What is the Fibre Saturation Point and why does it matter?

The Fibre Saturation Point (FSP) is the moisture content at which cell walls are fully saturated but no free water remains in the cell cavities — roughly 28% for most temperate species. Wood only shrinks or swells when moisture changes below the FSP. Above 28% MC, the cell walls are already fully swollen, so drying a green board from 80% down to 28% causes no dimensional change. All shrinkage happens in the 28-to-0% range.

What is the difference between tangential and radial shrinkage?

Radial shrinkage occurs across the growth rings (toward and away from the centre of the tree), while tangential shrinkage occurs along the growth rings (around the circumference). Tangential shrinkage is almost always larger than radial — typically 1.5 to 2.5 times larger — which is why flatsawn boards cup and check more than quartersawn boards of the same species.

Why does flatsawn lumber move more than quartersawn lumber?

In a flatsawn board the growth rings run roughly parallel to the face, so the face width shrinks at the higher tangential rate. In a quartersawn board the rings run perpendicular to the face, so the width shrinks at the lower radial rate. The difference can be dramatic with high T/R ratio species like beech (11.9% T vs 5.5% R) and is the main reason quartersawn flooring and furniture panels are more dimensionally stable.

What is the T/R ratio and what does it tell me?

The tangential-to-radial (T/R) ratio compares a species' two shrinkage directions. A ratio below 1.4 is excellent stability, 1.4-1.7 is good, 1.7-2.0 is moderate, and above 2.0 is poor. Low-ratio species like yellow birch (1.30) and paper birch (1.37) are prized for joinery because they move proportionally in both directions, reducing the tendency to cup or warp. Note that teak (2.32) and western red cedar (2.08) both have high T/R ratios — their outdoor reputation comes from very low absolute shrinkage values (teak: R 2.5%, T 5.8%), not from a balanced T/R ratio.

How do I read the formula this calculator uses?

The USDA standard shrinkage formula is: shrinkage% = S × (MC_start - MC_final) / FSP, where S is the total green-to-oven-dry shrinkage coefficient for that species and direction, MC values are in percent, and FSP is 28. This is a linear interpolation — the assumption is that wood shrinks uniformly between FSP and oven-dry. Real-world shrinkage is slightly non-linear at very low MC, but the linear model is accurate enough for all practical workshop calculations.

Does wood shrink along its length?

Very little. Longitudinal (along-the-grain) shrinkage is typically just 0.1-0.3% for straight-grained wood, roughly 30-50 times less than tangential shrinkage. The calculator uses 0.2% as the standard longitudinal coefficient. Spiral grain, reaction wood, and juvenile wood can push longitudinal shrinkage higher, but for normal sawn timber it is effectively negligible compared to width and thickness changes.

What moisture content should I target for indoor furniture?

Interior furniture in a heated home typically equilibrates to 6-8% MC in dry climates and 8-11% in humid ones. Kiln-dried lumber at 8% is therefore a good starting point for most indoor work. For outdoor furniture or decking, target 12-15% to match average ambient conditions. Always allow freshly kiln-dried stock to acclimatise to the shop for at least a week before cutting joints.

Wood Shrinkage Calculator

A wood shrinkage calculator covering 32 hardwood and softwood species with USDA Wood Handbook shrinkage coefficients. Enter your board’s green (wet) dimensions, the starting and target moisture content, and the saw orientation — the calculator returns exact width, thickness and length loss, volume shrinkage percentage, and a dimensional stability rating based on the tangential-to-radial ratio.

How it works

Wood shrinkage is governed by one key biological fact: the cell walls of wood can only hold a limited amount of bound water. That limit — the Fibre Saturation Point (FSP) — is approximately 28% moisture content for most temperate species. Above the FSP the wood dimensions are stable; below it, every percentage-point drop in MC causes measurable shrinkage.

The USDA Wood Handbook (Table 4-3) publishes two total shrinkage coefficients per species, measured from green (FSP) to fully oven-dry (0% MC):

Radial (R): shrinkage across the growth rings
Tangential (T): shrinkage along the growth rings (always larger)

For any moisture change below FSP, the standard linear interpolation formula gives:

shrinkage% = S × (MC_start − MC_final) / FSP

where S is the species coefficient (R or T depending on orientation), and FSP = 28. The calculator applies this formula independently to width and thickness based on the saw orientation you select. Flatsawn boards (growth rings roughly parallel to the face) shrink primarily at the tangential rate in the width direction; quartersawn boards (rings perpendicular to the face) shrink at the lower radial rate — which is why quartersawn stock is more stable. Riftsawn uses the average of R and T for width.

Longitudinal (along-grain) shrinkage uses a fixed coefficient of 0.2%, the accepted standard for straight-grained sawn timber.

Worked example

You mill a flatsawn white oak board:

Green dimensions: 6 × 1 × 96 inches (face × thickness × length)
Drying from 28% MC down to 8% MC (kiln-dry)
White oak: R = 5.6%, T = 10.5%

Applying the formula to width (tangential):

shrinkage% = 10.5 × (28 − 8) / 28 = 7.5%

Width loss = 6 × 0.075 = 0.450 inches → final width = 5.550 inches

Thickness (radial):

shrinkage% = 5.6 × (28 − 8) / 28 = 4.0%

Thickness loss = 1 × 0.040 = 0.040 inches → final thickness = 0.960 inches

If instead the same board were quartersawn, the width would shrink at only 4.0% (radial), losing just 0.240 inches — 47% less movement across the face.

Species	Orientation	MC change	Width loss (6”)
White Oak	Flatsawn	28% → 8%	0.450”
White Oak	Quartersawn	28% → 8%	0.240”
Hard Maple	Flatsawn	28% → 8%	0.421”
Teak	Flatsawn	28% → 8%	0.247”
Beech	Flatsawn	28% → 8%	0.507”

Formula note

The linear-interpolation model is accurate for practical wood-movement prediction across the FSP range. Real shrinkage is slightly non-linear at very low MC values (below 5%), but for workshop and construction purposes the USDA linear formula is the accepted industry standard. The T/R ratio shown for each species is a useful dimensional stability index: species below 1.4 (yellow birch: 1.30, paper birch: 1.37) are dimensionally stable and resist cupping; species above 2.0 (beech: 2.16, teak: 2.32, western red cedar: 2.08) have higher cupping risk and benefit from quartersawn orientation in demanding applications. Note that teak and western red cedar are still valued outdoors because their absolute shrinkage values are very low, even though their T/R ratios are high.

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