Calculator
Example data table
| Species | MC start → end | Width | Direction | Estimated width change |
|---|---|---|---|---|
| Western Red Cedar | 14% → 9% | 6.000 in | Tangential | ≈ -0.051 in |
| Oak (White) | 12% → 7% | 8.000 in | Tangential | ≈ -0.150 in |
| Douglas Fir | 10% → 16% | 5.500 in | Radial | ≈ +0.057 in |
Formula used
Most dimensional change happens below the fiber saturation point (FSP). The calculator assumes shrinkage (or swelling) is linear with moisture change under FSP.
Stotal is the total shrinkage from FSP to oven-dry, chosen by species and grain direction. Tangential values generally apply to flat-sawn faces; radial values to quarter-sawn faces.
How to use this calculator
- Pick your units, then select a wood species preset or choose Custom.
- Enter starting and ending moisture contents for your outdoor conditions.
- Enter your board dimensions and choose grain directions for width and thickness.
- For slats or decking, set the number of boards and any fixed gap allowance.
- Press Calculate to see size changes, set width change, and downloadable reports.
Moisture change drives movement
Wood shrinks and swells as moisture content (MC) moves below the fiber saturation point (about 28–30%). If a board drops from 18% MC to 12% MC, the change is 6 points. Multiply that change by the species shrink rate to estimate dimension loss. This calculator focuses on predictable, reversible movement from seasonal drying and re-wetting. For planning, use stable averages, not short-lived surface spikes.
Typical outdoor moisture ranges
In many gardens, covered storage may sit near 10–14% MC, while exposed outdoor conditions often hover around 14–20% MC. After rain cycles, surface MC can spike higher, then return toward equilibrium. Designing for a realistic MC swing, such as 12% to 18%, usually gives safer allowances than using extreme one-day readings. A pin meter reading taken 6–10 mm deep is usually more reliable.
Width shrinks more than thickness
Across the grain, movement is strongest. Flat-sawn boards move mainly in the tangential direction, commonly around 6–10% from green to oven-dry for many species, while quarter-sawn boards track the radial direction, often 3–6%. Along the grain, length change is small, frequently under 0.2%, so plan gaps and joinery around width and thickness.
Planning gaps and fasteners
For slats, decking, and planter cladding, small gaps prevent buckling. A common practice is starting with 3–6 mm spacing and letting boards swell in wet months. Use elongated screw holes or slots across the grain, and avoid trapping water at end grain. The calculator’s “set width change” helps estimate total movement across multiple boards. Predrill near edges and prefer corrosion-resistant fasteners outdoors.
Using results in garden projects
Raised-bed sides, trellis frames, and bench tops benefit from movement planning. If the predicted width shrinkage is 2.5 mm per board and you have 8 boards, total change can reach 20 mm across the panel. Allocate that movement as gaps, expansion room in a frame, or floating tenons. Re-check results when switching species or board orientation. Finishes can slow moisture change, but they will not eliminate movement.
Frequently Asked Questions
1) What moisture contents should I enter for outdoor garden boards?
Use readings that represent your normal seasons. For many gardens, 12% to 18% is a practical swing; covered storage may be lower, exposed boards higher. Avoid using a wet surface reading taken right after rain.
2) Which grain direction should I select?
Choose tangential for flat-sawn faces and radial for quarter-sawn faces. If you are unsure, tangential is the conservative choice because it predicts larger movement across the board’s width.
3) Why does my board length barely change?
Wood movement along the grain is very small compared with cross-grain shrinkage. Most length change is typically under a few tenths of a percent, so garden joinery problems usually come from width and thickness changes.
4) How do I plan gaps for decking or slats?
Calculate the set width change across all boards, then decide where that movement can go. Add gap allowance, use spacer blocks, and let fasteners slide in elongated holes so boards can swell without buckling.
5) Can finishes stop shrinkage?
No. Finishes can slow moisture exchange and reduce the speed of movement, but they do not prevent it. Plan for the calculated shrinkage and swelling even when sealing all faces and edges.
6) What if I don’t know the species shrink rate?
Use a close preset species if available, or pick Custom and enter a conservative rate. When uncertain, choose values on the higher side to avoid tight joints and seasonal crowding in outdoor assemblies.