A dimensional change test has three standards in it. ISO 6330 washes and dries the specimen. ISO 3759 says how to prepare, mark and measure it. ISO 5077 ties the two together and says how to calculate and report the change.
Laboratories invest in the first and improvise the second. Yet a shrinkage limit of 3 % on a 350 mm gauge length is about 10 mm. A ruler read at an angle, a specimen nudged flat with the palm, or a mark that bled into a blot can move the result by a millimetre or two — a large share of the tolerance.
What ISO 3759 asks for on fabric specimens
The long-standing requirements for flat fabric specimens are straightforward:
| Item | Requirement |
|---|---|
| Specimen | 500 mm × 500 mm, edges parallel to length and width |
| Position in the piece | Not from the ends of the piece (at least 1 m in) |
| Marks | At least three pairs in each direction |
| Distance between a pair | At least 350 mm |
| Distance from edges | No mark closer than 50 mm |
| Reading | To the nearest 1 mm, on a smooth flat surface, without tension |
The current edition is ISO 3759:2011. Older texts excluded fabrics that stretch easily in use, such as many knits; the 2011 edition covers woven and knitted fabrics and made-up articles. For garments, ISO 3759 lists measuring points by garment type — body length, sleeve, chest width, waist, inside leg — preferably along seams or between seam intersections.
The marks themselves
ISO 3759 allows several marking methods: indelible ink with a template, fine contrasting threads sewn into the fabric, and, for thermoplastic materials, small holes made with a heated wire.
Problems we see in practice:
- Ink that is not indelible enough fades or spreads in a hot wash. A dot that has grown to 2 mm wide cannot be read to 1 mm.
- Marks placed by eye instead of with a template. If one pair is 352 mm and the next 347 mm, nothing is wrong yet — but a template makes misreadings easier to spot later.
- Marks too close to the edge, where fraying and edge curl distort the fabric. The 50 mm margin is there for a reason.
Mark each pair clearly with an identifier, so that the after-wash reading is paired with the right before-wash reading.
Timing: measure conditioned, not "dry"
Both measurements — before and after washing — are made on conditioned specimens in the standard atmosphere of ISO 139. A specimen that comes out of the tumble dryer warm and overdry is not ready to measure; neither is a line-dried specimen that "feels dry" after a night in a humid room.
Bring specimens to equilibrium in the standard atmosphere before measuring, and lay them flat, separately, so air reaches both faces. If the specimens were conditioned in a cabinet, take them out one by one rather than stacking them on the measuring table while you work through the batch.
Tension: the invisible error
ISO 3759 says to remove wrinkles without stretching. In practice, operators smooth a specimen outward with the flat of the hand — which stretches knits and loose weaves by a millimetre or two in exactly the direction being measured.
Better habits:
- Let the specimen settle on the table for a moment before you touch it.
- Smooth from the centre with a light touch, or not at all; most fabrics flatten under the rule itself.
- Do not let part of the specimen hang over the table edge.
- Use a rule or tape that is long enough, laid down without dragging the fabric.
Fibreglass measuring tapes drift with use. Older editions of ISO 3759 advised checking their accuracy at least every six months; include them in your calibration plan with the same seriousness as a balance.
The sign convention
ISO 5077 reports dimensional change as a percentage of the original dimension, with shrinkage negative and extension positive:
Dimensional change % = (final − original) / original × 100
The trap is a spreadsheet written years ago that calculates (original − final) / original. The magnitudes are identical; the signs are reversed. A report that says "+3.2 %" without the word "shrinkage" next to it will eventually be read the wrong way. Write the word as well as the sign.
How many specimens?
The precision of a single specimen is modest. The informative annex of the older EN 25077 text, based on a 1970s interlaboratory trial with a different wash method, put the 95 % confidence limits for one test on one specimen at roughly ±1.7 percentage points, falling below ±1 with four specimens. Those figures are dated, but the lesson is current: one swatch cannot decide whether a fabric passes a 3 % limit.
Test more than one specimen, report length and width separately, and report how many specimens and cycles were used.
Checklist for the measuring table
- Specimen 500 × 500 mm, at least 1 m in from the piece end?
- Three pairs of marks in each direction, at least 350 mm apart, 50 mm from the edges?
- Marks that survive the wash and can be read to 1 mm?
- Both measurements on conditioned specimens?
- No hand-smoothing tension, no overhang?
- Shrinkage written as negative and labelled in words?
Our reference washing machines and conditioning cabinet are made in our own workshop in Istanbul; the measuring table, though, is where your laboratory decides whether their accuracy reaches the report.