What shrinkage is, and why you must measure it yourself
Shrinkage is the fraction of a dimension that disappears the first time fabric is wetted, agitated and dried. It is not a property of the fibre alone. It comes from the tension the fabric was held under during weaving, dyeing and finishing, which the mill stretched in and which water lets go. That is why two 100% cotton wovens from the same shelf can shrink 2% and 8%, and why no published table can tell you your number.
So you measure it. Mark a swatch, wash and dry it exactly the way the finished item will be washed and dried, measure the same marks again, and the fraction lost is your shrinkage. AATCC 135 is the formal version of that procedure for home laundering, and it exists because the answer depends on the wash temperature, the agitation and — most of all — the drying method. A tumble dryer on high will pull far more out of a cotton knit than line drying will.
The calculator then does two things with the number. It converts each finished dimension into a cut dimension, so pieces land on size after washing. And it converts the project's yardage into the yardage to buy, so prewashing the cloth does not leave you short.
Divide by what survives — do not multiply by what is lost
Shrinkage is (before − after) ÷ before. An 18 in benchmark that comes back at 17.1 in has lost 0.9 in, and 0.9 ÷ 18 = 5%. Straightforward.
The allowance is where it goes wrong. The instinct is to add the shrinkage back: 30 in finished, 5% shrinkage, so cut 30 × 1.05 = 31.5 in. Wash that and you get 31.5 × 0.95 = 29.925 in — a little short. At 5% the miss is under a tenth of an inch and nobody notices. At 15% it is not small: 34 × 1.15 = 39.1 in, which washes down to 39.1 × 0.85 = 33.235 in, three-quarters of an inch under a 34 in target.
The correct move is to divide by the fraction that survives. If 5% goes, 95% stays, so the cut must be 30 ÷ 0.95 = 31.5789 in. Check it: 31.5789 × 0.95 = 30.0000. The two methods diverge by exactly the factor 1/(1−S) against (1+S), which is 1.1111 against 1.1000 at 10% shrinkage — about one percent of the piece, growing fast as shrinkage rises.
Length and width get separate treatments because woven fabric shrinks unevenly. Warp yarns run lengthwise and are held under the most tension on the loom, so lengthwise shrinkage is usually the larger of the two on a woven. Knits often behave the other way, and can grow in one direction while shrinking in the other as the loops relax. The calculator never averages the two.
Area shrinkage, 1 − (1 − S_L)(1 − S_W), is the product rather than the sum. At 5% and 2.78% the area loss is 1 − 0.95 × 0.9722 = 7.64%, slightly less than the 7.78% you would get by adding, because the two losses overlap.
Worked example: a cotton with 5% length and 2.8% width shrinkage
You mark an 18 in square on a quilting cotton, wash it warm and tumble dry it, and remeasure: 17.1 in lengthwise and 17.5 in across. The project needs pieces finishing 30 in long and 20 in wide, and the pattern calls for 3 yd.
- Lengthwise shrinkage. (18 − 17.1) ÷ 18 = 0.9 ÷ 18 = 5.00%.
- Widthwise shrinkage. (18 − 17.5) ÷ 18 = 0.5 ÷ 18 = 2.78%.
- What survives. 1 − 0.05 = 0.95 lengthwise, 1 − 0.0278 = 0.9722 widthwise.
- Cut length. 30 ÷ 0.95 = 31.579 in, so cut 31⅝ in and lose a sixteenth to rounding.
- Cut width. 20 ÷ 0.9722 = 20.571 in, so cut 20⅝ in.
- Area shrinkage. 1 − (0.95 × 0.9722) = 1 − 0.92361 = 7.64%.
- Yardage to buy. 3 ÷ 0.95 = 3.158 yd, so buy 3¼ yd — 0.158 yd, or 5.7 in, more than the pattern says.
Check the length allowance by running it forwards: 31.579 in cut, times 0.95, is 30.000 in finished. If you had instead cut 30 × 1.05 = 31.5 in, the finished piece would be 29.925 in — visibly fine here, and the same mistake at 15% shrinkage would cost you three-quarters of an inch.
Reading the result and deciding what to do about it
Under 2% in both directions is a stable fabric. Most preshrunk quilting cottons and finished shirtings land here. Cut to the finished sizes and prewash or not as you prefer.
2% to 7% is the ordinary range for untreated cotton wovens. Prewash the yardage, then cut normally — that is nearly always simpler than cutting oversize, because oversize cutting must be applied to every piece and every seam allowance, and it distorts any piece that is not a rectangle.
Above 7%, and especially above 10%, prewash twice. Cotton flannel, linen, and loosely constructed cotton knits all reach double digits on the first wash, and a good deal less on the second. The second wash tells you what the garment will keep doing over its life, which is the number that matters for a fitted item.
