Why wood moves

Wood is hygroscopic — it absorbs and releases moisture until it reaches equilibrium with the surrounding air (the "equilibrium moisture content" or EMC). As it gains moisture, it swells; as it loses moisture, it shrinks. This change is mostly across the grain, not along the length (a 10-foot board might move 0.1" in width but only 0.005" in length).

Two key concepts most online calculators miss:

Direction matters. Tangential (flatsawn) movement is roughly twice radial (quartersawn) movement. For the same species, a 6" flatsawn board can move 3x more than a 6" quartersawn board across the seasons.
EMC is what counts, not RH. Humid air at 70°F and 50% RH corresponds to roughly 9% EMC in wood. But cold winter air at 20% RH can produce 4-5% EMC indoors (because heating air without humidification drops the RH), while summer air at 80% RH produces 14-16% EMC.

How we calculate

The standard formula:

ΔL = L × (ΔMC × CS / 100)

Where ΔL is dimensional change, L is the original dimension, ΔMC is change in moisture content, and CS is the coefficient of shrinkage (radial or tangential). Species coefficients are taken from the USDA Forest Products Laboratory Wood Handbook (FPL-GTR-190).

What to actually do with this number

If your panel moves 1/8" across a 12" width over a year:

For a tabletop: Allow movement in the breadboard end. Use figure-8 fasteners or elongated holes so the screws can slide.
For a frame-and-panel door: Make the panel 1/4" smaller than the frame groove on each side, and let it float.
For a drawer front: Allow a 1/16" gap on each side; the drawer will fit year-round.
For a breadboard edge: Mortise the elongated slots for the tenons — they should slide, not bind.

Reading the visualization

The bar chart shows how your board's width changes through the year using a typical 4-season indoor EMC curve for a temperate climate. If you live in a dry desert or a humid coastal area, your bars will look more extreme; if you run a whole-house humidifier, they'll be flatter.

When to ignore the calculator

Plywood and MDF move roughly 1/10 as much as solid wood. The cross-grain layers stabilize the panel.
Slab wood with bark inclusion moves unpredictably. Let it sit a full year in your shop before flattening.
Wood stored in a non-climate-controlled space is not at indoor EMC. Move it inside, wait, then measure with a moisture meter.

Common questions

My board "moved" 1/4". Is that bad?

That's normal for an 18" flatsawn white oak board going from a 5% winter EMC to a 14% summer EMC. The board isn't defective — you just didn't allow for movement. Next time, build for it.

Should I buy quartersawn or flatsawn?

Quartersawn moves about half as much, looks more figured, and costs 2-3x more. For most furniture projects, flatsawn is fine — just design for movement. For wide tabletops (24"+), quartersawn is worth considering.

Can I prevent movement?

No. You can slow it (with sealants on end grain and equal-thickness finishes on all faces), but you cannot stop it. The wood you bought grew on a tree and behaved like one for 50 years before it met you.