M&S Bra Size Calculator: Measure & Find Your Perfect Fit

Getting the right bra size can feel like a guessing game, but Marks & Spencer has made it simpler with their dedicated bra size calculator and free in-store fitting service. With over 200 stores offering professional fittings and band sizes from 28 to 40 inches, finding your fit is more accessible than ever.

M&S stores offering free bra fittings: Over 200 ·
Band sizes available at M&S: 28–40 inches ·
Cup sizes at M&S: AA to K

How to measure bra size M&S?

Step 1: Measure your band size

Using a soft tape measure, wrap it around your ribcage directly under your bust. The tape should be level and snug – not too tight. Marks & Spencer (official bra fitting guide) recommends ensuring the underband sits level and feels comfortably snug without tightness. If the band rides up during wear, you likely need a smaller band size.

Step 2: Measure your bust size

Measure around the fullest part of your bust, keeping the tape parallel to the floor. Take the measurement to the nearest ¼ inch for accuracy, as advised by Calculator.net (bra size calculator tool). The difference between your bust and band measurement determines your cup size.

Step 3: Calculate your bra size using M&S calculator

Input your band and bust measurements into the M&S bra size calculator (available in inches and cm). The calculator returns a UK size. For example, a 30-inch band with a 36-inch bust gives a 6-inch difference – a D cup, resulting in size 30D. Note that M&S cup sizes run from AA (less than 1 inch difference) to K (10 inches difference).

How to calculate your correct bra size?

Using the M&S bra size calculator

The M&S official bra size calculator is accessible directly on their website. It accepts measurements in both inches and centimetres, making it useful for shoppers in the UK and Ireland.

Inputting measurements correctly

• Band measurement: measure under the bust, snug and level.
• Bust measurement: measure at the fullest part, tape parallel to floor.
• Record both to the nearest ¼ inch or ½ cm.

Understanding your size output

The calculator shows your band size (e.g., 34) and cup letter (e.g., C). A 34C means a 34-inch band with a 37-inch bust (3-inch difference = C cup). If the result feels off, check the cup fit signs: bulging means go up a cup; gaping or wrinkling means go down. M&S notes that the centre of the bra must sit flat against the ribcage – if it lifts away, a bigger cup is needed.

Which is bigger, 36C or 38C?

Sister size explanation

A 38C has a larger cup volume than a 36C because the band is larger. The cup letter on its own doesn’t indicate volume – it’s the difference between band and bust. M&S official guide states that sister sizing involves going down one band size and up one cup size to maintain the same volume. So 36C and 38C are not sisters; 38C is a full cup volume larger.

Band vs cup volume

Cup volume increases as band size increases. A 36C holds less volume than a 38C, even though both share the same letter. This is why bra sizing can be confusing – two women with the same cup letter may have very different breast volumes if their bands differ.

Chart of sister sizes

One pattern across b or 2 sizes, but the volume stayed constant.

The pattern: to go up one band size, drop one cup letter to keep the same cup volume. For example, a 34C has the same volume as a 36B and a 32D, as explained in the M&S official bra fitting guide.

How to calculate bra size in Ireland?

M&S bra fitting services in Ireland

Marks & Spencer offers bra fitting appointments in Ireland, including stores in Dublin and Cork. These are free and can be booked in advance. The same in-person method used in UK stores is applied, ensuring a consistent fit experience. M&S official guide covers the same principles for all locations. Find your perfect bra size with the M&S Bra Size Calculator, which uses the same principles as their in-person fitting services. Sainsbury’s plant pioneer steaks

Using the online calculator with cm/inches

The M&S bra size calculator accepts both inches and centimetres, so Irish shoppers can measure in either unit. The calculator returns a UK size, which is the standard for M&S stores in Ireland.

Finding M&S stores in Dublin, Cork

M&S has multiple stores in Dublin (Grafton Street, Liffey Valley, Dundrum) and in Cork (Mahon Point, Opera Lane). Each offers bra fitting services. To book, visit the M&S website or ask in-store.

What are sister sizes and how do they work?

Sister size chart

A sister size is a different band and cup letter combination that holds the exact same cup volume. M&S official bra fitting guide gives the example: 34C has the same cup volume as 32D or 36B.

How to find your sister size

• To go to a smaller band: go down one band size and up one cup letter. Example: 34C → 32D.
• To go to a larger band: go up one band size and down one cup letter. Example: 34C → 36B.

Examples: 32C to 34B to 30D

Take 32C: its sisters are 34B (band up, cup down) and 30D (band down, cup up). All three have identical cup volume. This is useful when your exact size is unavailable, but M&S cautions that the band fit must still be correct – a too-loose sister size will still ride up.

Quotes from the experts

“In-person bra fitting is the most accurate method for determining bra size.”

— M&S bra fitting team (Marks & Spencer official bra fitting guide)

“For measurement accuracy, take all measurements to the nearest ¼ inch or ½ cm.”

— Calculator.net sizing guide (Calculator.net bra size calculator tool)

For women in the UK and Ireland, the choice is clear: use the M&S bra size calculator for a reliable starting point, then visit a store for a professional fitting to catch any fit issues a tape measure might miss. Without that hands-on check, you risk ending up with a bra that feels almost right – but almost never delivers the support your body actually needs.

Related reading: How to measure bra size · Bra size calculator

For those who prefer an in-person fitting, the M&S bra fitting guide details appointments and store locations across Ireland.

Frequently asked questions