How to Measure Fabric Shrinkage With a Wash Test
Most fabric labels and listings say nothing useful about shrinkage. "Pre-wash before sewing" is common; a percentage, in each direction, after a stated wash, is rare outside mill spec sheets. Yet shrinkage is the single most common reason a carefully sewn garment, curtain or cushion cover comes out of its first wash the wrong size. The fix is a simple wash test, done before you cut. This guide shows how to do one properly, how to read the result with the site's Fabric Shrinkage Calculator, and what to do with the number once you have it.
Why fabric shrinks
Most shrinkage in the first wash is relaxation shrinkage. During weaving, knitting and finishing, fabric is held under tension and stretched, and the yarns are set in that stretched state. When the fabric is wetted and agitated, the yarns relax back towards their natural length, and the cloth contracts. Cotton, linen, viscose and other cellulose fibres also swell when wet, which shortens the crimped yarns further.
Some fabrics also show progressive shrinkage, a smaller further loss over repeated washes, and wool can felt, a different process in which the fibre scales lock together under heat, moisture and agitation. Felting can shrink wool dramatically and is not reversible.
Finishes such as sanforising for cotton and heat-setting for synthetics reduce shrinkage, but they do not always remove it, and you often do not know what finishing a fabric has had.
What you need
- A sample at least 50 cm by 50 cm, or the full width by 50 cm for a narrow fabric. Bigger samples give more accurate percentages.
- A ruler or tape measure marked in millimetres.
- A way to mark the fabric that will survive washing: a permanent fabric marker, or tailor's tacks in a contrasting thread.
- Your usual washing machine and dryer, or the method you will use for the finished item.
The method
- Square up the sample. Cut along the grain, with the selvedge along one side if possible, so you know which direction is the warp (length) and which the weft (width).
- Mark two lengths. Mark a 40 or 50 cm distance along the warp and the same across the weft, at least 5 cm in from the edges. Mark the pairs at a couple of places if you can, and label which is which.
- Let the fabric rest flat for a few hours before marking if it has been on a tight roll, so you do not measure tension that would have relaxed anyway.
- Wash and dry it exactly as the finished item will be treated: same temperature, same detergent, same spin, same drying method. Tumble drying usually shrinks more than line drying.
- Let it settle, then lay it flat without stretching and measure each marked distance again.
Laboratories use standard procedures for this, for example the ISO methods for domestic washing and drying and for measuring dimensional change, which fix the load, temperature and the way samples are marked and conditioned. A home test is less precise but answers the question that matters: what will happen in your machine.
Reading the result
Shrinkage percentage is the change in length divided by the original length, times 100. The calculator's measure mode does the arithmetic and tells you whether the fabric shrank or grew. Some example readings:
| Direction | Before | After | Result |
|---|---|---|---|
| Length (warp) | 50 cm | 48 cm | 4% shrinkage |
| Width (weft) | 50 cm | 49 cm | 2% shrinkage |
| Width of a knit | 50 cm | 50.5 cm | grew by 1% (shown as −1%) |
The first two rows are typical of a woven cotton: shrinkage along the warp is often larger than across the weft, because the warp is held under more tension during weaving and finishing. The third row shows that a fabric can also grow in one direction, which is common for knits that shrink in length and spread in width. The calculator reports growth as a negative shrinkage.
If your marks were 50 cm apart and you can only measure to the nearest millimetre, each measurement is good to about 0.2% of the length. On a 20 cm mark it is only good to about 0.5%. That is why bigger samples matter.
Repeat the test if it matters
For a single wash test the result is a good guide, not a guarantee. If the finished size really matters, for fitted covers, curtains to a precise drop, or garments made to measure, wash and dry the sample two or three times and measure after each. If it keeps shrinking, use the figure after the last wash, or pre-wash the whole piece several times before cutting.
What to do with the number
Option 1: pre-wash the whole piece
If the fabric can be washed before cutting, washing and drying the whole length the way you will treat the finished item takes most of the shrinkage out before it matters. You still need to buy enough to allow for it. The guide to how much extra fabric to buy for shrinkage covers that.
Option 2: cut larger
If you cannot pre-wash, for example because the fabric must stay crisp until it is made up, or the item will be dry-cleaned after it is made, cut larger to allow for the expected shrinkage. The calculator's pre-cut mode works out how much: to end up with a 200 cm finished length from a fabric that shrinks 5%, cut 210.53 cm, an allowance of 10.53 cm. At 3%, cut 206.19 cm. Notice that the allowance is a little more than 5% or 3% of the finished length; that is because the shrinkage percentage applies to the longer, cut length.
Option 3: choose a different fabric
If a fabric shrinks a great deal, or differently in each direction, or keeps shrinking wash after wash, it may simply be the wrong fabric for a fitted item.
Common sources of error
- Stretching while measuring. Lay the sample flat, smooth it gently, and do not pull. Knits are especially easy to stretch.
- Measuring wet. Wait until the fabric is fully dry and has settled.
- Testing a different process. A cool hand wash will not tell you what a hot machine wash and tumble dry will do.
- Testing too small a piece. Small samples make small measuring errors into large percentages.
- Ignoring the second direction. Warp and weft shrinkage are rarely the same, and a pattern piece cut on the bias is affected by both.
Fibres that need special care
Wool can felt if washed warm with agitation; test with the gentle method the care label specifies, never hot. Viscose and other rayons can shrink noticeably and are weaker when wet; handle them gently. Linen often shrinks in the first washes. Blends with elastane should be tested at the temperatures they will actually see, because heat damages elastane. For anything labelled dry-clean only, wet testing may damage it; follow the care label.
What the calculator covers
The calculator turns a before-and-after measurement into a shrinkage or growth percentage, and turns a known percentage into a pre-cut length for a target finished size. It cannot predict how a fabric will behave before you test it, and it treats each direction separately: run it once for the length and once for the width. The test itself is the real work; the arithmetic just makes the result usable.