Fabric Shrinkage Calculator

What it is for

It turns a wash test on a swatch into numbers: how much the fabric shrinks in length, width and area, and how much extra you need to buy. It helps anyone sewing cotton, linen or knit pieces avoid a garment that comes out too small after its first wash.

How it works

Cut a swatch, measure its length and width, wash and dry it the way the finished item will be cared for, and measure again. Shrinkage is the difference divided by the original size. Lost area combines both directions. For buying, the tool divides the metres you need after washing by one minus the length shrinkage, giving the unwashed length to purchase.

Worked example

A swatch of 20 by 20 cm comes out at 19 by 19.6 cm. Length shrank 5%, width 2% and total area 6.9%. If the project needs 2 m of washed fabric, divide 2 by 0.95: buy about 2.11 m, which is 0.11 m more.

Limits and caveats

The result applies to the fabric and wash method used in the test. Hotter water, a dryer or repeated washes may shrink it further, and knits can also stretch. Measure the swatch lying flat, without pulling it.

Frequently asked questions

Do I need to wash fabric before sewing?

For natural fabrics such as cotton, linen and viscose it is almost always worth it, because the finished item will be washed and will shrink. Pre-washing avoids surprises in fit. Synthetics shrink little, but a swatch test removes any doubt.

How do I measure the swatch properly?

Cut a square of 20 to 30 cm, mark the measurements with a pen or pins, and measure with the fabric lying flat and unstretched. Wash and dry it as you plan to care for the item, let it rest and measure the same marks again.

Why use length shrinkage when buying?

Because fabric is sold by length at a fixed width. If it shrinks 5%, each washed metre yields 0.95 m, so you divide your need by 0.95. The narrower width should also be considered when laying out your pieces.