How to size underwear without trying it on (and get it right first time)
Underwear is one of the few things you can't try on before you buy. For obvious hygiene reasons, no brand worth trusting will accept returns or exchanges of opened underwear. Which means you have one shot at getting your size right.
Here's the four-step sizing method that gets it right on the first order, the most common mistakes, and the single rule that almost every brand gets backwards.
Step 1: measure your hip, not your waist
Most underwear sizing is keyed to hip measurement, not waist. The hip measurement is taken at the widest part — usually 18-22cm below the natural waist. For underwear, hip is what matters because the leg openings sit at the widest part of the body.
How to measure:
- Stand in front of a mirror, weight evenly on both feet
- Wrap a soft tape measure horizontally around the widest part of your hip / bum
- The tape should be snug but not compressing — flat against skin or thin fabric
- Read the measurement at the front (or in the mirror)
Don't measure your waist for underwear sizing unless the brand specifically tells you to. Waist size and hip size aren't the same number, and underwear sizes are calibrated to hip.
Step 2: cross-reference with the brand's actual chart, not generic AU/UK sizing
"Size M" varies by 4-6cm across brands. Trust the chart for the specific brand you're buying, not your usual size from a different brand. Specifically look for hip measurement ranges in cm.
For example, our chart for The Gym G:
- XS: AU dress 6-8, hip 85-90 cm
- S: AU dress 10, hip 90-95 cm
- M: AU dress 12, hip 95-100 cm
- L: AU dress 14, hip 100-106 cm
- XL: AU dress 16+, hip 106-112 cm
If your hip is 96cm, you're in the M range. If it's 99cm, you're at the top of the M range — close to L. The next step matters here.
Step 3: if you're between sizes, size DOWN
This is the rule that almost every brand gets backwards in their messaging.
The conventional advice you'll see in 80% of size guides: "If you're between sizes, size up for comfort." This is bad advice for performance underwear specifically.
Here's why:
- Performance fabric (recycled nylon with spandex) has memory and stretch. It softens and gives slightly with wear.
- A pair that feels "firm" on day one feels perfect by day three and slightly soft by day fourteen.
- A pair that feels "comfortable" on day one feels loose by week two — the elastic has settled into the larger size.
- Loose underwear isn't more comfortable. It rolls down at the waist, shifts during movement, and creates ridges where it bunches.
For cotton underwear, "size up" is debatable advice (cotton doesn't stretch much). For performance underwear, it's just wrong. Always size down when between, unless you have a specific reason not to (recent significant weight change, post-pregnancy, etc.).
Step 4: choose the right cut for your body, not just the right size
Three different sizes (XS / S / M) might fit you depending on how the underwear is cut:
- Low-rise thong: Sits at the hip. Doesn't cover the natural waist. Hip measurement is the only thing that matters.
- Mid-rise thong (most common): Sits below the navel. Hip measurement matters most, waist matters slightly.
- High-cut brief: Sits at or just below the natural waist. Both hip and waist matter — pick the size that fits your hip; if your waist is also at the top of the range you might want one size larger.
Most everyday performance underwear is mid-rise thong cut. Stick with hip measurement.
Common sizing mistakes
"I always wear M in clothes so I'll order M"
Underwear sizing isn't necessarily aligned with clothing sizing, especially for performance underwear. Use the measurement chart, not your usual size. AU 12 in tops doesn't necessarily mean M underwear.
"I'll order S because I want them tight"
Underwear that's actually too small creates four problems: rolling at the waist, digging into the leg openings, riding up at the back, and chafing. None of which is "tight" in a good way. Performance underwear is designed to fit firmly at your correct size — going below your size doesn't make it firmer, it makes it ill-fitting.
"I'll order both sizes and try"
You can't return either, so this just doubles your spend and you keep one. If you're seriously between sizes, look at our sizing-by-style notes (above) and pick based on the cut. If still unsure, email us — we'd rather help you pick than have you stuck with one that doesn't fit.
"I haven't measured in years, my old size is fine"
Bodies change. Most women are 1-2 cm different at hip from where they were three years ago. Take the 90 seconds and measure before ordering. Especially if you've had a baby, gone through significant weight change, or just turned a decade.
The "between sizes" decision tree
If your measurement is exactly at the boundary between two sizes (e.g. 95.0cm — between S and M):
- If you want firm support and the fabric is performance synthetic: size DOWN
- If the fabric is cotton or has minimal stretch: size UP
- If you carry weight more in the front: size UP
- If you carry weight more in the back/hip: size DOWN
- If you're post-pregnancy and your body is still changing: size at your CURRENT measurement, not pre-pregnancy
What if it doesn't fit when you try it on at home?
For our underwear specifically, and for most performance underwear, the rule is: try wearing them for a full day before deciding. Performance fabric softens with body heat and movement. A pair that feels firm on the first 30 seconds often feels perfect after lunch.
If after a full day they're still genuinely uncomfortable — pinching, rolling, leaving marks — they probably are the wrong size. Email us so we can recommend better for next time.
If they're invisibly comfortable by hour two, you got the size right and you're set.
What we make, in case you're wondering
The Gym G is sized in AU dress equivalents (XS/S/M/L/XL) keyed to hip measurement. Performance fabric (recycled nylon and spandex) — softens with wear. If between sizes, size down. Available in singles ($37) or in 4 / 7 / 10-packs at increasing savings.
Further reading
- The complete guide to underwear that disappears under everything — our pillar guide
- The right underwear for hot Pilates — sizing for movement-heavy use
Questions? Email hello@barethrills.com — we read every message.
