There is no international standard called “squat-proof”. It is a result — a finished garment, on a body, in a deep squat, under real light — and it fails most often in bulk, not in the sample. This is how we evaluate it, and the opacity test protocol we recommend writing on the PO.
Labs can measure light transmission, but approval is a finished-garment visual test under agreed conditions.
The seat of a legging can run far more strain than the relaxed fabric shows. Screen fabric at the strain your pattern creates.
Approve the lightest colour on the same base, not just black. Light shades fail first.
Re-test after the care cycle you sell. Pre-wash-only approvals cause bulk disputes.
Why leggings go see-through
Opacity depends on how much light passes between the yarns. Stretching opens the knit loops; a deep squat stretches the seat and back thigh more than any other movement. So a fabric that looks solid on the table can turn sheer exactly where customers look.
Four variables decide it: the fabric (weight, structure, yarn), the colour (light and bright shades transmit more), the pattern (how much negative ease the size chart builds in) and the light (daylight, store LEDs and phone flashes all behave differently).
Our three-gate evaluation
- Fabric screening at working strain. We calculate how far the seat panel stretches on the body from the pattern’s hip measurement against the target body, and stretch the fabric to that level over a ~400 lux light box. Screening relaxed fabric would pass almost anything.
- Finished-garment squat test. On the proto, a wearer holds a full squat; front and rear are checked by eye and photographed at a fixed distance under daylight and store-type LED. The hero colour and the lightest colour are tested on the same base.
- Post-wash re-check. The same test after the home-laundering cycle on the care label. Washing can relax or shrink a construction and change the result.
Only when all three gates pass does a base and colour go forward to PPS for a squat-proof programme.
Opacity Test Protocol buyers should put on the PO
- Colour set — approve the hero black and the lightest colour on the same base.
- Pose — full squat, thighs at least parallel, front and rear, on a finished garment (not a swatch).
- Lighting — one daylight and one store-LED photo, camera distance locked, no flash. Eye and camera both noted.
- Wash — re-run after the care cycle you will sell.
- Seam map — front rise, gusset and waistband roll called out; opacity is not only a fabric claim.
- Bulk check — the same test on garments from bulk fabric, per dye lot, before shipment.
Fabric levers that fix see-through
| Lever | Effect | Trade-off |
|---|---|---|
| Weight (g/m²) | More yarn per area, more cover | Warmer, heavier; GSM alone does not guarantee opacity |
| Structure | Double knit / interlock covers better than light single jersey | Firmer hand, higher cost |
| Gauge | Finer gauge = smaller loops = less light through | Mill-dependent; changes hand |
| Yarn lustre | Dull or full-dull yarn scatters light; bright yarn looks thinner when stretched | Changes sheen — check against the brand look |
| Elastane level | Higher power reduces how far the seat stretches | Firmer fit; size chart may need to move |
| Brushing / peach finish | Raises surface fibres for a softer, more covered look | Can increase pilling; test it |
| Pattern ease | Less negative ease at the seat = less strain | Changes the fit the brand wants |
PowerForm™ 215 (81% nylon 66 / 19% Lycra®) is our structured, squat-proof-checked base; AirTouch™ 190–200 g/m² is lighter and cooler, and needs more care on pale shades. Heavier 260–280 g/m² sculpting fabrics are a mill-development route, covered in best GSM for leggings.
Why the sample passed and bulk failed
The classic dispute: the approved sample was opaque, the bulk is not. The usual causes are a lighter bulk fabric lot (weight drifted within tolerance), a different dye lot or shade, a changed finishing setting, or a size range where the largest sizes stretch the fabric further than the sample size. Write weight tolerance and a per-lot opacity check into the PO, and test the largest size, not only the sample size.
A full reconstruction of this failure — and how it was fixed — is in the sheer leggings rescue case.
It means the specific base and colour passed the three gates above on your garment. It is not a blanket property of a fabric name, and we do not print it as a hangtag claim on your behalf.
Methods and sources cited
- No ISO/ASTM ‘squat-proof’ standard exists — opacity is agreed per programme
- ISO 6330 — domestic washing procedures for post-wash re-checks
- EN 14704-1 / ISO 20932-1 — strain levels and elasticity
Standards are named as the methods buyers commonly specify. Factory results are only those stated as on file; programme results are reported per brief.