Finding the right bra size is frustrating enough, but when it comes to M&S (Marks & Spencer), things can get even trickier. Over the years, I’ve helped many people navigate the confusing world of bra sizes, and I’ve seen how M&S sizing doesn’t always follow standard rules.

So, if you’ve ever put on an M&S bra and thought, “Wait, why doesn’t this fit like my usual size?”—you’re not alone.

Let’s go step by step on how to measure your bra size for M&S, what mistakes to avoid, and what real people have discovered about their sizing.


The M&S Bra Size Calculator

M&S has a bra fitting tool that suggests a size based on your measurements and fit preferences. But here’s the thing—it’s not always accurate.

Many people have noticed that the M&S calculator adds extra inches to the band size—four inches, to be precise.

Adding four inches to your measurement isn’t unusual; it’s actually been an industry practice for over 50 years.

For example, let’s say a customer measures 36 inches around their underbust.

Instead of recommending a size 36 band, M&S might suggest a 40 or even 42 band size.

This discrepancy can lead to bras that are too loose around the back, causing insufficient support and discomfort.

I mean, think about how fabric technology has evolved over time. That’s right—fabric matters too when choosing the right bra size!

But, like I always say—one size won’t fit all! The fit can be all over the place, and honestly? Sister sizes often work better than what the online calculators spit out.

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Why? Because boobs aren’t just numbers! Your breast shape plays a HUGE role in how a bra actually fits.

Wanna crack the code on sister sizes? Check this out! And while you’re at it, try our bra size calculator to see if your “perfect fit” from M&S really holds up… or if a sister size is about to change your life. 

So, What About In-Store Bra Fitting?

M&S Bra Fit, Source: M&S

Well..

I’ve had clients walk into M&S and swear by their fittings, while others left ready to scream into the void.

One of my clients—a nurse at THR hospital—was measured as a 30DD, but guess what? She actually fit better in a 28E. Classic case of trust, but verify!

Oh, and she bought two bras, kept one, and returned the other. Smart move. Here’s why you should always buy two when trying a new brand, fabric, or style.

Then there were those who were nudged (not at gunpoint, of course 😆) into going up multiple band sizes, which left them with loose, unsupportive bras.

But hey, try arguing your doubt with an in-store “bra expert” (no shade to the fitters—they’re just following company rules).

Always Measure Yourself at Home First

Before heading to M&S or any other store, it’s helpful to measure yourself at home. This will give you a clear idea of your band and cup size, so you can confidently try on bras that are more likely to fit.

You can find thousands of artilces and video on how to measure bra size. Here’s how to measure yourself anyways:

  1. Band Size: Wrap a soft measuring tape snugly around your ribcage, right under your bust. Ensure the tape is level and not too tight. The number you get is your band size. For example, if you measure 34 inches, your band size is likely 34.
  2. Bust Measurement: Measure around the fullest part of your bust, keeping the tape level across your back. Subtract your band size from this measurement. Each inch represents a cup size (e.g., 1 inch difference = A cup, 2 inches = B cup, and so on).
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For example, if your underbust is 34 inches and your bust measures 38 inches, the difference is 4 inches, making you a 34D.

What Real People Have Discovered About M&S Bras

If your M&S 34C feels too big, don’t panic—try a 32D instead. A lot of people find that going down a band and up a cup makes M&S bras fit way better.

And here’s the thing—style matters. Yep, even within the same brand!

Full-cup bras? Usually roomier.

Plunge bras? Often run smaller.

A 34G in one style might fit completely different from a 34G in another.

Bottom line? The M&S size calculator is a gamble. My advice? Buy your suggested size AND the sister size. If you’re in-store, know your measurements and try both! Trust me, it’s worth it.