Period Pants Size Guide: Your Perfect UK Fit (4–18)

What size period pants should you buy?

The size whose hip and waist ranges contain your own measurements in centimetres, not the size on your jeans label. Dress sizes are a rough translation that varies from shop to shop, and period pants need a closer fit than ordinary knickers because the gusset has to stay where the flow lands.

Here is Annahitah’s adult chart.

If your waist and hips land in different sizes, go with the measurement that matters for the cut. Hips decide the fit of briefs, Brazilians and thongs, because that’s where they sit. Waist matters more for high-waist styles, which sit at or near your natural waist and will roll or dig if that band is wrong. Annahitah’s high-waist styles are Shanti, Odessa, Athena, Valentina, Lilah, Luna and Gaia.

How do you measure your waist and hips?

Stand relaxed, use a soft tape, and measure hips at the widest point of your bottom and waist midway between your lowest rib and the top of your hip bone, breathing out normally. It takes two minutes and saves an exchange.

For hips, Brand A’s method is sound: stand with your feet together, find the widest part of your hips and bottom, keep the tape level all the way round and parallel to the floor, and don’t pull it tight. Brand B suggests feeling for your pelvic bones to locate the widest point. Measure over bare skin or thin underwear, not jeans. Do it twice, or ask someone to hold the tape at the back.

For waist, the British Heart Foundation’s method works for underwear as well as health checks: find the bottom of your ribs and the top of your hip bone, wrap the tape midway between the two, make sure it’s snug but not digging in, and breathe out naturally before you read it.

No tape measure? Wrap a piece of string or a phone charger cable round you, mark where it meets, and lay it against a ruler or a tape measure app.

How do UK dress sizes translate to centimetres?

A major UK clothing retailer’s own-brand chart puts a UK 10 at 72 cm waist and 96 cm hips, but every retailer cuts differently, which is why period pants brands ask for measurements instead. The same chart runs UK 8 at 63 cm waist and 88.5 cm hips, UK 12 at 73 and 98.5, UK 16 at 83 and 108.5, and the retailer itself says other brands vary.

Set that next to Annahitah’s chart and the “generous” note makes sense. Annahitah’s M covers UK 10-12 with a waist of 76-80 cm, which sits above where that retailer places a UK 10-12 waist (72-73 cm). The hip ranges are closer. The practical lesson is to trust the centimetres on Annahitah’s chart over the dress-size column, and to expect a slightly roomier cut than a high-street label gives at the same nominal size.

How do period pants sizes run, small or large?

It depends on the brand, and the two sizing habits are opposites. Annahitah describes its sizes as more on the generous side than the tight side, for comfort, and says that if you’re in doubt you should pick the smaller size. Brand A says the reverse: between sizes, go up. Brand B says period pants should sit snugger than ordinary knickers and to size up if that’s uncomfortable.

None of them is wrong; they’re describing their own cuts. What goes wrong is carrying a habit across. If you sized up with another brand and do the same with Annahitah, you’ll end up a size too big.

Whatever the brand, too big causes more trouble than slightly too small, because a loose gusset moves. Brand C makes the same point from its own fitting advice: pants that are too big sag at the seams and leak.

Should period pants feel tighter than normal knickers?

Yes, slightly, but never tight enough to dig in or leave marks. The absorbent gusset works by sitting against your body, so the pants need enough hold to keep it there when you sit, walk and sleep.

Brand C describes the two ways fit fails. Too big, and the pants sag at the seams, the gusset drifts and blood reaches the edge of the absorbent zone. Too tight, and moisture gets pressed to the seams and wicks out through the fabric. The right size feels like well-fitting underwear: you forget it’s there, and the gusset is flat against you when you check.

Bear in mind that your body changes across the month. Bloating in the days before and during a period can add to your waist and hip measurements, and postpartum bodies change week by week. Measure on an ordinary day if you can, and remember that Annahitah’s note about generous sizing already builds in some room.

Why do period pants leak at the front or back?

Because the fit has moved the gusset away from where the flow lands, not because the absorbency has failed. A Consumer NZ trial of two brands found that some testers felt the padding stopped short at the front and a few leaked there; others find blood runs backwards when they lie down and reaches past the gusset.

Front leaks point to a gusset that’s too short for your body or pants that sit too low; a high-waist style brings the absorbent zone higher. Back leaks at night point to the need for coverage up the back and, on heavy nights, a Super Heavy level. Annahitah says its Super Heavy styles are designed for overnight protection. If you’ve got the size right and still leak at an edge, try a different cut before a different size, because gusset length and position vary between styles.

If a waistband rolls down, the usual culprits are a waist measurement that sits above the band’s range or a high-waist style worn below the natural waist. Measure your waist and check it against the chart; if you’re at the top of a size, the next one up will hold better at the band even though the hips feel roomier.

What size fits teens and smaller frames?

Annahitah’s teen chart starts at XXXS, which fits a UK 2-4 with a waist of 60-64 cm and hips of 79-83 cm, and the brand describes it as a young average-size teenager.

One thing to know before ordering: only Valentina, the organic cotton style Annahitah calls its signature teenage pant, comes in XXXS. The other teen styles (Athena, Shanti, Lilah, Serendipity, Odessa) start at XXS, and so does the First Period Starter Set. For context, Brand A’s smallest teen size covers hips of 75-90 cm and Brand B’s youngest teen size 74-79 cm, so Annahitah’s XXXS sits in the same territory as other UK brands’ smallest sizes. Measure a teen the same way as an adult, over bare skin, and go by hips; age and height charts are a poorer guide than centimetres. Fit for ten to twelve year olds has its own guide in this series.

What if the size is wrong?

Unworn pants in perfect condition can be returned or exchanged within 30 days of purchase, so a wrong size is a swap, not a loss. Annahitah offers a full refund or exchange; you pay the return postage. Try the first pair on over clean underwear and check the fit before you wear it for real, because worn pants can’t go back.

To avoid the exchange altogether, measure before you order, read the centimetres rather than the dress size, and choose the smaller size if you’re between two. If you’re buying a bundle, note that one size applies to every pair in the set. When you’re ready, find your size on the chart and order against your hip and waist measurements.