Wet Felting Shrinkage Calculator

Wet felting typically shrinks wool 25–40% as friction locks the fibres together. Starting from a target finished size, you need to lay out a larger piece — at 30% shrinkage, a 30 cm finished piece starts around 43 cm.

Your numbers

Finished size(cm)Shrinkage(%)

Pre-felt size

42.9cm

Formula

Pre-felt size = Finished size ÷ (1 − Shrinkage ÷ 100)

The rate swings a lot with fibre (merino shrinks more, coarser wool less), soap, and how hard you rub. Test-felt a small swatch first if you have not worked with that wool before.

What to enter

Input	Default	Accepted range
Finished size (cm)	30	1 and up
Shrinkage (%)How much it shrank; pre-washing spends this before you cut.	30	10 ~ 45

Step by step

FormulaPre-felt size = Finished size ÷ (1 − Shrinkage ÷ 100)

With the default numbersPre-felt size = 30 ÷ (1 − 30 ÷ 100)

AnswerPre-felt size = 42.9 cm

Quick reference table

Results when only Finished size (cm) changes and everything else stays put.

Finished size (cm)	Pre-felt size (cm)
15	21.4
22.5	32.1
30	42.9
45	64.3
60	85.7

What each result means

Result	At default values
Pre-felt size (cm)The larger starting size you lay out before shrinkage, ahead of the finished size.	42.9

Common mistakes

The rate swings a lot with fibre (merino shrinks more, coarser wool less), soap, and how hard you rub. Test-felt a small swatch first if you have not worked with that wool before.

Glossary

Shrinkage: How much it shrank; pre-washing spends this before you cut.
Pre-felt size: The larger starting size you lay out before shrinkage, ahead of the finished size.

Frequently asked questions

QHow is Wet Felting Shrinkage Calculator calculated?

Pre-felt size = Finished size ÷ (1 − Shrinkage ÷ 100) — Wet felting typically shrinks wool 25–40% as friction locks the fibres together. Starting from a target finished size, you need to lay out a larger piece — at 30% shrinkage, a 30 cm finished piece starts around 43 cm.

QCan you walk through an example?

With Finished size 30cm, Shrinkage 30%, the answer is Pre-felt size 42.9cm.

QWhat do I need to enter?

Enter Finished size, Shrinkage. The result recalculates as you type, and an empty box counts as zero.

QHow much does the answer move if I change a number?

Changing only Finished size (cm) moves the answer to Finished size (cm) 15 → Pre-felt size (cm) 21.4 and Finished size (cm) 60 → Pre-felt size (cm) 85.7. The table below lays out five steps.

QHow are the numbers rounded?

Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.

QAnything to watch out for?

The rate swings a lot with fibre (merino shrinks more, coarser wool less), soap, and how hard you rub. Test-felt a small swatch first if you have not worked with that wool before.

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Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.