Quick answer: Leggings go sheer under stretch because stretching spreads the same yarn over more area. Squat to parallel and a panel typically extends about 40% in length and 30% in width, so its area grows 1.82× — and a 260 GSM fabric is, for that moment, covering the area with the mass of roughly 143 GSM. Every opacity floor you have ever read is quoted for fabric lying still. The only test that predicts the mirror is a stretched one.
The arithmetic nobody puts on the hangtag
GSM — grams per square metre — is a measure of mass spread over area. Both halves of that matter, and only one of them is printed anywhere. When you stretch a knit, the mass is fixed: no yarn is added. The area is not. So the number that decides whether light gets through falls the moment you move, and it falls by the product of the stretch in both directions, not the sum.
Here is the whole effect in one table. Read down to the movement, across to the fabric weight, and the cell is roughly what that fabric is behaving like at that moment.
| What you are doing | Stretch (length × width) | Area becomes | 200 GSM → | 230 GSM → | 260 GSM → | 280 GSM → |
|---|---|---|---|---|---|---|
| Standing, walking | 20% × 15% | 1.38× | 145 | 167 | 188 | 203 |
| Lunge, climbing stairs | 30% × 20% | 1.56× | 128 | 147 | 167 | 179 |
| Squat to parallel | 40% × 30% | 1.82× | 110 | 126 | 143 | 154 |
| Below parallel, or a deep forward fold | 50% × 40% | 2.10× | 95 | 110 | 124 | 133 |
| Extreme extension — or a size too small | 60% × 50% | 2.40× | 83 | 96 | 108 | 117 |
Two things to hold on to. First, this is a first-order geometric model: real knits also change thickness as they extend, filaments flatten and spread, and light transmission does not fall in a perfectly straight line with areal density. The numbers show the size and direction of the effect, not a laboratory reading. Second, and less comfortably — the effect is large enough that the modelling error hardly matters. Any fabric loses roughly 45% of its effective weight in a parallel squat. Nothing in the specification list closes a gap that big; it can only start from a high enough place that the gap does not reach through.
Why the same stretch ruins nude and survives black
Dye does work that weight does not. A deep black yarn absorbs light that would otherwise pass between the loops, so a black fabric can be lighter and still read solid. A nude or pale shade has almost nothing doing that job, and every gram has to be structural. That is the entire reason our own floors are not one number.
| Shade family | Our relaxed-state floor | Effective at squat stretch (1.82×) | What that means in the mirror |
|---|---|---|---|
| Black and true darks | 190 GSM | 104 GSM | Depends almost entirely on the dye depth doing the work |
| Mid-tone colours | 220 GSM | 121 GSM | Fine until the fabric is also thinned by wear |
| Light shades and nudes | 240 GSM | 132 GSM | Survives most of what a class asks of it |
These floors are Yesseam production data — what we will and will not ship — not an industry standard; there is no ISO or ASTM method that scores apparel opacity directly. Read them next to the second column and the shape of the problem is clear: a pale legging has to start almost a third heavier than a black one to arrive at the same place under load.
The five things that decide whether yours survives the squat
| Variable | What it actually does | How you can check it |
|---|---|---|
| Fabric weight (GSM) | Sets the starting point every other variable works from. It is necessary and nowhere near sufficient. | Weigh nothing yourself — ask for the figure and the test method behind it (ASTM D3776 covers most fabrics). |
| Shade depth | Dye in the yarn absorbs light that would otherwise pass through. A nude legging has almost no dye doing that work, which is why its floor is 50 GSM higher than black. | Hold the seat up to a window before you ever put it on. |
| Knit structure | A dense double-knit closes the gaps between loops; an open rib or pointelle opens them. Two fabrics at the same GSM can differ visibly. | Stretch a small area and look for the loop pattern separating into a grid. |
| Yarn and filament count | Finer multifilament yarns pack more filaments into the same mass, covering better than a coarse yarn at identical weight. | You cannot see this — it is why identical spec sheets still produce different fabrics. |
| Recovery (growth) | A fabric that does not snap back sits permanently stretched after a few wears, so its effective weight never returns. ASTM D2594 is the method that puts a number on it. | Pull the waistband firmly, let go, and watch whether the loops close again immediately. |
Only two of those five are things a shopper can assess, which is the honest reason this failure keeps happening at retail. The rest are decided at the mill, months before anything reaches a shelf. If you are briefing a factory rather than buying a pair, the companion piece on how squat-proof leggings are actually built covers the same five from the production side, with the test protocols.
Four tests you can run in sixty seconds
| Test | How | A pass looks like | What it proves |
|---|---|---|---|
| The window test | Hold the seat panel flat against daylight, unstretched. | Light passes but you cannot read shapes through it. | Only that the fabric is dense at rest. It is the weakest of the four and the one most shops rely on. |
| The two-hand stretch | Grip the seat panel and pull it to roughly one and a half times its width against a light background. | The knit stays a solid field, not a visible grid of loops. | This is the one that matches what a squat does. If it fails here it will fail in a class. |
| The full squat, in daylight | Put them on, squat to parallel in front of a mirror in natural light — not shop lighting. | No change in colour depth across the seat. | The only test that includes your own body and your own size. Shop fluorescents flatter fabric; windows do not. |
| The bend-and-hold | Fold forward and hold for thirty seconds, then stand and look at the seat. | The fabric closes back immediately with no lingering pale patch. | Recovery. A pale patch that fades slowly is growth, and growth is what turns a good legging sheer by month three. |
Do the second one in the shop. It is slightly awkward and it is the only one of the four that costs nothing and predicts something.
