How Do I Find My Size in Formal Trousers?

By Ella Blake, Tellar Stylist

Finding your size in formal trousers comes down to three measurements: your waist, your hips, and your inside leg. Get those three numbers right, match them to the specific brand you're buying from, and you'll avoid the most common formal trouser disasters — the dreaded seat pull, the waist gap, and the ankle that's somewhere around your mid-shin. I've had all three. In a job interview. We don't need to dwell on it.

The reason formal trousers are trickier than jeans or casual bottoms is that the fabrics have very little give. There's no stretch to bail you out if you're between sizes. What you measure is what you get — which is actually brilliant news once you know your numbers, because you can shop with real confidence rather than playing changing room roulette.

The Three Measurements You Need

Grab a soft tape measure and note these down before you shop for anything:

Waist: Measure at your natural waist — the narrowest part of your torso, usually a couple of inches above your navel. Keep the tape snug but not tight. Don't hold your breath. Your trousers will need to survive a full day of normal human activity.
Hips: Measure around the fullest part of your seat, typically 8–9 inches below your natural waist. This measurement is crucial for formal trousers because a structured waistband will not stretch over a hip it wasn't cut for.
Inside leg (inseam): Measure from your crotch down to your ankle bone. With formal trousers the length matters enormously — too short and the whole silhouette is ruined; too long and you're shuffling around on pooling fabric.

Store these on your phone. They are your secret weapon for online shopping and will save you an unreasonable amount of time.

How Formal Trouser Sizing Actually Works

Unlike casual trousers which typically use UK sizing (10, 12, 14), formal and tailored trousers — especially from premium brands — often use a dual sizing system: waist in inches paired with a length in inches. So you might see W30/L30, or W32/L32. This is far more precise than generic UK sizing, and once you know your numbers it makes shopping infinitely simpler.

High street brands tend to stick to UK sizing with short/regular/long length options. As a rough conversion:

UK 8 = approx W26–27
UK 10 = approx W28–29
UK 12 = approx W30–31
UK 14 = approx W32–33
UK 16 = approx W34–35

I say "approx" because sizing varies significantly between brands — and with formal trousers, that variance has real consequences. A size 12 at Reiss cuts smaller than a size 12 at Phase Eight. Neither is wrong; they're just cut to different fit philosophies. This is exactly why matching your actual measurements to specific brands matters so much.

Finding the Right Cut for Your Shape

Fuller Hips and Thighs

The number one issue with formal trousers for curvier frames is the hip-to-waist ratio. Most trousers are cut for a relatively straight silhouette, so if you have a more pronounced curve, you'll often find that sizing up to fit the hip leaves you with excess fabric at the waist. The solution? Look for brands that have specifically addressed this with a shaped waistband or adjustable tabs. Hobbs — a brand beloved by stylists for its precise, grown-up tailoring — does this particularly well, with trouser cuts that accommodate a fuller hip without sacrificing the waist. Me&Em is another excellent option; their tailored trousers have a slightly higher rise that sits beautifully on curvy figures and prevents the dreaded seat pull.

Petite Frames

Length is everything here. Even a "short" length in regular sizing can hit awkwardly on a petite frame. Always shop the petite range where possible — Massimo Dutti offers an excellent petite edit of their tailored trousers, and their quality is genuinely exceptional for the price point. LK Bennett is another strong pick; their petite range is consistently well-reviewed for proportion and fabric quality, and their tailored styles work beautifully for both the office and formal occasions.

Taller Frames

Regular lengths will hit mid-calf and ruin the entire line. Hugo Boss is outstanding for tall women — their tailored trousers come in longer inseam options as standard, and the quality of the cut makes them worth every penny. Ted Baker also tends to run slightly longer in the leg across their tailored range, which works well if you're 5'8" and above.

Straighter, More Athletic Builds

If you have a straighter hip-to-waist ratio, formal trousers will likely be your easiest trouser category — the cuts are typically designed with a less exaggerated curve. A wide-leg or straight formal trouser will look incredibly sleek. Banana Republic does an exceptional straight-leg tailored trouser that's become something of a workwear staple for good reason, and Cos offers minimal, beautifully-cut formal styles with slightly relaxed proportions that suit an athletic frame perfectly.

