Every piece of underwear has a small label with numbers and fiber names on it. Most of the time, nobody looks at it. And yet that label tells you almost everything about how the piece will actually feel when you wear it: softness, stretch, breathability, how well it holds up over time. Knowing how to read it means you stop picking underwear at random and start choosing with intention.
This guide explains how to decode the fiber content label on seamless underwear, fiber by fiber, and what each percentage actually means for your everyday comfort.
Why fiber content labels are mandatory in Canada
Under Canada's Textile Labelling Act, every textile item sold in the country must show its fiber content on a legible label that you can check before you buy. Fibers must be listed in descending order of percentage by total weight of the fabric. It's a legal requirement, not the manufacturer's choice.
Here's what that means for you as a shopper: the first fiber listed is always the one present in the largest amount. And the total always has to add up to 100%. If a label says "63% nylon, 37% spandex," the fabric is made of exactly those two fibers, with no other material hidden in the main fabric.
Some underwear also lists the gusset's content separately, because that piece is often cotton and differs from the main fabric. It's good manufacturing practice and a sign of transparency.
The most common fibers in seamless underwear
Nylon: lightweight, soft and durable
Nylon is a synthetic fiber made from polyamide. In seamless underwear, it's usually the main fiber, the one that gives the fabric its lightness, its soft feel and its resistance to wear. A high-nylon fabric is thin, glides over your skin without catching and keeps its shape wash after wash.
Nylon dries quickly, which makes it really pleasant in warm weather and on active days. It doesn't soak up moisture the way cotton does; it lets it evaporate, which keeps the fabric dry against your skin.
Quality seamless underwear usually contains between 60% and 80% nylon. Below 60%, the fabric tends to be less fluid and less soft. Above 80%, it can lack stretch if there isn't enough spandex to make up for it.
Spandex: stretch and shape recovery
Spandex, also called elastane or Lycra depending on the brand, is the fiber that gives fabric its stretch. It can stretch to several times its original length and snap back to its original shape, which is called elastic recovery. That property is what creates the "second skin" feel: the fabric follows every move your body makes without ever squeezing.
In seamless underwear, the spandex percentage is key to support and how the piece feels on. Too little, roughly under 15%, gives you a fabric that stretches but doesn't recover as well, which can lead to slipping or fabric that gets baggy over time. A high percentage, like 37%, gives very pronounced stretch and firm support without stiffness.
It's also the fiber most sensitive to heat and chemicals. Washing in hot water or tossing it in the dryer gradually breaks down the polyurethane chains that make up spandex, which permanently reduces the fabric's stretch. To dig deeper into this, the article why your seamless underwear loses its stretch explains the mechanisms in detail.
Cotton: natural breathability for the gusset
Cotton is a natural fiber whose main strength is breathability. It absorbs moisture, lets air circulate and is gentle on sensitive skin. That's why it's recommended for the gusset, the lining at the crotch, even in underwear whose main fabric is synthetic.
A 100% unblended cotton gusset keeps all of those qualities. A cotton gusset blended with polyester is less breathable and less absorbent, two qualities that matter a lot for intimate health. When you read a label, check whether the gusset's content is listed separately. A line like "gusset: 100% pre-shrunk cotton" is a good sign: it means the cotton was treated so it won't shrink in the wash, which keeps the fit of your underwear intact over time.
Cotton isn't ideal as the main fabric for seamless underwear, though. It absorbs and holds on to moisture, loses its shape more easily without spandex to support it, and doesn't lend itself well to the circular knitting that gets rid of side seams. That's why pure cotton works better as a complement than as the main material in seamless underwear.
Microfiber: softness and fineness
Microfiber isn't a fiber in the strict sense; it's a way of processing nylon or polyester to get extremely fine yarns. Microfiber fabric often feels softer than standard nylon, because the ultra-fine fibers create a smoother surface.
On a label, you won't see "microfiber" listed as a fiber: you'll see "nylon" or "polyester," sometimes with "microfiber" mentioned in the product description. Microfiber is loved for its lightness and softness, but its stretch always depends on the percentage of spandex it's paired with.
Polyester: durable and affordable
Polyester is a common synthetic fiber in entry-level underwear. It stands up better than nylon to certain kinds of abrasion, but it's less soft and less fine. In seamless underwear, polyester sometimes partly or fully replaces nylon to cut manufacturing costs.
On a label, a blend like "80% polyester, 20% spandex" points to a functional seamless fabric, but usually one that's less soft and less lightweight than nylon. For underwear you wear every day, nylon tends to offer a better experience.
Modal and bamboo: natural softness
Modal is a synthetic fiber derived from beechwood cellulose. Bamboo, for its part, is processed into a fiber similar to viscose. Both are known for their softness and breathability, and they're especially well suited to sensitive skin or skin that reacts to conventional synthetic fibers.
