How to Find Your Size in Blazers — and Never Get It Wrong Again

By Ella Blake, Tellar Stylist

Finding your size in a blazer is genuinely one of the trickiest things in women's fashion — and I say that as someone who has stood in a fitting room, one arm stuck in a sleeve that's too narrow, wondering how I got a size so spectacularly wrong. The answer, as I'll explain, is that blazer sizing is its own language — and once you learn to speak it, everything clicks.

Why Blazer Sizing Is So Confusing

Unlike a jersey top or a pair of leggings, a blazer is a structured garment. It's cut to a specific silhouette, and it won't forgive a bad fit the way a soft fabric will. Add in the fact that every brand sizes differently — sometimes wildly so — and you've got a recipe for fitting room frustration.

The core problem is that most women dress their waist and hips when shopping, but a blazer fits from the shoulders down. If the shoulders don't sit correctly, nothing else will — not the lapels, not the hem, not the sleeve length. It's the one area where you genuinely cannot tailor your way out of trouble (well, not without spending a fortune).

Step One: Measure Your Shoulders and Bust First

Before you even look at a size label, you need two measurements:

• Shoulder width — measure across the back from shoulder point to shoulder point (where the seam of a T-shirt would sit). This is your blazer's governing measurement.

• Bust — measure at the fullest point, over your bra. This determines whether the front will close comfortably and whether the back will pull.

If you've got a broader back or a fuller bust, size up and have the waist taken in by a tailor — it costs very little and transforms the fit. I've done this with a Reiss blazer I bought two sizes up; once altered at the waist, it looked like it was made for me.

Understand the Cut Before You Buy

Blazers come in several distinct cuts, and each one fits differently on the body:

• Oversized / relaxed — cut generously through the shoulders and body. Brands like Mango and COS do this particularly well. Generally, go to your usual size or one down if you want the sleeve to hit the right place.

• Fitted / tailored — cut close to the body with a defined waist. Think Massimo Dutti or Jigsaw. These run true to size but can be narrow across the back, so if you're broad-shouldered, always check the measurements before buying online.

• Single-breasted longline — a longer cut that skims the hip. Whistles and Me&Em nail this silhouette. Longer blazers amplify any fit issue at the shoulder, so get this right above everything else.

• Double-breasted — structured and formal. Ted Baker and LK Bennett do beautiful versions. These are cut wider at the front to allow for the overlap, so your usual size often works well.

The Shoulder Test — The Only Test That Matters

Put the blazer on and do one simple check: stand in front of a mirror and look at where the shoulder seam falls. It should sit right at the edge of your shoulder — not drooping down your arm, and not pulling up towards your neck. If it droops, the blazer is too big in the shoulder. If it pulls, it's too small. No amount of wishful thinking — or tailoring — will fix a shoulder that's off.

Once the shoulder fits, check:

• Can you move your arms forward without the back riding up?

• Does the front lie flat without gaping across the bust?

• Do the sleeves hit just above the wrist bone (or wherever you prefer)?

If you tick those three boxes, you've found your blazer size.

Brand-by-Brand: What to Expect

Phase Eight blazers tend to run slightly generous — if you're between sizes, go down. Hobbs cuts beautifully for a classic British fit, true to size with a slightly narrow shoulder, so if you're broader across the back, go up. Reiss is consistently well-cut for a fitted silhouette and runs true, but the back can be narrow if you're athletic. Boden is wonderfully generous through the bust and shoulder — ideal if you've historically struggled to get both right at once.

For a more fashion-forward option, Anthropologie carries some brilliant blazers from independent labels — the sizing varies by brand, so always check the individual measurements listed on each product page. And Zara — love it or loathe it for its sizing inconsistency — is worth checking regularly. Their blazers sell out fast and the quality at the price point is hard to beat. Size up if in doubt; their tailored styles run slim through the body.

Two independent brands I've been quietly recommending to everyone lately: Albaray (a beautifully understated British sustainable label with genuinely impeccable tailoring — the kind of blazer that people ask about) and Never Fully Dressed (bolder, more colourful, and surprisingly well-proportioned for curvier frames). Both are worth bookmarking now.

The Half-Size Hack Every Woman Should Know

If you land between sizes — the shoulders fit in a 12 but it's baggy through the body, or a 10 fits the body but won't go across your shoulders — here's my rule: always buy for your shoulders. A skilled tailor can take in the body, raise the hem, and shorten the sleeves for well under £50 at most local tailors. That is infinitely cheaper and more effective than buying a blazer that fits the waist but makes your arms look like they belong to someone else entirely.

I've had this done with blazers from M&S (brilliant quality at the price, but the tailored styles can come up long in the body) and from French Connection, whose cuts I adore but whose sizing occasionally runs narrow. Two alterations later, both now fit like a dream.

Never Guess Your Blazer Size Again — Use Tellar

This is exactly where Tellar.co.uk becomes genuinely useful. Tellar is the UK's leading free sizing tool — you enter your measurements once and it instantly matches you to your correct size across 1,500+ brands. No more guessing whether to size up at Jigsaw or down at Zara.

Here's how it works:

• Measure once — bust, waist, and hip, or just use a brand size you already know fits well

• Use the Store Size Lookup tool to get your precise size at any brand — COS, Reiss, Arket, Everlane, and hundreds more

• It's always free, no account needed, works in-browser on any device

And while you're there, explore the Tellar Fashion Hub — a library of honest, unsponsored style guides written by real stylists. No ads, no affiliate bias, no fluff. Just straight-up fashion advice.

Some posts worth reading alongside this one:

• The Ultimate Clothing Sizing Guide — everything you need to know about how sizing works across brands

• Jeans Trends 2026 — the cuts worth buying right now

• Ultimate Guide to Dresses & Best Buys

• Ultimate Guide to Jackets & Best Buys

The blazer is one of those wardrobe pieces that, when it fits properly, genuinely transforms everything — it makes jeans look intentional, gives a dress authority, and brings an entire outfit together in a way nothing else quite does. Get the shoulder right, know your cut, and use Tellar to skip the guesswork. You'll wonder how you ever shopped without it.