Wool Felting Calculator

Felting is a compounding process: each contributor (fiber type, heat, agitation, pH, and superwash treatment) either accelerates or dampens the same underlying scale-interlocking mechanism, rather than adding an independent effect on top. Multiplying the factors together means that if any single condition is mild — say, low agitation even at high heat — the overall shrinkage stays proportionally lower, which matches how felting behaves in practice: a hot soak with no agitation felts far less than the same temperature in a machine cycle.

Area shrinkage describes how much surface area the fabric loses overall, but each edge of the fabric only needs to shrink by the square root of that area loss to produce it — a square that loses 36% of its area only has sides that are 20% shorter (1 minus the square root of 0.64). That's the exact relationship the calculator applies to convert area shrinkage into linear shrinkage, so a large area-shrinkage number always corresponds to a noticeably smaller per-dimension change.

No — the calculator only cuts the felting factor to about a tenth of untreated wool, it doesn't zero it out. At high enough heat and agitation, the chlorine-Hercosett coating can still be overwhelmed and superwash wool can felt, just far more slowly than untreated fiber, which is why the tool still returns a nonzero risk level and a recommended safe temperature even with superwash set to yes.

A low reversibility score (5-15) reflects that once wool scales have physically interlocked, gentle stretching or reblocking usually can't fully separate them again, so the calculator treats double-digit area shrinkage as effectively permanent. Below about 3% area shrinkage the score climbs to 80 because that light a felting can often be worked back out by soaking, stretching, and reblocking the piece while it's still wet — the score is really a rough proxy for how much of the felting has become structural versus still relaxable.