Bra Size Calculator Online – Find Your Perfect Fit

Wearing the wrong bra size is far more common than most people realise — and it can affect comfort, posture, and confidence throughout the day. The challenge is that bra sizing can feel confusing, especially when shopping across different brands or sizing systems. Our Bra Size Calculator takes the guesswork out of the process entirely.

Enter your underbust and bust measurements in either centimetres or inches, and the calculator instantly determines your bra size — including both the band number and cup letter — using standard sizing charts. The result comes with a clear explanation and a full breakdown table showing every step of the calculation, so you understand exactly how your size was arrived at.

Why Getting Your Bra Size Right Matters

An ill-fitting bra is one of the most common wardrobe issues, yet many people continue wearing the wrong size simply because they’ve never had an accurate measurement taken. A band that’s too loose fails to provide support, while one that’s too tight causes discomfort and restricted breathing. A cup that’s too small leads to spillage and poor shape, while one that’s too large leaves gaps and reduces lift.

Knowing your correct size makes shopping far easier, reduces returns when buying online, and makes an immediate difference to how your clothes fit and how you feel wearing them.

How to Take Your Measurements Correctly

Accurate measurements are the foundation of a correct size calculation. Taking them properly takes only a minute and makes a significant difference to the accuracy of your result.

How to Measure Your Underbust

Wrap a soft measuring tape directly under your bust, where the band of a bra would normally sit. Keep the tape snug against your skin and level all the way around — not too tight and not sagging at the back. Breathe normally and note the measurement.

How to Measure Your Bust

Wrap the measuring tape around the fullest part of your bust, typically across the nipple line. Keep the tape relaxed — not pulled tight — and make sure it remains level and parallel to the ground all the way around. Note this measurement.

For the most accurate results, take both measurements while wearing a non-padded bra or no bra at all, and stand up straight with your arms relaxed at your sides.

Who Should Use This Calculator

Online Shoppers

Knowing your size in both imperial and metric formats makes it much easier to shop confidently across international brands and size charts without second-guessing yourself.

First-Time Buyers

If you’ve never had a proper fitting or are unsure of your size, this calculator gives you a reliable starting point based on your actual measurements rather than guesswork.

Anyone Whose Body Has Changed

Weight changes, pregnancy, breastfeeding, and age can all affect bra size. If it’s been a while since your last fitting, this tool gives you an up-to-date result in seconds.

Shoppers Switching Brands or Sizing Systems

Band and cup sizing can vary between UK, US, EU, and other international systems. This calculator provides a consistent result you can use as a reference across different sizing conventions.

How to Use the Bra Size Calculator

Getting your result takes less than a minute. Follow these simple steps.

Step 1: Select Your Preferred Unit

Choose between centimetres (cm) or inches (in) depending on whichever unit you used to take your measurements.

Step 2: Enter Your Underbust Measurement

Type in the measurement you took directly under your bust. For example, 73.5 cm or 29.0 inches.

Step 3: Enter Your Bust Measurement

Type in the measurement taken around the fullest part of your bust. For example, 88.0 cm or 34.5 inches.

Step 4: Click Calculate Bra Size

The tool processes your inputs against standard sizing charts and delivers your result instantly.

Step 5: Review Your Results

You’ll see your bra size displayed clearly — for example, 34B or 75C — along with a plain-language explanation of what the result means. The full breakdown table shows your entered measurements, the difference between bust and underbust, the calculated band size, the calculated cup size, and your final bra size.

Understanding Your Bra Size

A bra size consists of two components: a number (the band size) and a letter (the cup size). Understanding what each represents helps you make better choices when shopping.

The Band Size

The band size is derived from your underbust measurement. It represents the circumference of your ribcage just below the bust and determines how much of the bra’s support comes from the band itself. A well-fitting band should feel snug but not restrictive, sitting level and flat all the way around your body.

The Cup Size

The cup size is determined by the difference between your bust measurement and your underbust measurement. Each inch or centimetre of difference corresponds to a cup letter — the greater the difference, the larger the cup. Cup sizes typically progress from AA through to G and beyond, with each step up representing approximately one inch or 2.5 cm of additional difference.

Cup Size Reference Guide

The following differences between bust and underbust measurements correspond to standard cup sizes:

A less than 1 inch difference typically indicates an AA cup. A difference of around 1 inch corresponds to an A cup. Two inches indicates a B cup, three inches a C cup, four inches a D cup, five inches a DD or E cup, and six inches an F cup — with larger letters following from there. In centimetres, each cup step is approximately 2 to 2.5 cm of difference.

Why This Calculator Stands Out

Unlike basic size guides that give you a rough estimate, this calculator applies standard sizing chart formulas to your exact measurements and shows you the full working in a breakdown table. It supports both metric and imperial units, works on any device, and requires no registration or payment. Whether you’re shopping for everyday basics or something special, having your correct size at hand makes the entire experience faster, easier, and more likely to result in a fit you’ll love.