Illustrative dimensions only. This is not a test of a particular fabric or a promise that the same percentage will repeat.
Start with the measured change
Suppose the lengthwise reference distance changes from 20 cm to 19 cm, and the width changes from 20 cm to 19.4 cm. The length retains 19 ÷ 20 = 0.95 of its starting measurement, while the width retains 0.97.
| Direction | Before | After | Change |
|---|---|---|---|
| Length | 20 cm | 19 cm | 5% shrinkage |
| Width | 20 cm | 19.4 cm | 3% shrinkage |
Work backwards from the length you need
If the full length behaves like the sample, every metre before washing leaves 0.95 metres afterwards. To retain 2 metres:
2 ÷ 0.95 = 2.105263… metres before washing
Adding 5% to 2 metres gives 2.10 metres. But 2.10 × 0.95 = 1.995 metres: 5 mm short of the target. The inverse uses the retained fraction because the percentage loss is measured against the longer, original piece.
Round for the shop
If the shop sells quarter-metre increments, 2.105263… becomes 2.25 metres. At the same 5% loss, it would retain 2.1375 metres. If your shop permits a different increment, enter that in the planner. Do not round the minimum down.
Width is a separate constraint
Suppose the usable width before washing is 140 cm. A 3% width loss would leave 135.8 cm. If your pattern layout requires 140 cm of usable width, the length calculation alone does not solve the problem. Check or revise the layout.
Add an allowance only when you need it
The default planner uses zero extra usable length. If you choose a 10 cm usable allowance, convert it to 0.10 metres and include it in the target before reversing the shrinkage: (2 + 0.10) ÷ 0.95 = 2.210526… metres. This still rounds to 2.25 metres at 0.25-metre increments. Check what your pattern already allows for before adding more.
The planner opens with 2 m, 5%, zero extra and a 0.25 m increment. Change these to your measurements. Calculation method and limits.