre-saturation point
FSP is around 30% MC for most species (22% for teak and true mahogany — Wood Handbook 2021 Table 13-5 note b). Above FSP, water sits in the cell cavities, not the cell walls, and the wood does not change dimension. So freshly felled timber at 80% MC behaves the same as wood at 31% MC: no shrinkage yet. The Wood shrinkage tool clamps start and end MC to FSP automatically — if you see a 'clamped' note, the start MC was above FSP and the calculation treats it as FSP.
Tangential is bigger than radial, always
Tangential shrinkage (across the growth rings, the width of a flat-sawn board) is typically 1.5–2× the radial shrinkage (along the rays, the thickness of a flat-sawn board). For white oak: 10.5% tangential vs 5.6% radial green→OD (WH Tables 4-3/4-4). For rubberwood: 5.1% vs 2.3%. This anisotropy is the root cause of cupping, bowing and twist — the two faces of a board shrink at different rates.
From green to 8% MC, expect 4–8% width loss
A 250 mm wide white-oak board loses about 7.7% in width going from 30% MC (treated as green) to 8% MC: final 230.7 mm. A 200 mm rubberwood board loses about 3.7%: final 192.6 mm. Planing allowance (2.4–3 mm per face) and saw kerf (1.8–2.5 mm band saw) sit on top of the green dimension — see Rough sizing for the combined calculator.
Quarter-sawn and rift-sawn shrink more evenly
Quarter-sawn stock has the radial direction across the face, so both dimensions of the cross-section shrink at the radial rate — less cup, but more expensive to produce (waste kerf). Rift-sawn sits in between. For wide table tops that must stay flat, furniture makers pay a 30–60% premium for quarter-sawn; for construction framing where the board is hidden in a wall, flat-sawn is fine.
Match the build MC to the in-service EMC, not to a fixed number
If you build at 8% MC and ship to a 12% EMC climate, the wood swells; to a 6% EMC climate, it shrinks. The 1–2 pp difference at the build target becomes visible at door panels and tabletops. Run the EMC tool against the destination climate, then target a build MC 1–2 pp below that EMC so the wood moves toward equilibrium in the customer's room, not away from it.