How Do I Find My Size in Trousers? Your No-Nonsense Guide

By Ella Blake, Tellar Stylist

To find your size in trousers, you need three measurements: your waist, your hips, and your inside leg — and once you have those three numbers, you'll never have to guess again. It sounds obvious, but you'd be amazed how many of us (myself included, for years) were winging it based on a vague "I'm usually a 12" and then standing in a changing room wondering why nothing fits properly.

Let me share a personal low point. I once ordered four pairs of trousers from four different brands in what I believed was "my size." Two were borderline unwearable, one needed a belt the size of a hula hoop, and one — one! — fitted beautifully. I had wasted a Saturday afternoon and a small fortune in return postage. It was that afternoon that made me properly obsessive about understanding trouser sizing. Consider this your shortcut.

The Three Measurements You Actually Need

Grab a soft measuring tape (the kind from a sewing kit — not a DIY tape measure, trust me) and measure the following:

Waist: Measure around the narrowest part of your natural waist — usually about an inch above your belly button. Keep the tape snug but not tight. Stand naturally; don't hold your breath in!
Hips: Measure around the fullest part of your bottom and hips. This is roughly 8–10 inches below your waist. This measurement is crucial for wide-leg and straight-leg cuts — it's where most trousers are actually fitted.
Inside leg (inseam): Measure from your crotch down to your ankle bone along the inside of your leg. If you have a well-fitting pair of trousers, you can measure the inseam of those instead — lay them flat and measure from the crotch seam to the hem.

Write those three numbers down. Photograph them on your phone. You will use them constantly.

Why UK Trouser Sizing Is So Confusing

Here's the frustrating reality: UK trouser sizing is wildly inconsistent between brands. A size 12 in M&S (legendary for generous cuts and excellent trouser fits across multiple leg lengths) can be noticeably smaller than a size 12 in Zara, which tends to run on the slimmer side, especially through the hip and thigh. This is called vanity sizing, and brands do it deliberately — it's a surprisingly calculating bit of consumer psychology designed to keep you loyal to the brand whose sizing "always works" for you.

This is why checking a brand's actual size guide against your measurements is so important, rather than assuming your usual number will translate.

How to Use a Brand Size Guide Properly

Most brands publish a size guide on their website — but there are a couple of things to watch out for:

Check whether the measurements are "to fit" or "garment measurements." "To fit" means your body measurement; "garment measurement" means the actual fabric dimensions (which will be larger to allow for ease of movement). Always use "to fit" measurements to find your size.
Go by your largest measurement. If your hips put you in a size 14 but your waist says 12, size up and use a belt if needed. Trousers that are too tight across the hip will never look good regardless of how much you want them to.
Don't ignore the leg length. Brands like Next and M&S offer short, regular, and long leg options — use them. A petite length isn't just for petite women; it's simply a shorter inseam, and if yours measures under 29 inches, petite trousers will sit properly and not bunch at the ankle.

Where to Shop — By Budget

High street: Mango is brilliant for tailored trousers with a Continental cut — they photograph beautifully and hold their shape well. COS does clean, architectural wide-leg and straight-leg styles in quality fabrics; sizing is reliable and consistent. Whistles has some of the best-cut smart-casual trousers on the high street, with particularly flattering mid-rise options. Jigsaw is excellent for linen and wool-blend styles — their sizing leans classic and accommodates a fuller hip comfortably. Hobbs is a go-to for workwear tailoring with consistent sizing and a good range of leg lengths. Me&Em invests in quality fabrics and is a reliable choice for straight-leg and wide-leg cuts that photograph well and wear even better. Reiss offers beautifully constructed trousers with a slightly elevated feel without the designer price tag — their tailored styles are consistently well reviewed.

Premium: Paige is widely considered one of the best brands for fit — they use a sizing system based on actual waist measurements in inches, which makes finding your number far more precise than a generic UK 10/12/14. Citizens of Humanity offers similar precision sizing and their wide-leg styles are exceptionally flattering across different body shapes.

Independent brands worth knowing: Albaray is a quiet gem — a UK brand focused on quality natural fabrics with a relaxed, elevated feel. Their trousers come in considered cuts with a clear size guide that's genuinely accurate. Beaumont Organic is another independent label I've been recommending for years; organic cotton, ethical production, and trousers that actually fit like they were cut by someone who understands a real body. Both are worth bookmarking.

A Quick Note on Rise

Rise — the distance between the crotch seam and the waistband — makes an enormous difference to comfort and silhouette and almost nobody talks about it enough. A low-rise trouser will sit on your hip bones; a high-rise will sit at or above your natural waist. If you find trousers constantly gape at the back waistband, a high-rise cut will usually fix it. If you find they pull across the stomach, a mid-rise with a little stretch in the fabric will be kinder. Don't just focus on waist and hip — check the rise measurement in the size guide too.

The Shortcut: Use Tellar to Find Your Exact Size Across 1,500+ Brands

Never Guess Your Trouser Size Again — Use Tellar

Tellar.co.uk is the UK's leading free sizing tool — and it's an absolute game-changer for trouser shopping. You enter your measurements once, and it matches you precisely to your size across 1,500+ brands instantly. No more size guides, no more guesswork, no more return parcels.

Measure once — bust, waist, hips, or your existing brand size
Use the Store Size Lookup tool to get your precise size in any brand — COS, Reiss, M&S, Mango, and 1,500 more
Always free — no downloads, no subscriptions, works in-browser

And while you're there, explore the Tellar Fashion Hub — a library of free, honest, unsponsored style guides written by real stylists. No ads, no brand deals, no fluff. Just genuinely useful advice.