Bra Size Calculator
Estimates a UK starting bra size by a named, documented method, with sister sizes; converts a known size only through a chart you supply.
Measurements
Around the ribcage just under the bust, tape level and snug but comfortable, after breathing out.
Standing upright, around the fullest part of the bust, tape level and not pulling.
Starting size
Enter your snug underbust and standing bust.
Before you rely on this: Use this as a quick guide and check the result against your own working before you rely on it.
How to use this calculator
- Choose cm or inches. Measure your snug underbust (around the ribcage under the bust, after breathing out) and your standing bust (fullest part, tape level).
- Read the UK starting size, the reasoning behind its band and cup, and its sister sizes. Optional extra measurements are shown as differences for your own fitting.
- To convert a size you already know, paste the shop's own conversion table — sizes are looked up, never calculated.
How it works
The Zerdly starting-size method is a named, proposed heuristic — not a clinical or universal standard. Band: the nearest even number to your snug underbust in inches; an underbust exactly between two bands shows both. Cup: the nearest whole number of inches by which your standing bust exceeds that band, read on the UK sequence AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K.
A difference beyond K is reported as outside the method, never clamped to K. Sister sizes stay in the same sizing family: one band down and one cup up, or one band up and one cup down — a similar nominal cup volume, not the same fit.
Worked example
32 in snug underbust, 36 in standing bust. Band 32; the bust is 4 in more, which the method reads as D: UK 32D, with sister sizes 30DD and 34C. With a 37 in bust it would be 32DD, and a 33 in underbust shows both 32 and 34 rather than hiding the tie.
Tips
- Measure in a non-padded bra or unclothed, keeping the tape level.
- When trying on, the band should sit level and not ride up, the cups should not spill or gape, and the straps should not carry the weight.
Frequently asked questions
What are sister sizes?
Sizes with a similar nominal cup volume on a different band: one band down and one cup up, or one band up and one cup down. Useful when the cup is close but the band is too tight or too loose.
Why will it not convert to US or EU on its own?
Because the labels differ between systems and brands, especially above D, and Zerdly has not verified the right to republish a brand's table. Paste the shop's own conversion table and the tool looks your size up in it.
Is my measurement data stored?
No. Everything is calculated in your browser; nothing is saved, sent or put in the address. A measurement card prints only if you ask for one.
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Cite this tool
Zerdly. (2026). Bra Size Calculator. Retrieved from https://www.zerdly.com/tools/bra-size-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.