Inputs
Example data table
| Species | MC (%) | Pattern | Dims (mm) | ΔW (mm) | ΔT (mm) |
|---|---|---|---|---|---|
| Douglas fir | 19 → 12 | Flat-sawn | 2400 × 200 × 50 | ≈ -3.55 | ≈ -1.12 |
| Oak (white) | 16 → 8 | Quarter-sawn | 1800 × 150 × 40 | ≈ -2.24 | ≈ -4.19 |
| Cedar (western red) | 14 → 10 | Rift-sawn | 1200 × 100 × 25 | ≈ -0.31 | ≈ -0.31 |
Examples are approximate and depend on actual timber condition.
Formula used
Timber movement is modeled mainly below the fiber saturation point (FSP). Moisture values are clamped to 0…FSP before calculation.
- Effective moisture change: ΔMC = clamp(MCᵢ,0,FSP) − clamp(MC𝒻,0,FSP)
- Fraction of FSP range: k = ΔMC / FSP
- Directional shrinkage (%): p = S_total × k
- Final dimension: D₁ = D₀ × (1 − p/100)
Use tangential or radial values across grain, and longitudinal along grain.
How to use this calculator
- Select a species preset, or choose Custom and enter shrinkage values.
- Set initial and final moisture contents for your site conditions.
- Pick a sawing pattern to map tangential and radial directions.
- Enter length, width, and thickness in millimeters.
- Press Submit to see results above the form.
- Use Download CSV or Download PDF for documentation.
Moisture content and the fiber saturation point
This calculator estimates size change only within the shrinkage range below the fiber saturation point (FSP). It clamps both moisture inputs to 0–FSP, then uses the effective change ΔMC to scale movement. If timber drops from 19% to 12% with FSP 30%, k equals 0.233. That fraction is applied to total shrinkage coefficients to predict dimensional change for planning.
Directional shrinkage and grain orientation
Wood shrinks differently tangentially and radially, so sawing pattern matters. Flat-sawn boards typically move more in width because width aligns with tangential shrinkage. Quarter-sawn members often place radial movement on width and tangential on thickness. The calculator lets you map directions automatically or set custom directions, helping you compare detailing choices for cladding, flooring, and structural blocking.
Using presets versus project-specific testing
Preset species values are practical for early estimates, budgeting, and coordination. However, actual shrinkage depends on growth conditions, density, drying schedule, and machining. When performance is critical, replace presets with test-based tangential and radial shrinkage from your supplier or lab data. Keep longitudinal shrinkage small but nonzero for long members, especially where cumulative movement affects alignment.
Interpreting results for tolerances and detailing
Outputs include final dimensions, absolute changes (ΔL, ΔW, ΔT), and a derived volumetric change from L×W×T. Use these to set joint gaps, slot lengths, and allowance at connections. For example, predicted width loss of 3.6 mm over 200 mm suggests allowing clearance at trims or using adjustable fasteners. Where swelling is possible, design for both directions.
Quality checks and field measurement tips
Confirm moisture content with calibrated meters at multiple depths and faces, not just the surface. Record temperature and storage conditions, then recheck after acclimation. If initial moisture exceeds FSP, expect little movement until drying reaches the FSP threshold. Validate predicted movement with a control sample, and document each run using the CSV or PDF export for site records. These steps reduce rework and improve durability, safety, performance.
FAQs
1. What moisture values should I enter?
Use measured initial and target in-service moisture. If you only know relative humidity, convert it using local guidance or conditioning charts. The calculator caps movement above FSP to avoid overstating change.
2. Why does width change more than thickness sometimes?
Because the width may be aligned with tangential shrinkage, which is usually larger than radial shrinkage. Change the sawing pattern or set custom directions to match your board orientation.
3. Do the presets guarantee accuracy?
No. Presets are typical averages for early planning. For critical tolerances, replace them with supplier or lab values for your batch, and consider site-specific conditioning.
4. What if the timber gets wetter after installation?
If final moisture is higher than initial, the calculator reports swelling estimates within the FSP range. Design joints, fixings, and clearances to tolerate both drying shrinkage and wetting expansion.
5. How should I use the volumetric change result?
Treat it as a derived indicator, not a specification. Use directional outputs for detailing, but volumetric change can help compare species stability and highlight high-movement scenarios.
6. Can I export results for documentation?
Yes. After a successful run, use Download CSV for spreadsheets or Download PDF for a clean report. Exports include key inputs, mapped directions, and computed dimensions.