Studies consistently show that around 80% of women are wearing the wrong bra size. That is not a small number — it means the vast majority of us are going through each day in a bra that is too tight, too loose, or simply the wrong shape for our bodies. The good news is that measuring your bra size at home is straightforward, takes less than five minutes, and requires nothing more than a soft measuring tape and this guide.

Whether you have never been professionally fitted or you simply want to double-check after a weight change, pregnancy, or birthday, this complete UK guide will walk you through exactly how to calculate your bra size — the right way.

What You Will Need Before You Start

Before you begin, gather the following:

Measuring over a heavily padded bra will give you inaccurate results. If you want to measure over a bra, use an unpadded one that fits as closely as possible. Measuring without a bra is perfectly fine and often gives the most accurate result.

Step One — Taking Your Underbust Measurement (Band Size)

The underbust measurement is the foundation of your bra size. It determines your band size, which is the number in your bra size (for example, the 34 in 34D).

How to Measure Your Underbust

Stand up straight and breathe normally. Wrap the measuring tape around your ribcage, directly underneath your bust. The tape should sit level all the way around — not higher at the back than the front — and it should be snug but not digging in. You should be able to slide two fingers underneath it.

Take the measurement in inches. Most UK bra sizes are based on inches, so stick with inches throughout this process.

Converting Your Underbust to a Band Size

Once you have your underbust measurement in inches, convert it to a band size using the following method:

Many modern UK bra fitters use the underbust measurement directly, without adding inches. Our bra size calculator uses this modern method and gives you a recommended starting size along with alternatives to try.

Step Two — Taking Your Bust Measurement (Cup Size)

Your bust measurement determines your cup size — the letter in your bra size (for example, the D in 34D).

How to Measure Your Bust

Keep the measuring tape level and wrap it around the fullest part of your chest. This is usually across your nipples, though for some body types it may be slightly higher or lower. Do not pull the tape tightly — let it sit comfortably around your bust without compressing it.

Again, note this measurement in inches.

Step Three — Calculating Your Cup Size

Subtract your band size from your bust measurement. The difference in inches corresponds to your cup size.

A Worked Example

Let us say your underbust measures 30 inches and your bust measures 36 inches.

Common Measuring Mistakes to Avoid

Even with the best intentions, it is easy to get a slightly off reading. Here are the most common errors and how to avoid them:

Measuring Too Loosely

A tape that hangs away from your body will add inches to your underbust measurement and give you a band size that is too large. The band should be firm — that is where the majority of a bra's support comes from.

Measuring Over the Wrong Bra

Padding adds volume. If you measure over a push-up bra, your bust measurement will be inflated and your cup size calculation will be too large. Measure over nothing, or over a plain, unlined bra.

Not Standing Upright

Slouching changes the position of your ribcage and bust. Stand tall with your arms relaxed at your sides when measuring.

Rounding Without Testing

If you measure between two sizes, try both and see which feels better. Bra sizing is a starting point, not a fixed formula. Different brands and styles can fit differently even within the same nominal size.

What to Do Once You Have Your Size

Your calculated size is your starting point. When you try on a new bra, use the outermost (loosest) hook at the back. As the bra stretches with wear over time, you can move to the inner hooks to maintain the same fit. If you need the inner hooks straight away, the band may be too large.

After trying on a bra, run through a quick fit checklist:

If you notice any of these issues, read our guide on signs your bra does not fit for specific advice on how to adjust your size to fix each problem.

When Should You Re-Measure?

Your bra size is not fixed for life. It changes with your body, and there are several key moments when a re-measure is a very good idea:

As a general rule, remeasuring once a year is sensible even if nothing dramatic has changed, because gradual body changes are easy to overlook.

Try Our Free UK Bra Size Calculator

Once you have taken your measurements, plug them straight into our free UK bra size calculator to get your recommended size instantly. The calculator uses your underbust and bust measurements, gives you a primary recommendation, and also shows your sister sizes — alternative sizes with the same cup volume that may be worth trying if your first choice is unavailable or uncomfortable.

Getting the right bra size is genuinely life-changing. A well-fitting bra is more comfortable, more supportive, and will look far better under clothing. Take five minutes to measure today — your back, shoulders, and posture will thank you for it.

Ready to find your perfect bra size? Try our free UK calculator.

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