Your breasts and rib cage change during pregnancy and after birth, and a bra that fitted in the first trimester may not in the ninth month. Nursing bras add cups that open for feeding, and most brands suggest buying your first one in the last weeks of pregnancy. This calculator turns your measurements into a current bra size and a suggested first nursing bra size, using published retailer guidance.
How to use the nursing bra size calculator
- Measure your underbust snugly around the ribcage right under the bust, with the tape horizontal.
- Measure your bust around the fullest part, over the nipples, with the tape horizontal and a little looser.
- Enter both in inches or centimetres and choose whether you want your size now or a first nursing bra.
- Read the sizes. You get your current size and, for the nursing option, the size one band and one cup larger.
Nursing bra size formula
cup = bust − band, in inches (1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, 6 = DDD/F …)
first nursing bra = band + 2 in, cup + 1 letter
The measuring method is the one given by Bravado and Leonisa (band rounded to an even number, cup from the bust minus band); the older “add 4 inches to the band” method is no longer recommended, so it is not used. The cup letters follow the US scale in Wikipedia’s over-the-bust table (retrieved 2026-10-15). The “one band and one cup up” step is Leonisa’s guideline for a first nursing bra from the late-pregnancy size.
Example conversions (late pregnancy → first nursing bra)
| Late-pregnancy bra | First nursing bra (one band and one cup up) |
|---|---|
| 32B | 34C |
| 34C | 36D |
| 36C | 38D |
| 36D | 38DD |
| 38D | 40DD |
Worked example
Underbust 32 in, bust 37 in, measured in late pregnancy.
- Band: 32 (already even).
- Cup: 37 − 32 = 5 inches, which is a DD.
- Current size: 32DD.
- First nursing bra: band 34, cup one letter up → 34DDD.
What changes and what to expect
- Band: the rib cage can expand in later pregnancy, so a nursing bra with several hook positions lets you adjust.
- Cup: breasts often increase when milk comes in, and then settle; some women need a smaller nursing bra after a month or two, and mothers of twins may need a larger one.
- Re-measure often. Size is not fixed; measure again after the milk has come in and again as feeding patterns settle.
- Comfort first: wire-free or soft-wire styles are common choices, and stretchy cups cope with changes.
If you want the European or UK version of your size, use the Bra Size Converter (US/UK/EU/AU/FR/JP) and the Sister Size Calculator. This page cannot judge milk supply or health and does not replace a professional fitting; ask a fitter or lactation consultant if you are unsure.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify the published charts.
- Leonisa — what size nursing bra to buy (one band and one cup up; measuring method) — checked 2026-10-15
- Bravado Designs — fit guide (band and bust measurement, rib cage in later pregnancy) — checked 2026-10-15
- Wikipedia — Brassiere measurement (US cup letters by inch difference) — checked 2026-10-15
Frequently asked questions
What size nursing bra should I buy?
Leonisa advises starting from the maternity bra that fits you late in pregnancy and going one band size and one cup size up for your first nursing bra: a 34C becomes a 36D. This is a retailer’s guideline for a starting point, not a measurement of your milk supply.
When should I buy a nursing bra?
Retailer guidance suggests buying one or two in the last weeks of pregnancy, with adjustable bands, because your size can change as your milk comes in. Many women find they need a smaller nursing bra after the first month or two.
Why does the band size go up?
Maternity bra guidance describes the rib cage expanding in later pregnancy, so the band may need to be larger. It usually settles back after nursing, so adjustable hooks are useful.
How do I measure for a nursing bra?
Measure under the bust for the band, rounding to the nearest even inch, and around the fullest part of the bust for the cup. Subtract the band from the bust: one inch is an A cup, two a B, three a C. This is the method in Bravado’s and Leonisa’s guides.
Is this a medical or professional fitting?
No. It is general information. A fitter or lactation consultant can check the real fit. See also Bra Size Converter (US/UK/EU/AU/FR/JP) to translate a result to other countries.