How Do I Find My Size in Prom Dresses?

By Ella Blake, Tellar Stylist

To find your size in a prom dress, you need three measurements — bust, waist, and hips — and you should always size to whichever measurement comes up largest on the brand's chart, then have the dress altered to fit the rest of you. Prom dresses are not sized like your everyday high street clothes, and assuming your usual number is going to fit is the single most common mistake I see — and it causes a lot of unnecessary panic about three weeks before the big night.

I've helped a lot of people navigate the prom dress process over the years, and the panic is almost always the same: "I ordered a 10 and it won't zip up" or "I'm swimming in it." Neither is a disaster — both are completely fixable — but knowing how to size correctly from the start saves you a lot of stress, a lot of money, and a lot of last-minute visits to an emergency seamstress. So let's do this properly.

Why Prom Dress Sizing Is So Different

Prom and occasion dresses — especially formal ball gowns and structured corset styles — are often cut to a sizing chart that bears almost no resemblance to your usual high street size. A few reasons for this:

They use US sizing on many brands, which runs smaller than UK sizing. A US 8 is roughly a UK 12. If your dress is coming from a US brand or marketplace, always convert carefully.
Structured bodices have no stretch. A fitted corset or boned bodice needs to fit your bust and waist exactly — there's no elastane to help you out.
Floor-length skirts with full volume are generally more forgiving on the hip, but fitted column and mermaid styles are extremely unforgiving and require precise measurements.
Many prom dress brands size small — particularly those sold via online marketplaces. Always read the brand's own size chart, not just the label.

The Three Measurements You Must Take

Before you even start browsing, grab a soft tape measure and get someone to help you — measuring yourself accurately is genuinely difficult on your own. You need:

Bust: Measure around the fullest part of your chest, keeping the tape parallel to the floor. Wear the bra or strapless bra you plan to wear on the night — this matters more than you'd think.
Waist: Measure around the narrowest part of your torso, usually an inch or two above your belly button. Stand naturally — don't hold your breath or suck in.
Hips: Measure around the fullest part of your hips and bottom, usually about 8–10 inches below your waist.

Write all three down. Then, when you look at a size chart, check each measurement against the chart and note which column you fall into. If your bust says size 12, your waist says size 10, and your hips say size 14 — you order the size 14. Always go with the largest measurement. You can take a dress in; you cannot let it out if there's no seam allowance (and many cheaper dresses have very little).

Where to Shop — and What to Expect From Each

The good news is that the UK high street and premium market have genuinely stepped up their occasion wear game, and you don't have to overspend to find something stunning. Here's where I'd point you, depending on your budget:

Coast is one of the best places to start for prom and formal dresses on the UK high street. Their sizing is generous and fairly consistent, their fabrics are lovely — lots of chiffon and structured satin — and their size guides are detailed. Generally true to size.
ASOS is brilliant for prom because the sheer volume of styles means you can find almost anything. Their own-brand occasion wear is priced accessibly, and the customer reviews with photos are invaluable for assessing fit before you buy. ASOS also carries a wide range of sizes up to a UK 30.
River Island has quietly become very good for occasion dresses — think ruched satin midis and more structured corset styles at accessible prices. Their sizing runs fairly true to the UK standard.
Monsoon is worth a look if you want something with more of a romantic, embellished feel — lots of lace, floral appliqué and floaty silhouettes. They tend to cut generously, so if you're between sizes go with the smaller one.
Phase Eight is excellent for a slightly more grown-up or classic prom look — their floor-length gowns are beautifully made and well-priced in the premium bracket. Sizing is reliable and consistent.
Reiss offers more minimal, sophisticated occasion wear — think elegant column dresses and sleek satin styles. Cut for a slimmer silhouette, so go with your largest measurement and be prepared to size up one.
Hobbs is another great premium option for a classic, elegant look — their fit is generally very true to size and the quality of fabric is excellent.
Ted Baker is a go-to for glamorous, feminine prom styles — lots of embellishment, flattering ruching, and beautiful colourways. Ted Baker sizes in numbers (0–5) rather than UK sizes, so always check their conversion chart: a Ted 2 is roughly a UK 10.
Anthropologie is a gorgeous option if you want something more unique and less likely to show up at prom on three other people. Their sizing is US-based, so convert carefully and read the fit notes on each style.

For something truly one-of-a-kind, I'd also strongly recommend looking at Needle & Thread — a British label known for extraordinary embellished tulle gowns that are genuinely breathtaking in person. And for something more directional and fashion-forward, Rotate Birger Christensen (often just called Rotate) make incredible occasion dresses with a cooler, more editorial feel that's perfect if you want to look dressed up but not like a traditional prom cliché.

Alterations — Your Secret Weapon

I cannot stress this enough: almost no one fits a prom dress perfectly off the rail. This is completely normal and is not a reflection of your body — it's a reflection of the fact that dresses are made to a standard pattern and humans are not standard. Budget for alterations from the start. A good local seamstress or tailor can:

Take in the waist or sides if the dress is too large through the body
Shorten the hem to the right length for your height
Add boning or cups to a strapless bodice for better support
Take in straps that are too long
Add a modesty panel or let out the back if the zip won't close

Simple alterations typically cost between £20–£50 and can make an inexpensive dress look truly expensive. Find a local seamstress (check Google reviews) and book them as soon as you have the dress — good ones get booked up fast in prom season.

A Word on Buying Online From Overseas Marketplaces

If you're tempted by the very cheap dresses you see on sites like Shein, AliExpress, or similar — please be cautious. The sizing is wildly inconsistent, the fabric quality is often very different to what you see in the photos, and returns can be complicated or impossible. If you do go down this route, order at least two sizes, factor in plenty of time before the event, and keep your expectations realistic. For something as important as prom, I'd always invest in a dress from a brand with a clear returns policy and a proper UK size guide.

Take the Guesswork Out of Sizing — Use Tellar

Prom dress sizing is genuinely one of the trickiest areas to navigate — different brands, US vs UK sizing, and structured cuts that leave zero room for error. That's exactly what Tellar.co.uk is built for. It's the UK's leading free sizing tool, matching your exact measurements to over 1,500 brands instantly — so you can shop with confidence, not crossed fingers.

Measure once — bust, waist, and hips — and Tellar does the rest.
Use the Store Size Lookup tool to find your exact size across hundreds of brands — from Coast and ASOS to Reiss and Ted Baker.
Always free. No downloads. Works straight in your browser.

And for more styling advice, head to the Tellar Fashion Hub — an honest, unbiased library of stylist-written guides. Check out the Ultimate Clothing Sizing Guide, the Ultimate Guide to Dresses, the Best Jean Trends for 2026, and the Ultimate Guide to Jackets. No ads. No sponsors. Just genuinely useful fashion advice, always free.