| Species | Units | Width dir | Thk dir | MC | Width start | Width final | Thk start | Thk final | Len start | Len final |
|---|---|---|---|---|---|---|---|---|---|---|
| White Oak | mm | Tangential | Radial | 18% → 10% | 100 mm | 97.00 mm | 50 mm | 49.20 mm | 800 mm | 799.77 mm |
Wood movement is modeled only below the fiber saturation point (FSP). Moisture above FSP mainly affects weight, not size.
MC_end = min(MC_final, FSP)
Dimension_final = Dimension_start − Change
- Measure your board width, thickness, and length in one unit set.
- Enter your current moisture and the target moisture for your season or storage.
- Choose species and grain directions: Tangential usually moves more than Radial.
- Click Calculate to see final dimensions and percent change.
- Use the clearance gap suggestion for slats, lids, and frames. Download the CSV or PDF for your build notes.
Moisture swings in garden structures
Outdoor projects face repeated wet–dry cycles. In many climates, boards can move from 18% moisture after rain toward 10–12% in dry heat. Most dimensional change happens below the fiber saturation point, typically near 28%. That is why the calculator caps moisture inputs at FSP before estimating movement. A 120 mm slat may shift about 1 mm across grain during a 6% moisture drop.
Directional shrinkage explains most “mystery gaps”
Across-grain movement dominates. Tangential shrinkage commonly ranges around 7–11% from FSP to oven-dry, while radial shrinkage is often about 2–6%. Length change is usually near 0.1–0.2% and can be ignored for many garden frames. Choosing tangential versus radial in the form helps match flat-sawn and quarter-sawn faces.
Inputs that reflect real seasonal targets
A practical method is to model two scenarios: a “wet season” target and a “dry season” target. For example, compare 18% to 12% for rainy months and 12% to 8–10% for peak heat. If both values sit above FSP, the tool warns you because size change is assumed minimal until moisture drops below the saturation threshold. Many outdoor softwoods stabilize near 12–15% moisture in sheltered storage, but exposed lumber may swing wider throughout the year.
Turning percentages into build decisions
The result table converts Δ% into an actual dimension change. Use the larger cross-grain change (width or thickness) to size clearance in slats, lids, and panel frames. A small safety margin, such as 10–20%, is often reasonable for exposure, finish wear, and uneven drying. The optional gap suggestion applies this margin automatically.
Recordkeeping for repeatable cuts and repairs
Garden builds are easier to maintain when dimensions are documented. The CSV export stores moisture used, grain direction, and start-to-final sizes for up to 20 runs in the session. The PDF summary is convenient for job folders, allowing you to compare species choices, validate joinery clearances, and plan replacement boards with the same assumptions next season. Keeping the file with your cut list reduces errors.
1) Does sealed wood stop shrinkage?
Sealers slow moisture exchange but rarely stop it. Expect reduced speed of movement, not zero movement. Design joints and clearances for change, then use finishes to improve durability and stability.
2) What direction should I pick for width?
If the wide face is flat-sawn, tangential is usually closer. If the wide face is quarter-sawn, radial is often a better match. When unsure, run both and plan for the larger change.
3) Why does the calculator cap moisture at FSP?
Above FSP, moisture is mostly in cell cavities and affects weight more than size. Most dimensional change occurs below FSP, so the calculation focuses on the range where shrinkage and expansion are meaningful.
4) Can this predict warping or cupping?
It estimates average dimensional change, not distortion. Warping depends on grain, growth stresses, uneven drying, and restraint. Use the outputs to plan gaps and joints, and reduce warp with balanced construction.
5) What if my final moisture is higher than the initial?
The tool will show expansion instead of shrinkage and will flag a warning. This is useful when modeling boards stored indoors then moved outside, or when wet weather raises moisture during installation.
6) How should I use the suggested clearance gap?
Use it as a starting point for slats, lids, and panel frames. Increase the safety margin for high exposure, end-grain checking, or unknown moisture. Decrease it for sheltered builds with stable storage conditions.