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.
- 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.
- 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.
- Fabric preset is a shortcut that drops mid-range published figures for a common fibre and construction into the two percentage fields. It is a starting point only; editing either percentage afterwards switches the preset back to custom.
- Units switches between centimetres and inches. Switching converts the dimensions you have already entered as well as the result, so the scenario itself is preserved rather than silently relabelled.
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 and anchored to a common corner, 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.
Below the diagram, the Cut Allowance cutting-table game turns the same arithmetic into a hands-on exercise: you size a pattern piece on a bolt of fabric, commit it, and watch an animated wash-and-dry cycle pull it in further along the grain than across it. Both the calculator and the game are teaching estimates built on published dimensional-change figures, not a specification for any particular bolt of cloth.
Formula for shrinkage in each direction, and the inverse
With an original length , an original width , and shrinkage percentages and 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 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 , 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.
| Fabric | Length (warp) | Width (weft) | Notes |
|---|---|---|---|
| Cotton woven, untreated | 3–6% | 1–3% | Most of it in the first wash; hot water and tumble drying push toward the top of the range |
| Cotton woven, Sanforized | ≤1% | ≤1% | Residual shrinkage warranted not to exceed 1% in either direction |
| Raw denim, unsanforized | 7–10% | 3–5% | Sold deliberately untreated; sizing assumes a soak before wear |
| Linen | 3–5% | 2–4% | Relaxes further over several washes |
| Wool woven | 3–8% | 3–8% | Agitation causes felting, which is irreversible and not modelled by any percentage |
| Cotton jersey knit | 5–8% | −2 to +3% | Frequently widens as it shortens; enter a negative width figure |
| Polyester and poly blends | 0–2% | 0–2% | Thermoplastic fibres are heat-set and largely dimensionally stable |
| Viscose and rayon | 5–10% | 3–7% | Loses substantial strength when wet; handle gently |
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. The international equivalents are ISO 5077, which defines the dimensional-change determination, and ISO 6330, which defines the domestic wash and dry procedures the determination runs on.
All of them define the reported figure the same way: a signed percentage of the original dimension, with the benchmark distance measured before laundering as and after as , computed once for length and once for width.
A specimen that shrinks therefore reports a negative dimensional change. Trade practice, and this calculator, flip the sign and speak of shrinkage as a positive number, so a spec sheet reading −5.0% length change is 5% length shrinkage here. Note also that the divisor is the original dimension, which is precisely why the inverse problem needs a division rather than an addition.
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.
Sources. The arithmetic is elementary and derived in full above. The external anchors are the standards that define how a dimensional-change percentage is measured and reported, and the trademark that defines what pre-shrunk guarantees.
- Definition of the reported figure: ISO 5077:2007, Textiles — Determination of dimensional change in washing and drying, which reports the change as a signed percentage of the original dimension, determined separately for length and width.
- Wash and dry conditions behind that figure: ISO 6330:2021, Textiles — Domestic washing and drying procedures for textile testing, which specifies the reference machines, wash programmes, detergents, ballast, and the six drying procedures (line, line drip, flat, flat drip, flat press and tumble).
- North American equivalent: AATCC TM135, Dimensional Changes of Fabrics after Home Laundering, which measures length and width change separately from benchmarks applied before laundering, across four wash temperatures, three agitation cycles and four drying procedures, and reports each to the nearest 0.1%.
- Pre-shrunk definition: the Sanforized controlled compressive shrinkage process, a registered trademark under which residual shrinkage is warranted not to exceed 1% in either direction under the reference wash test.
- Typical fibre ranges in the table are observed household-laundering values and should be treated as starting estimates, not specifications. The calculator and the Cut Allowance game are learning tools, not a substitute for a swatch test on the cloth in front of you.
Questions sewers and upholsterers ask about shrinkage
Why can I not just add the shrinkage percentage to my target size?
Because the percentage applies to the cut size, not to the finished size, so adding it back undershoots. To finish at 100 cm after 10 percent shrinkage you must cut 100 divided by 0.90, which is 111.1 cm, not 110 cm. Cutting 110 gives you 99 cm. The error is small at 3 percent and grows quickly: at 20 percent shrinkage the naive method leaves you 4 percent short.
Why does the calculator ask for two shrinkage percentages?
Woven fabric almost never shrinks equally in both directions. Warp yarns run the length of the bolt and are held under the most tension during weaving and finishing, so they have the most stored relaxation to release and typically shrink more than the weft. Standard test methods report length and width change separately for exactly this reason, and averaging them into one number loses information you need when cutting.
What does Sanforized or pre-shrunk actually guarantee?
Sanforized is a registered trademark for a controlled compressive shrinkage process, and fabric carrying it is warranted to have residual shrinkage of no more than 1 percent in either direction under the reference wash test. Pre-shrunk without that mark is not a defined term and carries no numeric guarantee, so it is worth testing a swatch rather than assuming the shrinkage is negligible.
How do I measure my own fabric's shrinkage?
Mark a square on the fabric before washing, at least 25 cm on a side, with the sides parallel to the warp and weft. Launder and dry it exactly the way the finished piece will be cared for, then measure both directions again. Shrinkage is the change divided by the original marked length, times 100, computed separately for each direction. A larger marked square reduces measurement error.
Does the same fabric shrink again on the second wash?
Most of the relaxation happens in the first laundering, but cotton and linen typically continue to lose a little in the next two or three cycles before stabilising. Test methods usually specify multiple wash cycles for this reason. If a project has tight tolerances, wash the test swatch three times rather than once and use the cumulative figure.
Can fabric grow instead of shrinking?
Yes, and knits do it routinely. A jersey knit that shortens in length often widens across the course at the same time, so the width change is negative shrinkage. Enter a negative percentage in that direction and the calculator will handle it correctly, reporting a larger finished width and a smaller area loss than the length change alone would suggest.
Status messages will appear here.
Cut Allowance: size the piece before the wash, not after
A bolt of cloth runs down the cutting table with the lengthwise grain, the warp, pointing from top to bottom. Each order names one garment piece, the size it must measure after laundering, and the fibre and care method it will live with. Size the cut rectangle, commit it, and an animated wash-and-dry cycle pulls the piece in — further along the grain than across it, which is the part most people miss. The pale ghost outline is the finished target: if the shrunken piece still covers it in both directions the piece is good, and if it falls short in either one it is scrap. Extra allowance is safe but eats the bolt, and the bolt runs out.
The trap is deliberate. A fabric that shrinks 5% needs an allowance of , not 5%, because the percentage is taken off the cut size. Dial in exactly the spec figure and the piece lands a hair under target every time.
Keyboard: focus the cutting table, then Up and Down change the cut length along the grain, Left and Right change the cut width across the grain, and holding Shift gives fine 0.1 cm steps. Space or Enter commits the cut and runs the wash, then advances to the next order. H chalks the exact required cut onto the cloth but halves the points for that piece, and R restarts the run. Pointer or touch: drag the round handle at the lower-right corner of the pattern piece to resize both directions at once, or drag anywhere inside the piece; tap the table once to give it keyboard focus.
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Press Start cutting to load the first order onto the table.