Wood Movement Calculator

Frequently Asked Questions

What MC should I build at?

Match the wood to the in-service environment before milling. Most heated interiors stabilize furniture around 6-9% MC.

How do I find the equilibrium MC at my location?

Equilibrium MC depends on temperature and relative humidity. A conditioned home cycling between 30% and 50% RH moves wood between about 6% and 9% MC seasonally.

Can I stop movement with a thick finish?

No finish fully stops movement. Film finishes slow the rate but cannot eliminate seasonal swings. Always design for movement.

Which way does a flat-sawn board cup?

Flat-sawn boards tend to cup away from the heart side as the growth rings shrink. The bark side becomes concave in drying.

Is plywood immune to movement?

Yes, for practical purposes. Its cross-banded plies cancel cross-grain movement, making it ideal for large stable panels.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How It Works

Solid wood is hygroscopic: it gains and loses water with the surrounding air and changes size as it does. Below the fiber-saturation point (approximately 28% moisture content) every 1% change in moisture content (MC) produces a roughly proportional change in width. This tool estimates seasonal movement with the formula: change = width x dimensional-change coefficient x delta MC, where width is the board's starting width, the coefficient is specific to the species and grain orientation, and delta MC is the difference between the starting and ending equilibrium moisture content.

You pick the species and whether the face is flat-sawn (tangential, the larger mover) or quarter-sawn (radial, about half as much), enter the width and the two MC values, and the result is the expected width change in inches, the final width, and the equivalent movement in sixteenths of an inch for direct shop comparison.

Tangential (flat-sawn) coefficients are roughly 1.5 to 2 times the radial (quarter-sawn) values, which is why rift- and quarter-sawn stock is prized for stability. Movement happens almost entirely across the grain - length change along the grain is negligible for joinery purposes.

Use Cases

Wood movement calculations are essential for designing joints and attachments that accommodate seasonal change:

Sizing the elongated slots in tabletop fasteners so a dining table top can expand and contract without splitting or pulling the base joints apart.

Calculating the panel float gap needed in a frame-and-panel door or cabinet side so the panel can move freely without cracking the frame.

Predicting whether a built-in bookcase or cabinet will bind against the adjacent walls or floor in high-humidity months.

Deciding how wide to make breadboard ends on a wide tabletop and how long to make the elongated holes for the breadboard pins.

Estimating the movement of a solid-wood floor or countertop between installation in a dry winter shop and humid summer in-service conditions.

Tips

Design for movement, not against it:

Acclimate wood to the in-service environment before final milling. A board that equilibrates in the shop before cutting will move much less after installation. A pin or pinless moisture meter is essential.

Never glue or rigidly screw a wide solid-wood panel cross-grain. Use elongated slots, figure-8 fasteners, or wooden buttons that let the panel slide as it moves.

Quarter-sawn stock moves roughly half as much as flat-sawn in the same species. If movement is a concern, choose the cut for stability over figure.

Finish does not stop movement - film finishes only slow the rate slightly. Design for the full calculated movement, not a reduced value because the piece is finished.

For glue-ups, orient all boards in the same grain direction so the assembly moves as a unit rather than having opposing grain directions fighting each other.

FAQ

What MC should I build at?

Match the wood to the in-service environment before milling. Most heated interiors stabilize furniture around 6-9% MC. An unconditioned shop may hold lumber at 10-14%, which will shrink considerably in a heated room.

How do I find the equilibrium MC at my location?

Equilibrium MC is a function of temperature and relative humidity. A conditioned home cycling between roughly 30% and 50% RH takes wood between about 6% and 9% MC seasonally.

Can I stop movement with a thick finish?

No finish fully stops movement. Film finishes slow the rate of moisture exchange, slightly damping seasonal swings, but cannot eliminate them. Always design for movement.

Which way does a flat-sawn board cup?

Flat-sawn boards tend to cup away from the heart side (the growth rings try to straighten as they shrink). The bark side becomes concave in drying.

Is plywood immune to movement?

Yes, for practical purposes. Its cross-banded plies cancel cross-grain movement, which is why it is the standard choice for large stable case sides, backs, and furniture panels.

Wood Movement Calculator

Frequently Asked Questions

Related Woodworking Calculators