Why does wood move across the grain but not along it?

Wood cells are oriented along the grain, and their walls absorb and release moisture in the radial and tangential directions. Longitudinal movement is typically less than 0.1% and is negligible for most applications. Tangential (flat-sawn) and radial (quarter-sawn) movement can be 2–10% from green to dry.

What is the difference between flat-sawn and quarter-sawn movement?

Flat-sawn (tangential) boards show 1.5–2x more movement than quarter-sawn (radial) boards because the growth rings run nearly parallel to the face. Quarter-sawn boards are more dimensionally stable and less likely to cup, making them preferred for tabletops and flooring.

What moisture content should I use for installation?

Use the actual MC of the lumber at installation — typically 6–9% for kiln-dried lumber indoors or 12–15% for wood delivered in a humid climate. The equilibrium moisture content (EMC) for your region determines the service MC. Indoor heated spaces typically reach 6–8% EMC in winter.

Why is the fiber saturation point (FSP) important?

Wood only changes dimensionally when its MC is below the FSP (~28%). Above the FSP, free water fills cell cavities but the cell walls are already saturated, so no further shrinkage or swelling occurs. This calculator clamps values above 28% to 28% for the formula.

How accurate are the results?

The coefficients are average values from the USDA Wood Handbook. Actual movement varies within a species by 10–20% based on growth rate and density. Always add a safety margin — especially for wide panels — and leave at least the calculated gap plus 10–15% extra.

Can I use this for plywood or engineered wood?

No. Plywood and engineered products are cross-laminated, which nearly cancels out wood movement. This calculator applies only to solid lumber. For plywood, consult manufacturer specs — typical movement is less than 0.05% per % MC change.

Wood Movement Calculator

Name: Wood Movement Calculator
Availability: InStock
Author: Nham Vu

Calculate expected wood expansion and contraction across the grain from moisture content change using species-specific shrinkage coefficients.

Wood Movement Parameters

Wood Species

Unit

Board Width (in)

Board Thickness (in)

Installation Moisture Content (%)

Service Moisture Content (%)

Movement Results — Oak, Red

MC change: 5.0% • Formula: ΔD = D × S × ΔMC ÷ 100 • FSP clamped at 28%

Flat-Sawn (Tangential)

Width movement —

Thickness movement —

Coefficient used —

% of width

Quarter-Sawn (Radial)

Width movement —

Thickness movement —

Coefficient used —

% of width

Recommended installation gap (flat-sawn): —

Calculated movement + 15% safety margin. Leave this gap at each fixed edge for full-width boards.

Understanding Wood Movement Direction

Tangential (flat-sawn): Growth rings run roughly parallel to the face. The width dimension moves tangentially — the direction with the most movement. Flat-sawn boards cup more but are cheaper and wider.

Radial (quarter-sawn): Growth rings run roughly perpendicular to the face. Width moves radially — typically 40–60% less movement than flat-sawn. Quarter-sawn boards are dimensionally stable and show medullary rays (figure) on oak and sycamore.

Thickness: For flat-sawn boards the thickness dimension is radial; for quarter-sawn it is tangential. This calculator uses the same logic — the opposite coefficient applies to thickness.

Source: USDA Forest Products Laboratory, Wood Handbook (2010 ed.). Coefficients are %/% MC change below FSP.

Summary

Calculate expected wood expansion and contraction across the grain from moisture content change using species-specific shrinkage coefficients.

How it works

Select the wood species from the dropdown — each species has unique tangential and radial shrinkage coefficients from USDA data.
Enter the board width and thickness (the dimensions that change across the grain).
Enter the installation moisture content — the MC of the lumber when it is installed.
Enter the expected service moisture content — the EMC the wood will reach in its final environment.
Choose inches or millimeters as your output unit.
The calculator displays movement for flat-sawn (tangential) and quarter-sawn (radial) orientations and shows the minimum gap to leave during installation.

Use cases

Determining expansion gaps for solid hardwood flooring installation.
Sizing wood movement slots and buttons for solid tabletops and panels.
Planning seasonal movement allowance in door and window frames.
Comparing flat-sawn vs. quarter-sawn movement to choose the right cut for a project.
Calculating mortise-and-tenon joint tolerances in furniture construction.
Estimating shrinkage of green lumber as it dries to indoor EMC.
Specifying reveal gaps on solid wood siding and paneling.
Teaching woodworking students about cross-grain wood movement principles.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu