What moisture content should I build furniture at?

Build at the moisture content your finished piece will live at — for most heated/air-conditioned interiors in North America that is 6–9% MC. Let lumber acclimate in your shop for a week or two and check it with a moisture meter before final milling.

Which moves more, flatsawn or quartersawn lumber?

Flatsawn boards move more. Their width lies along the growth rings (tangential direction), and tangential shrinkage typically runs 1.5–2× radial shrinkage. Quartersawn boards put the rings on edge, so their width changes in the smaller radial direction — that stability is a main reason quartersawn stock costs more.

Does finishing stop wood movement?

No. Film finishes slow moisture exchange, which evens out short-term humidity spikes, but over a season the wood still reaches equilibrium with its environment and moves accordingly. Design for movement — floating panels, elongated screw holes, breadboard tenons with pinned center only — rather than trying to seal it away.

What humidity swing should I plan for indoors?

A common design range for heated homes is roughly 6% MC in winter (dry, heated air) to 12% MC in humid summer — the 6-point default in this calculator. Homes with tight climate control see less; garages, basements, and coastal or four-season climates can see more.

Does wood move along its length too?

Barely. Longitudinal shrinkage in normal wood is about 0.1–0.2% from green to oven-dry — one to two orders of magnitude less than across the grain — so length change is ignored in furniture design. Exceptions are juvenile wood and reaction wood, which can move enough lengthwise to bow a board.

How accurate is this estimate?

It is a planning estimate, typically good to within about 25%. The formula assumes shrinkage is linear below a 30% fiber saturation point, uses species-average shrinkage values, and real boards vary with grain angle, density, and drying history. For fine work, leave allowance on the generous side.

Wood Movement Calculator

Wood expands and contracts across the grain as its moisture content follows ambient humidity. Estimate the change with ΔD = D × (S ÷ 30 ÷ 100) × ΔMC, where D is the board's cross-grain width, S is the species' total green-to-oven-dry shrinkage percentage (tangential for flatsawn, radial for quartersawn), and ΔMC is the moisture-content swing in points. Pick a species below, set the width and expected swing (6%→12% is a common indoor range), or load a city preset to use its seasonal equilibrium-moisture-content range.

End-grain cross-section of your board. The width change is exaggerated for visibility — read the true figure in the result above.

Data: USDA Forest Products Laboratory, Wood Handbook FPL-GTR-282 (2021), public domain.

The formula, documented

ΔD = D × (S ÷ 30 ÷ 100) × ΔMC

ΔD — change in the board's cross-grain dimension, inches
D — current cross-grain dimension, inches
S — the species' total green→oven-dry shrinkage percentage: tangential for flatsawn boards, radial for quartersawn
ΔMC — moisture content change, in percentage points

Dividing S by 30 converts total shrinkage into shrinkage per 1% of moisture change, using the convention that wood stops moving above the fiber saturation point, taken as roughly 30% MC. Dividing by 100 turns the percentage into a fraction. This linear approximation is the standard shop estimate and tracks the Wood Handbook's dimensional change equation closely in the 5–15% MC range where furniture lives.

Worked example

A 12″ wide flatsawn white oak panel goes from 6% MC (winter shop) to 12% MC (humid summer). Tangential shrinkage for white oak is 10.5%:

ΔD = 12 × (10.5 ÷ 30 ÷ 100) × 6 = 0.252 in ≈ 1/4 in

A quarter inch on one panel. That is why solid tabletops are fastened with slotted clips, why frame-and-panel doors float their panels, and why a drawer fitted snug in July can seize in January.

Why wood moves — and how to design for it

Below the fiber saturation point, water leaves or enters the cell walls themselves, and the walls physically shrink or swell. Because cells are long tubes aligned with the trunk, almost all of that change happens across the grain: most in the tangential direction (along the growth rings), roughly half as much radially (across the rings), and almost none along the length. The tangential/radial ratio is why flatsawn boards cup — the ring side shrinks more than the pith side.

You cannot pick numbers that make movement go away; you can only choose where it goes. Run tabletop fasteners in slots, size breadboard-end tenons with glue at the center pin only, leave a seasonal gap around door panels sized from this calculator, and orient critical parts quartersawn when stability matters more than figure. Estimate with the species you actually use — a stable pine carcase and a lively oak top behave very differently in the same room.

Frequently asked questions

What moisture content should I build furniture at?: Build at the moisture content your finished piece will live at — for most heated/air-conditioned interiors in North America that is 6–9% MC. Let lumber acclimate in your shop for a week or two and check it with a moisture meter before final milling.
Which moves more, flatsawn or quartersawn lumber?: Flatsawn boards move more. Their width lies along the growth rings (tangential direction), and tangential shrinkage typically runs 1.5–2× radial shrinkage. Quartersawn boards put the rings on edge, so their width changes in the smaller radial direction — that stability is a main reason quartersawn stock costs more.
Does finishing stop wood movement?: No. Film finishes slow moisture exchange, which evens out short-term humidity spikes, but over a season the wood still reaches equilibrium with its environment and moves accordingly. Design for movement — floating panels, elongated screw holes, breadboard tenons with pinned center only — rather than trying to seal it away.
What humidity swing should I plan for indoors?: A common design range for heated homes is roughly 6% MC in winter (dry, heated air) to 12% MC in humid summer — the 6-point default in this calculator. Homes with tight climate control see less; garages, basements, and coastal or four-season climates can see more.
Does wood move along its length too?: Barely. Longitudinal shrinkage in normal wood is about 0.1–0.2% from green to oven-dry — one to two orders of magnitude less than across the grain — so length change is ignored in furniture design. Exceptions are juvenile wood and reaction wood, which can move enough lengthwise to bow a board.
How accurate is this estimate?: It is a planning estimate, typically good to within about 25%. The formula assumes shrinkage is linear below a 30% fiber saturation point, uses species-average shrinkage values, and real boards vary with grain angle, density, and drying history. For fine work, leave allowance on the generous side.