Fabric Shrinkage Calculator

Introduction: what actually happens to cloth in the wash

Shrinkage is not the fibre getting smaller. It is the fabric releasing tension that was built into it on the loom and in finishing. Warp yarns are pulled taut for the entire length of the weaving run, and the finishing line stretches the cloth again to get it flat and on-width. Water swells the fibres, heat and agitation give them the mobility to move, and the yarns crimp back toward the relaxed geometry they would have adopted if nothing had been pulling on them. The cloth gets shorter and thicker rather than losing material.

Two consequences follow directly, and both are why a single shrinkage percentage is usually the wrong tool. First, the direction that was under the most tension gives back the most, so woven fabric almost always shrinks more along the warp than across the weft. Second, knits can behave in the opposite sense across the width: a jersey that shortens in length frequently widens across the course as the loops relax sideways.

This calculator therefore takes separate percentages for the two directions, and it works in both directions of the problem. Forward, it tells you what a cut piece will measure after washing. Backward, it tells you what to cut so the finished piece lands on the measurement you actually want, which is the question most sewing and upholstery projects are really asking and the one most shrinkage calculators do not answer.

How to use the fabric shrinkage calculator in either direction

Pick the mode first, because it changes what the two dimension fields mean.

1. Predict finished size takes the dimensions of the fabric you already have and reports what it will measure after laundering. Use this when you have a fixed piece of cloth and need to know whether it is big enough.
2. Find cut size takes the finished dimensions you want and reports what to cut. Use this when you are buying yardage or cutting into a bolt. This is the mode most projects need.

Then fill in the rest:

• Length is the along-the-grain, along-the-bolt direction, which is the warp on a woven fabric and the wale on a knit.
• Width is the cross-grain direction, the weft or the course.
• Length shrinkage and width shrinkage are separate percentages. Take them from a swatch test if you can, from the fibre table below if you cannot. Negative values are allowed, for the knits that grow.
• Units toggles between centimetres and inches and converts the whole result, including the sample table.

The result reports both dimensions, the change in each, and the total area change, which is the number that matters when you are working out how much yardage to buy. The diagram beneath draws the before and after rectangles to scale, overlaid, so the asymmetry between the two directions is visible rather than merely stated. The address bar updates with a link that reproduces the whole scenario.

Formula for shrinkage in each direction, and the inverse

With an original length $L_0$, an original width $W_0$, and shrinkage percentages $p_L$ and $p_W$ applied independently to the two directions, the finished dimensions are:

Rearranging for the cut size given a target finished dimension is where most people go wrong. The percentage applies to the cut size, so you must divide rather than add it back:

The area change is not the sum of the two percentages, because the two reductions multiply:

For small percentages the difference between this and simply adding them is negligible, but at 10% in each direction the true area loss is 19%, not 20%, and at 20% each it is 36%, not 40%.

Why adding the percentage back is the classic mistake

Suppose you want a finished dimension of 100 cm from a fabric that shrinks 10%. The instinct is to cut 110 cm. That gives:

a centimetre short. The correct cut is $100/0.90=111.1$ cm. The error scales with the square of the percentage, so it is negligible on a 2% cotton and painful on anything heavily shrinking: at 20% the naive cut leaves you 4% short, and on a 150 cm curtain panel that is 6 cm of missing hem.

Worked example: a square tablecloth in unwashed cotton

You want a finished square tablecloth measuring 144 cm on each side. A swatch test on the cotton gives 4% shrinkage along the warp and 2% across the weft, which is a typical asymmetry for a mid-weight woven. Because the finished size is what is fixed, this is a cut-size problem:

So the piece you cut is not square. It is 150.0 cm along the grain by 146.9 cm across, and it becomes square only after washing. Cutting a 150 cm square instead would give a finished piece of 144.0 by 147.0 cm, visibly rectangular on a square table.

The area change is $100·(1-0.96·0.98)=5.9\%$, so the 2.20 m² you cut becomes 2.07 m² of finished cloth. Add hem allowance after this calculation, not before: a 2 cm hem on each edge means adding 4 cm to each finished dimension first, then dividing by the shrinkage factor, which gives a cut of 154.2 by 151.0 cm.

Swap in an 8% wool blend and the same 144 cm target needs a 156.5 cm cut, which is often the point where a project no longer fits the yardage you bought. Checking that before the scissors touch the cloth is the entire value of the exercise.

Typical shrinkage by fibre and construction

These are ranges observed in ordinary household laundering, not guarantees. The warp figure is generally the larger of the two for wovens.

The single most useful thing you can do with this table is not use it. Mark a square at least 25 cm on a side on your actual fabric, with its edges parallel to the warp and weft, launder it the way the finished piece will be cared for, and measure both directions again. That measurement beats any published range, because shrinkage varies between dye lots of nominally identical cloth by a full percentage point.

How the test standards define the number

The percentages quoted on a fabric spec sheet come from a defined procedure rather than a casual wash. AATCC Test Method 135, "Dimensional Changes of Fabrics after Home Laundering", applies benchmarks to the specimen before laundering and measures the pairs again afterwards, with four washing temperatures, three agitation cycles, and four drying procedures available so the test can mirror the care the garment will actually get. Length and width changes are reported separately, which is exactly the asymmetry this calculator preserves.

That matters when you compare numbers. A shrinkage figure is only meaningful alongside the wash conditions that produced it, and a fabric quoted at 2% on a cold gentle cycle with line drying may be at 5% on hot with a tumble dryer. When a supplier gives you one number without conditions, treat it as optimistic.

Limitations and assumptions of this shrinkage model

The arithmetic is exact. What it cannot represent is most of what makes real fabric interesting.

• Uniform shrinkage is assumed within each direction. Real cloth shrinks unevenly across a wide bolt, particularly near the selvedges where finishing tension differs, so a 300 cm-wide piece may not shrink identically at both edges.
• Skew and torque are not modelled. Twill fabrics and many knits rotate as they relax, so a rectangle can come out of the wash as a parallelogram. That is a separate measurement from dimensional change and standard test methods report it separately.
• Felting is not shrinkage. Wool subjected to heat and agitation mats irreversibly rather than relaxing. No percentage predicts it reliably, and the process cannot be reversed by blocking.
• Single-cycle assumption. The model applies one shrinkage event. Cotton and linen typically continue to lose a little over the next two or three washes; use a cumulative figure from a multi-wash swatch test if the tolerance is tight.
• Seams, interfacings, and trims shrink differently from the shell. A garment made from three fabrics with three different shrinkage rates will pucker at the seams regardless of how carefully each panel was cut.
• Fabric weight and hand change too. Shrinkage makes cloth denser and often stiffer, which alters drape in ways that no dimension in the result captures.
• Dye lot variation. Two bolts of the same fabric from different lots can differ by a full percentage point, so a swatch from one bolt does not certify another.

Questions sewers and upholsterers ask about shrinkage