Wood Movement Calculator
Predict seasonal wood expansion and contraction by species, grain orientation, and moisture content change.
Wood Movement Calculator
Predict seasonal expansion and contraction
Wood Species
Board Configuration
Moisture Content Range
Winter / dry season
Summer / humid season
Typical Indoor MC Ranges
Dry climate (desert): 4–9% MC
Average US climate: 6–12% MC
Humid climate (coast): 8–14% MC
Kiln-dried lumber: 6–8% MC at delivery
Predicted Movement
Species Data
Tangential
8.6%
flat sawn
Radial
4%
quarter sawn
Design Tip
Allow for this movement in tabletops, panel glue-ups, and frame-and-panel construction. Use elongated screw holes, breadboard ends with floating tenons, or expansion gaps.
Formulas Used
Dimension Change
Where W is board width, S is the total shrinkage percentage for that grain orientation, 30 represents fiber saturation point, and ΔMC is the moisture content change.
Shrinkage Coefficient
The shrinkage coefficient per 1% change in moisture content. Divide the green-to-oven-dry shrinkage by 30 (the fiber saturation point).
Understanding Wood Movement
Wood is hygroscopic — it absorbs and releases moisture in response to the ambient humidity. This causes dimensional changes that every woodworker must account for.
Why Wood Moves
Wood cells are like tiny tubes. When they absorb moisture, the cell walls swell. When they lose moisture, the walls shrink. This movement occurs primarily across the grain (width and thickness), with negligible change along the grain (length).
Critical concept: Movement occurs in the tangential direction (parallel to growth rings) and the radial direction (perpendicular to growth rings). Tangential movement is always greater.
Grain Orientation Matters
| Orientation | Direction of Movement | Relative Amount | |------------|----------------------|------------------| | Flat Sawn | Tangential (most movement) | 100% | | Rift Sawn | Mix of both | ~75% | | Quarter Sawn | Radial (least movement) | ~50% |
This is why quarter-sawn lumber is prized for tabletops, doors, and musical instruments — it moves roughly half as much as flat-sawn stock.
Species Comparison (Tangential Shrinkage)
| Species | Tangential | Radial | T/R Ratio | |---------|-----------|--------|----------| | Western Red Cedar | 5.0% | 2.4% | 2.1 | | Teak | 4.0% | 2.6% | 1.5 | | Genuine Mahogany | 4.1% | 3.0% | 1.4 | | Black Walnut | 7.8% | 5.5% | 1.4 | | Black Cherry | 7.1% | 3.7% | 1.9 | | Red Oak | 8.6% | 4.0% | 2.2 | | Hard Maple | 9.9% | 4.8% | 2.1 | | Hickory | 10.5% | 7.0% | 1.5 |
Lower T/R ratio = more predictable movement. Species like walnut and teak have low T/R ratios, meaning flat-sawn and quarter-sawn boards behave more similarly.
Practical Design Rules
- Never glue cross-grain joints — the seasonal movement will break the joint or split the wood
- Allow ⅛" per foot of width as a rough rule for wide panel expansion gaps
- Use mechanical fasteners that allow movement (slots, clips, elongated holes)
- Finish all sides equally — unfinished surfaces gain/lose moisture faster, causing cupping
- Acclimate wood to your shop's humidity before building (1-2 weeks minimum)
Frequently Asked Questions
Related Calculators
Shelf Sag Calculator
Check if your shelf will sag under load — calculates deflection, maximum recommended load, and gives species-specific stiffness data.
Wood Weight Calculator: Lumber in lb & kg
Estimate solid lumber weight from actual dimensions or board feet, using a selected species density or your own verified density.