Lumber Drying Time Calculator
The year-per-inch rule against a diffusion model, which squares the thickness — and why resawing before the stack quarters the wait.
This is a shape rather than a species-specific prediction. Real drying depends on the wood, the climate and how well the stack is built and stickered, and a moisture meter beats any calculation. What the arithmetic is good for is showing that a linear rule cannot hold across the range people apply it to. Nothing is uploaded.
A year per inch is a line through a curve
Moisture leaves a board by diffusing to the surface, and diffusion time goes with the square of the distance it has to travel. The traditional rule is linear, so the two agree at 4/4 — where the model is anchored — and part company above it. The multiple between them turns out to be the thickness itself.
| Stock | A year per inch | Diffusion model | Multiple | Extra wait |
|---|---|---|---|---|
| 4/4 (1") | 1.00 yr | 1.00 yr | 1.00× | — |
| 5/4 (1.25") | 1.25 yr | 1.56 yr | 1.25× | +0.31 yr |
| 6/4 (1.5") | 1.50 yr | 2.25 yr | 1.50× | +0.75 yr |
| 8/4 (2") | 2.00 yr | 4.00 yr | 2.00× | +2.00 yr |
| 10/4 (2.5") | 2.50 yr | 6.25 yr | 2.50× | +3.75 yr |
| 12/4 (3") | 3.00 yr | 9.00 yr | 3.00× | +6.00 yr |
| 16/4 (4") | 4.00 yr | 16.00 yr | 4.00× | +12.00 yr |
So 8/4 stock is 4 years rather than 2, and 16/4 is 16 rather than 4. The rule is not slightly optimistic about thick timber — it is optimistic by a factor that grows with exactly the thing you were worried about. Below an inch it errs the other way, which is the same curve seen from the other side.
Which is why resawing before the stack is worth it
Two one-inch boards dry in 1 year where one two-inch board
takes 4 — the same timber, cut before the stack rather than
after it. They dry simultaneously, so the wait is a quarter, and the ratio holds at any
thickness because halving a squared quantity quarters it.
The linear rule cannot see that at all: it says resawing merely halves the wait, which makes
it look like a marginal convenience rather than the difference between one summer and four.
There are good reasons not to — you lose a kerf, you commit to a thickness, and thick stock
cut wet can move as it dries — but "it barely helps" is not one of them.
How to use
- Enter the thickness of the stock.
- Adjust the years-per-inch anchor if your climate differs.
- Compare the rule against the diffusion model.
- Check what resawing first would do.
Frequently asked questions
Is a year per inch right?
At 4/4 stock, by construction. Above it the rule and the physics part company, because moisture leaves a board by diffusing to the surface and diffusion time goes with the SQUARE of the distance — so the multiple between them is the thickness itself.
How long does 8/4 stock take?
The rule says two years. Squaring the thickness gives four. And 16/4 is sixteen rather than four — the rule is not slightly optimistic about thick timber, it is optimistic by a factor that grows with exactly the thing you were worried about.
Does resawing before drying help?
A great deal. Two one-inch boards dry in a year where one two-inch board takes four, because they dry simultaneously and halving a squared quantity quarters it. The linear rule says resawing merely halves the wait, which makes it look like a convenience rather than the difference between one summer and four.
Should I always resaw then?
No — there are real reasons not to. You lose a kerf, you commit to a thickness before you know what the board wants to be, and stock cut wet can move as it dries. The point is that "it barely helps" is not among the reasons.
Is this a prediction for my wood?
No. It is a shape rather than a species-specific figure. Real drying depends on the wood, the climate, and how well the stack is built and stickered. What the arithmetic is good for is showing that a linear rule cannot hold across the range people apply it to.
How do I know when it is actually dry?
A moisture meter, and it beats any calculation. Air drying gets you to roughly the outdoor equilibrium — commonly 12 to 15% — and no further; getting to the 6 to 8% an interior piece needs takes a kiln or a long spell indoors.
What does the years-per-inch anchor do?
It scales both figures together, so you can set it from your own climate or your own experience. A damp maritime winter and a dry continental summer are not the same year, and the anchor is where that goes.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.