Watch the difference between the two directions as closely as the sizes themselves. When length and width shrink by different amounts, the fabric changes shape, not just size. A square becomes a rectangle, a bias-cut skirt hangs differently, and a quilt block that is 12.5 in one way and 12.2 in the other will not square up. That is also why you should not rotate pattern pieces to save fabric on a high-shrinkage cloth — the cutting layout calculator will happily turn pieces ninety degrees to gain a row, and on this fabric that puts the wrong shrinkage on the wrong dimension.
The extra yardage figure uses lengthwise shrinkage only, because yardage is sold by the length and the width comes as it comes. If width shrinkage is severe enough to take the usable width below what your widest piece needs, buying more length does not help — you need a wider fabric.
Cut allowance by shrinkage rate
| Shrinkage | Cut multiplier 1/(1−S) | Wrong multiplier 1+S | Cut for 30 in finish | Extra on 3 yd |
|---|---|---|---|---|
| 1% | 1.0101 | 1.0100 | 30.30 in | 0.030 yd |
| 2% | 1.0204 | 1.0200 | 30.61 in | 0.061 yd |
| 3% | 1.0309 | 1.0300 | 30.93 in | 0.093 yd |
| 5% | 1.0526 | 1.0500 | 31.58 in | 0.158 yd |
| 7% | 1.0753 | 1.0700 | 32.26 in | 0.226 yd |
| 10% | 1.1111 | 1.1000 | 33.33 in | 0.333 yd |
| 12% | 1.1364 | 1.1200 | 34.09 in | 0.409 yd |
| 15% | 1.1765 | 1.1500 | 35.29 in | 0.529 yd |
The third column is shown only to be avoided. It is close enough to ignore below about 3% and wrong by more than a quarter inch on a 30 in piece by the time shrinkage reaches 10%.
What the AATCC test method actually specifies
AATCC 135, Dimensional Changes of Fabrics after Home Laundering, is the reference procedure behind this calculation. It specifies benchmark pairs marked on a conditioned specimen, a defined wash cycle and water temperature, a defined drying procedure, and remeasurement after conditioning — and it reports the result as a signed dimensional change, negative for shrinkage. The companion method AATCC 150 does the same for whole garments rather than fabric. You are not running a laboratory test at home, but two things from the method are worth copying: use benchmarks of at least 10 in, and wash and dry the swatch exactly the way the finished item will be treated, because the drying step usually contributes more shrinkage than the wash does.
Where swatch tests go wrong
- Measuring the cut edges instead of marked benchmarks. Cut edges fray and curl in the wash. Mark two points well inside the swatch and measure between the marks.
- Using a swatch that is too small. An eighth of an inch misread on a 6 in benchmark is 2% of shrinkage; on an 18 in benchmark it is 0.7%. Bigger swatches are more honest.
- Stretching the swatch flat to measure it. Lay it down and let it settle. Pulling it out reports a shrinkage lower than the garment will actually experience.
- Testing a wash the item will never get. A cold delicate cycle and line drying is a different test from hot wash and high tumble. Test the treatment the finished item will really receive.
- Measuring while damp. Cotton reads larger damp than bone dry. Dry it fully, then leave it a few hours in the room where you sew before measuring.
- Applying one number to both directions. Warp and weft shrink differently on almost every woven. Measuring only one direction and using it for both is the fastest way to a piece that is right one way and wrong the other.
Prewash or cut oversize?
For most sewing, prewashing the yardage is better than cutting oversize. It removes the problem instead of compensating for it, it lets you cut to the pattern as drawn, and it also removes excess dye and sizing that would otherwise transfer in the first wash. The one number you still need is the extra yardage: buy 3 ÷ 0.95 = 3.16 yd so that after prewashing you still have the 3 yd the pattern wanted. Feed the prewashed width back into the fabric yardage calculator, because prewashed 44 in cotton often measures 42 in and that alone can change your cutting plan.
Cut oversize when prewashing is not an option — with fabrics whose finish or drape you would destroy, with pieced work where you want the crinkled texture that comes from washing a finished quilt, or with a fabric you have already cut. Quilters often deliberately skip prewashing for exactly that reason, and then plan for the whole quilt to shrink; if you go that way, run your block sizes through the quilt block count calculator at finished-after-wash dimensions.
The same arithmetic appears wherever a material is made oversize and then shrinks to size. Potters divide by the surviving fraction to decide how large to throw a pot, which is what the clay shrinkage calculator does; woodworkers use the reverse relationship for seasonal movement in the wood movement calculator. In all three, the error to avoid is identical: add the loss back and you land short, divide by what survives and you land on size.