Knit big, felt to fit — the math that gets you there
Felting (more correctly called fulling, when you start from knitted or crocheted fabric) is a little bit of magic and a little bit of risk. You take a loosely worked piece of animal-fibre wool, agitate it in hot water, and the fibres lock together into a dense, sturdy felt — perfect for bags, slippers, bowls, hats and toys that need to hold their shape. The magic is that the fabric becomes firm and seamless. The risk is that it also shrinks, often dramatically, and once it has shrunk you can never get the size back. That is why every felted project starts the same way: you knit it oversized, on purpose, so it finishes at the size you actually want. This calculator does the one piece of arithmetic that beginners most often get wrong — turning your target felted size into the larger size you have to make first.
How to use the calculator
- Pick your mode. Plan a project works backward from the finished felted size you want to the bigger size you have to knit first — the usual starting point. Predict a result runs the other way: tell it the size you have already knitted and it shows what that piece will felt down to. Same shrinkage math, both directions.
- Enter your size — the finished width and length you want (Plan), or the size you knitted (Predict). Switch between inches and centimetres at the top; the math works the same either way.
- Set your shrinkage. Pick a starting preset (Stockinette, Garter or Heavy felt) or type your own percentages. Crucially, width and length are separate — knit fabric rarely shrinks evenly in both directions.
- Or calibrate from a swatch. Open the swatch calibrator, enter your test piece's measurements before and after felting, and the tool computes your real shrinkage in each direction and loads it for you. This is the only way to get truly reliable numbers.
- Add your gauge (optional). Enter your loose, pre-felting stitches and rows per 4 inches and the calculator converts the oversized dimensions into the exact cast-on stitch count and number of rows to knit.
- Copy or print the plan and take it to your needles. Then felt in stages, checking often.
Why width and length shrinkage are different
This is the detail almost every other shrinkage calculator ignores, and it is the difference between a felted bag that comes out the right shape and one that comes out short and wide. As wool fulls, the knitted loops collapse — and they collapse more vertically than horizontally. In plain stockinette the rows squash together faster than the stitches pull in, so a piece typically loses more length than width: 35% length against 20% width is a common pattern. If you average those into a single 27% and apply it both ways, your finished piece ends up too tall and too narrow. By keeping the two directions separate, this calculator preserves the proportions of your design. Garter stitch and crochet behave differently again — often closer to even, or with less length shrinkage — which is exactly why you should calibrate from your own swatch rather than trusting any rule of thumb.
A worked example
Say you want a felted tote that finishes 12 inches wide by 12 inches tall, worked in stockinette with a typical 20% width and 35% length shrinkage. The calculator divides: 12 ÷ (1 − 0.20) = 15.0 inches wide to knit, and 12 ÷ (1 − 0.35) = 18.5 inches tall to knit. Notice that although the target is a perfect square, you knit a tall rectangle — because the length will shrink so much more. Add a loose felting gauge of 14 stitches and 18 rows per 4 inches and it converts those dimensions into 53 stitches to cast on and roughly 83 rows to knit before felting. Knit that rectangle, felt it down in stages, and it pulls in to your 12 × 12 target.
Predict mode: what will my finished knitting felt down to?
Sometimes the question runs the other way. You have already knitted a piece — maybe a swatch, maybe a whole bag — and you want to know the size it will become once it is fulled. Switch to Predict a result and the calculator multiplies instead of divides: felted size = knitted size × (1 − shrinkage), applied separately to width and length. A 18 × 18 inch knitted square at 20% width and 35% length shrinkage, for instance, felts down to roughly 14.4 × 11.7 inches — visibly shorter than it is wide, which is exactly what uneven shrinkage does to a piece that started square. Predict mode is the fastest way to sanity-check a pattern you did not write, to see whether a leftover oversized piece will land near a size you can use, or to understand what your own test swatch is telling you before you commit a whole project to those numbers. Treat the predicted size as the smallest the piece should reach: it is where full felting lands, and you can always stop earlier and block it a touch larger.
How much extra yarn does felting really cost?
Here is the part the simple calculators never tell you, and the reason felted projects so often run out of yarn: because you knit oversized in both directions, the fabric area you actually make is much larger than the finished piece. Area is width × length, so the two shrinkage percentages multiply. A piece that loses 20% of its width and 35% of its length keeps only 0.80 × 0.65 = 0.52 of its area — it loses about 48% of its fabric to felting. Flipped around, that means you have to knit roughly 1.9× the finished area, so you need close to 90% more yarn than an unfelted item of the same final size would take. The result panel shows this as the Knit area multiplier and an extra-yarn estimate so you can buy enough before you start — running short halfway through a felting project, where dye lots and exact gauge matter, is a genuinely expensive mistake. As a rule of thumb, when in doubt buy an extra skein; the yarn does not go to waste, it goes into the thickness and firmness of the felt.
Reading the firmness, area and density numbers
Two felted bags can finish at the identical size yet feel completely different — one floppy, one board-stiff — because firmness is set by how much you full, not by the final dimensions. That is what the result panel's Felt firmness and Area change readouts capture. A light full (under about 25% area loss) keeps some drape and visible stitch texture, good for scarves or soft hats. A soft-to-firm felt (roughly 25–60% area loss) is the sweet spot for bags, slippers and bowls that need to hold a shape without going rigid. A dense felt (above ~60% area loss) is stiff and self-supporting — right for structured vessels and soles, but it can feel like cardboard if you over-do it on a garment. Because the firmness number comes straight from your shrinkage percentages, you can dial in the feel you want at the planning stage: choose a heavier-felt preset (or calibrate a swatch you fulled harder) for a stiffer result, or stop short for something softer. The piece that comes off the needles always looks alarmingly large and loose — the firmness readout reassures you that all that openness is about to collapse into exactly the density you picked.
The science of fulling — why wool shrinks and acrylic doesn't
Felting is not the fibres melting or the yarn simply tightening; it is a physical, one-way migration of fibres that locks them permanently together. Every strand of animal wool is covered in microscopic overlapping cuticle scales, like roof tiles all pointing the same way toward the tip. When you add heat, moisture and agitation, those scales lift and the fibres begin to creep — and because the lifted scales catch in only one direction, each fibre edges root-first and is blocked from sliding back. Neighbouring fibres hook past one another, the whole web draws steadily tighter, and the tangle cannot release. That irreversible ratcheting is why felting can never be undone, why a quick hot-cold temperature shock accelerates it (the scales open and slam shut), and why agitation — old jeans or tennis balls in the machine — does most of the work. Superwash wool has had those scales chemically smoothed or polymer-coated specifically so they can survive a washing machine, which is exactly why it refuses to felt. Acrylic, cotton, silk and other plant or synthetic fibres have no lifting scales at all, so no amount of heat and agitation will full them. Understanding the mechanism is practical, not trivia: it tells you to knit loose (so the scaled fibres have room to migrate), to felt hot with friction, and to check your yarn band for "superwash" or "machine washable" before you waste a project on a fibre that physically cannot shrink.
Typical shrinkage by stitch and fibre
Use these as starting presets only — your yarn, water temperature and how hard you agitate will move them around. The presets in the tool are set to these stockinette, garter and heavy-felt values; always confirm with a swatch.
| Fabric / felting | Width shrink | Length shrink |
|---|---|---|
| Stockinette, moderate felt | ~20% | ~35% |
| Garter / textured | ~15% | ~25% |
| Heavy / dense felt | ~30–40% | ~40–50% |
| Crochet (denser fabric) | ~15–20% | ~20–30% |
Calibrating from a test swatch — the reliable way
Felting is not perfectly repeatable, but your yarn felted your way is consistent enough to design around. Knit a generous swatch — at least 8 inches square so measuring errors stay small — in the same yarn, stitch pattern and loose needle size as your project. Measure its width and length and write them down. Then felt the swatch exactly as you intend to felt the project: same machine or sink, same water temperature, same amount of agitation, fully fulled. Measure again. The calculator's swatch calibrator turns those four numbers into your true width and length shrinkage with the formula (before − after) ÷ before, and loads them into the shrinkage boxes so the rest of the project is sized to reality, not to a guess. Keep the swatch and its numbers in your project notes; the same yarn will behave the same way next time.
Common mistakes this prevents
Pro tips for felting on target
- Only non-superwash animal fibres felt — wool, alpaca, mohair. Superwash wool, acrylic and cotton will not shrink the way this calculator assumes, so check your yarn band first.
- Knit loose: go 2–3 needle sizes larger than the band suggests so the fibres have room to migrate and lock.
- Felt in short bursts. Pull the piece out, shape it, measure against your target, and put it back if it needs more. You can always felt more — never less.
- Hot water plus cold shock and a little soap speeds felting; a pair of old jeans or tennis balls in the machine adds the agitation that drives shrinkage.
- Block the finished felt over a form (a box, a bowl, rolled towels) while damp so it dries to exactly the shape and size you want.
- Stitch definition disappears as you full — don't waste a fancy lace or cable pattern on a piece you intend to felt.
Troubleshooting uneven or stubborn felting
Even with the right oversized dimensions, felting can misbehave — and most problems trace back to the fabric or the process rather than the math. If a piece won't shrink, check the fibre first: a hidden superwash or a wool-blend with too much nylon or acrylic simply cannot full, so confirm the band before blaming your machine. If it shrinks but stays floppy and never firms up, you usually have not agitated it enough or the water was not hot enough; add friction (jeans, towels, tennis balls), keep the water hot, and give it more, shorter cycles. Patchy or uneven felting — dense in places, loose in others — comes from knitting too tightly in spots or from the piece bunching against itself in the machine; knit loosely throughout and let the piece move freely. If your finished size is right in one direction but wrong in the other, your width and length shrinkage were not what you assumed — re-measure a swatch and recalibrate, because a single averaged percentage is the usual culprit. And if a project felts smaller than predicted, you simply fulled it past the point your swatch reached; stop earlier next time and block it out while damp. The unbreakable rule behind all of these: felt in short stages, measure against your target between each, and stop the instant you are on size — every felting failure that can't be fixed is a piece that went one cycle too far.
Where this fits in your project
Sizing a felted piece is a two-step problem: first decide the stitch and row counts that give the right pre-felting dimensions, then felt down to the finished size. This tool handles the felting half. For the first half — turning inches into stitches at your chosen gauge — pair it with a gauge calculator, and if you are felting a hat, the hat cast-on and crown-decrease tools give you the body and crown counts to oversize here. Working backward from a finished size, through the shrinkage, to a real cast-on number is exactly the kind of math that makes felted projects feel unpredictable. Do it once with the right formula and felting becomes a reliable, repeatable craft instead of a gamble.
Frequently asked questions
Is this calculator free?
Yes — free, no sign-up. Everything runs in your browser.
Does my data get saved?
No. There is no server and no tracking. Your numbers never leave your device.
Why divide instead of add the percentage?
The shrink percentage is taken off the larger pre-felting size, so you reverse it by dividing by (1 − shrink), not by adding it to your target. Adding always undershoots.
Should width and length really differ?
For stockinette, yes — length normally shrinks more than width as the rows collapse. Garter and crochet can be closer; calibrate from a swatch to be sure.
How do I get my own shrinkage numbers?
Felt a test swatch, measure before and after, and use the built-in calibrator. It computes (before − after) ÷ before for each direction.
Do I have to enter gauge?
No. Gauge is optional and only used to convert the oversized dimensions into cast-on stitches and rows. Leave it at zero to skip.
Can I felt superwash or acrylic?
No. Superwash wool is treated not to felt, and acrylic and cotton never felt. Use non-superwash animal fibres.
Does it work for crochet?
Yes — the math is identical. Enter crochet gauge for stitch counts; crochet often shrinks a little less in width.
How many felting cycles will I need?
It depends entirely on your yarn and machine. Felt in short stages and measure often rather than guessing a cycle count.
What if my piece felts too small?
You cannot un-shrink felt. Wet it and stretch/block it slightly larger while damp; otherwise size up next time. Stopping early is always safer.
How do I predict what my knitting will felt down to?
Switch to Predict mode, enter the size you already knitted and your shrinkage, and it multiplies to show the felted size. Treat that as the smallest the piece should reach and stop felting early if it gets close.
Why does felting need so much extra yarn?
You knit oversized in both width and length, and area multiplies the two. Losing 20% width and 35% length keeps just 52% of the area, so you knit ~1.9× the finished area — close to 90% more yarn. The result panel estimates this for you.