The Best Formal Trousers to Shop Right Now

Here's where I'd actually send clients, depending on occasion and budget:

Reiss — one of the best high street options for sharp, city-ready tailored trousers. Their Haisley and Joanne styles are consistently praised in Vogue and Grazia for their clean lines. Sizing runs true to fit, which I appreciate enormously.
Hobbs — the tailoring queen of the British high street. Their wide-leg formal styles are particularly good, and the fabrics feel genuinely elevated. Perfect for the office and smart occasions alike.
Me&Em — slightly more fashion-forward than Hobbs but equally precise in its tailoring. Their pull-on formal trousers are a lifesaver for long days when you need comfort without sacrificing polish.
Massimo Dutti — the brand that often flies under the radar but shouldn't. Their formal trouser quality rivals brands twice the price, and their fabric selection is exceptional. Worth the slightly higher high street price tag.
LK Bennett — a trusted choice for formal and smart-casual occasions. Particularly good for petite and standard frames. Their straight-leg styles are consistently elegant.
Phase Eight — brilliant for occasion dressing. Their wide-leg formal trousers in particular have been a consistent bestseller and are often recommended in style press for their flattering, relaxed cut.
Hugo Boss — for those wanting a true investment piece. Impeccably cut, excellent fabric quality, and the brand is particularly strong for women with longer legs. A pair of Boss formal trousers will outlast most of your wardrobe.
Ted Baker — smarter high street with a fashion edge. Their tailored trousers often have a contemporary detail — a subtle wide leg, an interesting waistband — that lifts a corporate look into something a bit more considered.

For two brilliant independent options, I'd also flag Kitri — a London-based brand making beautifully considered tailored pieces at genuinely accessible prices, with inclusive sizing and a strong commitment to quality fabrics. And Sézane, the French brand that has quietly become one of the most coveted labels for elevated basics, including formal trousers cut with that effortless Parisian precision that's very hard to replicate at high street price points.

Fabric Matters More Than You Think

Formal trousers span a huge range of fabrics, and each behaves differently on the body. A few things worth knowing:

Wool and wool-blend: The gold standard for structure. Holds its shape, looks expensive, and breathes better than most synthetics. Slight heaviness is normal and actually helps the trouser fall correctly.
Ponte: A stretchy knit fabric that mimics the look of tailored trousers but with significantly more comfort and give. Brilliant if you're between sizes — slightly more forgiving. Often used by brands like Me&Em and Phase Eight.
Viscose/polyester blends: More affordable, but watch the drape — cheap viscose can cling in unflattering ways. If the fabric looks shiny on the screen, it'll look shiny in real life. Always check fabric composition before you buy.
Linen: Beautiful for summer formal occasions, but it will crease. Accept this as part of the charm or bring a travel steamer.

Get Your Exact Size Before You Buy

All of this guidance is useful, but the most precise thing you can do is match your actual measurements to the brand you're buying from before you commit. That's exactly what Tellar does — and it's completely free.

Tellar is the UK's leading sizing tool, matching your measurements to over 1,500 brands instantly. So instead of guessing whether you're a 12 regular or a 12 short at Hobbs, you get your answer in seconds. No changing room required.

Here's how:

Measure once — waist, hips, inside leg, or use an existing brand size you know fits.
Use the Store Size Lookup tool to get your precise size in any brand — from Reiss and LK Bennett to COS and Arket.
Always free, no download, works in-browser.

If you're building a more complete workwear wardrobe, our Ultimate Guide to Jackets is worth a read alongside this — getting your jacket and trouser sizes aligned across the same brand makes building a proper suit feel much less daunting. And for getting your head around sizing more broadly, our Ultimate Clothing Sizing Guide covers everything you need to know in one place.

The Tellar Fashion Hub is full of free, unsponsored advice from real stylists — no ads, no affiliate bias, no fluff. Exactly the kind of resource that should have existed before every difficult changing room experience I've ever had.

Measure up, match your brand, and go and get the perfectly fitted trousers you deserve.