Fiber Saturation CalculatorFSP and Bound Water Analysis
Determine the Fiber Saturation Point - the critical moisture level where wood begins to shrink and its strength starts to change. Analyse bound water, free water and the state of your timber.
Why this calculation matters
Fibre saturation is the threshold where dimension starts changing. Above it timber loses water without moving; below it every point of MC matters.
Fiber Saturation Calculator
FSP and Bound Water Analysis
Sets typical FSP for the species.
Typical 26-30%. Adjustable per measurement.
Measured MC of the wood.
Leave 0 to skip mass-based MC.
After drying at 103°C to constant weight.
FSP drops ~0.1%/°C above 20°C.
| Property | Value | Meaning |
|---|
About Fiber Saturation Calculator
The Fiber Saturation Point (FSP) is the moisture content at which the cell walls of wood are completely saturated with bound water but the cell cavities hold no free water. It is the single most important threshold in wood science: above FSP wood does not shrink or change strength; below FSP wood shrinks, gains strength and becomes workable. FSP is typically 26-30% MC. It shows how much bound water remains above your target — and the fibre saturation point of wood is where all dimensional trouble begins.
Where Is This Used?
Formulas Used
MC from mass = (Wet mass - Oven-dry mass) / Oven-dry mass x 100FSP temperature adjust = FSP_20 - 0.1 x (T - 20) (T in deg C)Free water (above FSP) = max(0, MC - FSP)Bound water = min(MC, FSP)Below FSP: shrinkage and strength changes beginHow the calculation works
The fibre saturation point is adjusted slightly downward as temperature rises. Any moisture above that adjusted figure is free water sitting in the cell cavities; the rest is bound water held in the cell walls. Only the bound water affects dimension, which is why the split matters.
Show the formula
Temperature-adjusted fibre saturation point: FSPadj = FSP - 0.1 x (T - 20), with T in degrees C. Free water (%) = max(0, MC - FSPadj). Bound water (%) = MC - free water. Where sample masses are entered, MC = (wet mass - oven-dry mass) / oven-dry mass x 100.Dimensional change occurs only below fibre saturation. Above it, water leaves the cell cavities and the wood barely moves.
References and what each one provides
- USDA Wood Handbook (FPL GTR-190)USDA Forest Products LaboratoryPublished density, moisture relationships, shrinkage coefficients and mechanical properties for a wide range of species. The reference dataset behind most wood-property figures on this site.
- ASTM D4442ASTM InternationalStandard test methods for direct moisture content measurement in wood, including the oven-dry mass method.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Uses a species-independent baseline with a temperature correction. It does not account for species variation in fibre saturation point, extractives content, previous drying history, or measurement error in the sample masses.
Do not use this for Acceptance testing or laboratory reporting.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- FSP falls 0.1 points per °C above 20 °C
- Free water = max(0, MC − FSP); bound water = min(MC, FSP)
Formula checked against the sources above by an automated regression test (tests/verify-phase3a.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter your wood and climate details below, then press Calculate.
- Fill in fiber saturation point, current moisture content, current sample mass, oven-dry mass.
- Press Calculate.
Worked example
Common mistakes to avoid
- Assuming every species saturates at 30%. That is an average; real FSP runs roughly 25–35%.
- Expecting shrinkage above FSP. Wood only starts shrinking once it dries below fibre saturation.
- Confusing free water and bound water. Above FSP the wood loses weight but does not move.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers