Private: calculated on your device

UK bra size calculator

Four measurements, both band methods side by side, and an honest confidence score. It takes about five minutes with a soft tape measure.

How to measure No tape measure? How it works

How this calculator works

Most bra calculators take two numbers. This one takes four, because each one tells us something different about how a bra will fit.

Snug underbust sets your band. A bra band gives most of a bra’s support, so it needs to be firm.
Loose underbust shows how much soft tissue compresses under the tape. When your snug measurement falls exactly between two bands, a large gap tips the result to the firmer band.
Standing and leaning bust set your cup. Leaning forward lets breast tissue fall away from the chest wall, so the tape picks up tissue a standing measurement can miss. We use whichever is larger.

The four measurements, in the order the calculator asks for them.

Two methods, side by side

The modern method uses your snug underbust as the band. The traditional method adds 4 or 5 inches first, which gives a bigger band and a smaller cup. We recommend the modern result, and show both so you can see exactly why they differ.

A confidence score that doesn’t flatter

We check your four measurements against each other. A big gap between standing and leaning bust, a padded bra, or a result midway between two cups all lower the score, and we tell you why in plain English. If something looks like a measuring mistake, such as a bust smaller than an underbust, we ask you to re-measure rather than guess.

Your measurements stay on your device

The calculation runs in your browser. There is no account, no database and nothing to upload. Every formula is published on our methodology page, along with its limitations and a dated changelog, and our accuracy page explains what any tape measurement can and can’t tell you.

Frequently asked questions

Each measurement answers a different question. The snug underbust sets your band. The loose underbust shows how much soft tissue compresses under the tape, which helps choose between two bands. The standing and leaning bust measurements set your cup: leaning forward captures breast tissue that a standing measurement can miss, which matters most for fuller cups. We use whichever bust measurement is larger.

No. Your size is calculated by code running in your browser. Your measurements are never sent to our servers, never logged and never stored by us.

If you choose “Save to this device”, they are kept in your own browser’s storage, which we cannot see. You can delete them at any time from the size tracker.

The modern method uses your snug underbust, rounded to the nearest even number, as the band. The traditional method adds 4 or 5 inches to your underbust, which gives a bigger band and a smaller cup letter. We recommend the modern result and show both, so you can see why an older size chart might suggest something different.

When your bust measurement falls almost exactly between two cup sizes, rounding one way or the other would hide what is really a coin toss. We show both cups as equally likely so you know to try both.

Measure without a bra, or over a thin unpadded one. Padding adds to the bust measurement and can make the result about one cup too large, so the calculator warns you and lowers its confidence if you measured over padding.

A tape measurement is a starting point, not a guarantee. Brands, styles and breast shapes all vary, and a small change in where the tape sits can move your cup size. The confidence score tells you how consistent your four measurements are, and our accuracy page explains the limits in detail.

Before and after you measure

How to measure

The four measurements, step by step, with diagrams.

Methodology

The formulas, the reasoning, the limits and the changelog.

Accuracy and limitations

Know all about what a calculator can and cannot tell you.

Fit diagnosis

Easily tick your fit problems and get a specific size to try.

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