Wool Felting Shrinkage Calculator
Calculate how much wool fabric or fiber you need before wet felting or fulling, accounting for shrinkage rates by fiber type and felting method.
Wool felting is the process of matting and compressing wool fibers using heat, moisture, and agitation. The fibers have microscopic scales that lock together when agitated in warm, soapy water, and the fabric shrinks permanently as they do.
Shrinkage rates by fiber and method:
| Fiber / Method | Width Shrinkage | Length Shrinkage |
|---|---|---|
| Merino (100%), wet felt | 30–40% | 30–40% |
| Corriedale, wet felt | 25–35% | 25–35% |
| BFL (Blue-Faced Leicester) | 20–30% | 20–30% |
| Woven wool fabric, fulling | 15–25% | 15–25% |
| Knitted wool, felting | 25–40% | 35–45% |
| Roving (loose fiber) | 35–45% | 35–45% |
The formula:
If you want a finished piece of 30 × 40 cm and expect 30% shrinkage:
Starting size = Finished size ÷ (1 - shrinkage rate)
Starting width = 30 ÷ (1 - 0.30) = 30 ÷ 0.70 = 42.9 cm
Factors that increase shrinkage:
- Higher water temperature: hotter = more felting (up to boiling)
- More agitation: rubbing harder and longer shrinks more
- Longer felting time: more time = more shrinkage
- Higher percentage of merino: finer micron count felts more aggressively
- Acidic pH: vinegar rinses can increase felting speed
Factors that reduce shrinkage:
- Superwash treated wool: chemically treated to resist felting (will not felt at all)
- Acrylic or synthetic content: synthetics don’t felt, reducing overall shrinkage
- Gentle / cool washing: cool water with minimal agitation = minimal felting
- Wool batting vs roving: pre-processed batting shrinks more predictably
Testing before full project:
Always make a swatch first. Felt a 15×15 cm sample, measure after drying, and calculate your actual shrinkage rate. Every dye lot, every fleece, and every room temperature behaves differently.
Area, not linear, is what you lose
Shrinkage figures are quoted per edge, and that is the trap. A piece that goes 35% smaller across and 35% smaller down keeps 0.65 × 0.65 = 42% of its area. You lose 58% of what you laid out, not 35%. This one line accounts for most of the “I ran out of fibre” posts on felting forums.
Worked through: a finished 30 × 40 cm piece at 35% each way needs 46.2 × 61.5 cm of loose layout, which is 2,841 sq cm against the 1,200 sq cm you end up with.
Thickness change:
The wool does not go anywhere, so all of it ends up in that smaller area. At 35% each way the fibre is packed 1 ÷ 0.42 = 2.4 times as densely per square centimetre as it was in the loose layout.
The finished felt is not 2.4 times thicker, though, because felting also compresses. It ends up somewhere between the loose thickness and that 2.4, and where exactly depends on how hard you work it. Lay out more layers than feels necessary: thin felt is the more common mistake, and you cannot add wool afterwards.
Knitted wool is the odd one out
Every other row in the table above shrinks squarely, roughly the same amount in both directions. Knitted wool in a washing machine does not: it pulls in 25 to 40% across but 35 to 45% down, because the stitch structure gives up height more readily than width. Felting a jumper into a bag and finding it short is the usual result of ignoring that. The calculator now sizes the two directions separately for this option.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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