Why they were fine, and then weren’t
A pair that passed on day one can fail by month six without anything dramatic happening, and it is worth knowing which of the two mechanisms yours has.
Growth is the fabric not coming all the way back. Every knit recovers a little short of its original dimensions; a well-built one recovers to within a few per cent, a poorly built one settles several per cent larger and stays there. Once that has happened the garment is permanently at the top of the first row of our table before you have even moved. This is measurable — ASTM D2594 is the published method — and it is the single most useful question to ask a brand that talks about quality.
Laundering is the slower one. Repeated washing and drying changes dimensions and works the surface, and hot washes accelerate both. ISO 6330 defines the domestic wash and dry procedures textiles are tested under, and ISO 5077 defines how the resulting dimensional change is measured; between them they are why “tested after 30 washes” is a claim with content and “durable” is not. In practice: cold wash, no fabric softener, and never a hot tumble dry, which is the cheapest opacity insurance available.
Sweat is worth one sentence of its own, because it is often blamed and rarely guilty. Wet fabric transmits more light than dry fabric, so a legging can look darker and read more transparent in the same minute. It is a temporary state, not damage — how the fibre handles moisture is a separate property covered in how moisture-wicking fabrics actually work.
What to ask for — whether you are buying one pair or five hundred
Three questions get further than any amount of reading:
“What is the GSM, and in which shade?” A brand that answers with one number for a whole colour range has not tested the range. “What is the growth figure after repeated extension?” Under 8% is a reasonable expectation for performance knits and under 5% for compression; a brand that has the number will say it. “Was opacity checked at full stretch on the lightest colourway?” The lightest shade is the one that fails first, so it is the only one whose answer is informative.
If you are on the other side of this — specifying rather than buying — the numbers we hold production to are published in full in the QC thresholds a factory actually works to.
Sources
Opacity itself has no dedicated apparel test standard, which is exactly why the inputs matter. Each of these was opened and its current status checked on 8 September 2026.
| Standard | Why it is here |
|---|---|
| ASTM D3776/D3776M-20(2025) — Mass Per Unit Area (Weight) of Fabric | The test method behind any GSM figure worth quoting; applicable to most fabrics, knits included |
| ASTM D2594/D2594M-21 — Stretch Properties of Knitted Fabrics Having Low Power | How stretch and growth are measured on knits — growth is what makes a legging sheer over months |
| ASTM D737-18(2023) — Air Permeability of Textile Fabrics | A standardised read on how open a fabric structure is, which tracks with how much light gets through |
| ISO 20932-2:2018 — Determination of the elasticity of fabrics, Part 2: Multiaxial tests | The standard written for fabric deformed across its surface — the closest published method to what a squat does |
| ISO 20932-1:2018 — Determination of the elasticity of fabrics, Part 1: Strip tests | Single-direction elasticity, the figure most mills quote |
| ISO 6330:2021 — Domestic washing and drying procedures for textile testing | The wash cycles a fabric must survive before any durability claim means anything |
| ISO 5077:2007 — Determination of dimensional change in washing and drying | Puts a number on the shrink-and-grow that changes a garment’s effective weight after laundering |
| ISO 3801:1977 — Textiles: mass per unit length and mass per unit area | The ISO counterpart, cited here with its own caveat: it is written for woven fabrics, which is why knit GSM should reference ASTM D3776 |
FAQ
Why do my leggings only go see-through when I squat?
Because stretching spreads the same yarn over a larger area. Extend a panel 40% in length and 30% in width and its area grows to 1.82 times the original, so a 260 GSM fabric is momentarily covering that area with the mass of roughly 143 GSM. Nothing about the fabric has changed — only how thinly it is spread. That is why a pair can look perfectly solid on the hanger and fail in a mirror.
What GSM do leggings need so they are not see-through?
Our own production floors are 190 GSM for black and true darks, 220 GSM for mid-tone colours and 240 GSM for light shades and nudes, all measured relaxed. Those are starting points, not guarantees: at squat stretch a 240 GSM nude is behaving like about 132 GSM, so structure, yarn and dye depth have to carry the rest. Treat any single GSM number quoted without a shade and a knit structure as incomplete.
Can black leggings be see-through too?
Yes, though they have the most margin. Black gets help from dye depth absorbing light that lighter shades let through, which is why its floor sits 50 GSM below nude. Black still fails at extreme extension, in a size too small, or once the fabric has grown and thinned with wear — the failure is just less common and less obvious.
Why were my leggings fine at first and sheer a few months later?
Two things compound. Growth: a fabric that does not fully recover sits permanently stretched, so its effective weight never returns to the original figure — ASTM D2594 is the method that measures this. And laundering: repeated washing and drying changes dimensions and abrades the surface, which ISO 6330 and ISO 5077 exist to test under controlled conditions. A legging that failed neither test when new can still fail both after thirty washes.
How can I test leggings for see-through before I buy them?
Stretch the seat panel to about one and a half times its width against a light background and look for the loops separating into a visible grid. That single check predicts squat performance far better than holding an unstretched garment up to a window, which mostly tells you the fabric is dense at rest. If you already own them, squat in front of a mirror in daylight rather than in shop lighting.