A label reading "60% modal, 40% spandex" or "70% bamboo, 30% spandex" describes soft, breathable underwear that usually dries more slowly than nylon and may not hold up as well to long-term wear. They're good options if you're looking for a more natural alternative while keeping the stretch of seamless underwear.
Reading the percentages: what they really tell you
The label doesn't just list which fibers are there: it tells you their proportions, which completely changes the fabric's texture and behavior.
A fabric that's 95% cotton and 5% spandex will be breathable and soft, but not very stretchy and hard to work without seams. It won't make for quality seamless underwear.
A fabric that's 80% nylon and 20% spandex will be lightweight, soft and stretchy enough for most uses, but its support will be less pronounced than with a higher spandex content.
A fabric that's 63% nylon and 37% spandex, like the one in LICA Wear seamless underwear, strikes a balance designed specifically for the second-skin effect: enough nylon for softness and lightness, enough spandex for highly effective elastic recovery and firm support without stiffness. It's a ratio that requires top-quality materials to work properly, because low-quality spandex at 37% will give you a fabric that's too stiff or breaks down quickly.
So the fiber content on the label gives you a clue, but the quality of each fiber matters as much as the percentage. Two pairs with the same content on the label can feel very different depending on the quality of the materials and how the fabric is constructed. To understand how fabric construction affects comfort, the complete guide to fabrics for comfortable underwear covers these aspects in detail.
What the label doesn't always tell you
The fiber content label lists the fibers, but it doesn't say how the fabric was made. Two important points don't always show up on the label.
The type of knitting: a nylon-spandex fabric can be circular-knit, which eliminates side seams, or made as flat panels sewn together with seams. Both will have the same content on the label, but the first will be seamless and the second won't. That's why the words "seamless" in the product description are an essential complement to the fiber content label.
The edge finish: laser-cut edges, which get rid of any thick or stiff edge, don't show up on the label. That's something to look for in the product description or check visually in the photos.
Those two elements, circular knitting and laser cutting, are what set truly seamless underwear apart from underwear that's simply labeled that way. For everything about invisibility and what makes underwear truly comfortable, LICA Wear's complete guide to invisible underwear covers these design details.
Putting it into practice when buying seamless underwear
When you check the label on a pair of seamless underwear before buying, here are the three things to look at first.
The nylon-to-spandex ratio in the main fabric: aim for at least 15 to 20% spandex for functional stretch, and choose nylon over polyester for softness and lightness. A ratio of around 60-65% nylon and 35-40% spandex is a good benchmark for second-skin seamless underwear.
The gusset content: look for "100% cotton" or "100% pre-shrunk cotton" for the gusset. If the gusset's content isn't listed separately, it's worth asking or checking the product description.
The words "seamless" or "circular knit" in the description: the fiber content on the label isn't enough to confirm that a pair is truly seamless. How the fabric is made matters as much as what it's made of.
To see how these criteria apply to each cut, the article on which seamless underwear to choose for your body shape is a practical companion to this guide.
And to understand how fiber content affects care and longevity, the seamless underwear care guide explains why each fiber reacts differently to heat, detergents and drying.
FAQ: reading underwear fiber content labels
Why is the gusset's content sometimes listed separately on the label?
Because the gusset is often made from a different material than the main fabric. In quality seamless underwear, the main fabric is nylon and spandex for lightness and stretch, while the gusset is cotton for breathability and intimate health. The label then has to list both separately to comply with Canadian textile labelling regulations. If a label shows only one fiber content with no mention of the gusset, it may mean the gusset is made of the same material as the main fabric, which isn't ideal for comfort in the intimate area.
Does a high spandex percentage mean the underwear will feel tight?
Not necessarily. Compression depends on the size you choose, the quality of the spandex and how the fabric is constructed, not just the percentage. A fabric with 37% high-quality spandex, in the right size, will feel firm and supportive without squeezing. On the other hand, a fabric with 20% low-quality spandex in a size too small can feel tighter than a 37% fabric in the right size. The rule of thumb: if you're between two sizes, go with the smaller one, because high-spandex fabric adapts to your body without squeezing.
What's the difference between nylon and polyester on an underwear label?
Nylon and polyester are both synthetic fibers, but they feel different when you wear them. Nylon is generally softer, lighter and more durable for underwear. It has a silkier texture and does a better job of wicking moisture to the surface of the fabric. Polyester feels stiffer, is less fine and tends to trap more heat. For everyday underwear, nylon offers a better experience. Polyester can work in sports or entry-level underwear, but it doesn't reproduce the second-skin feel of a well-chosen nylon.
Reading a fiber content label means understanding what you'll actually be wearing before you even try it on. The nylon percentage tells you about lightness and softness. The spandex percentage tells you about stretch and support. A cotton gusset confirms breathability right where it matters most. Read together, those three pieces of information give you a clear picture of what the underwear will deliver day to day, well before the first wash.
