A strapless bra has a harder job than an everyday bra: with no straps to share the load, the band must support the bust entirely. That is why a strapless bra in your usual size often slips down by lunchtime. Fit specialists give consistent advice: go down a band. This calculator applies that advice to your size and shows the sister size with the same cup volume.
How to use the strapless bra size calculator
- Enter your usual band and cup, the size you wear in an everyday underwired bra.
- Say how the band feels: loose (you use the loosest hook), comfortable, or already firm.
- Read the sizes to try. Try the first, then the second if the cup is too small or large.
- Test the fit with the checks below before buying.
Strapless sizing formula
same cup volume: cup + 1 letter (sister size)
alternative: same cup letter on the smaller band
Journelle’s guide says to go down one band size when going strapless and gives 34C → 32C or 32D as the example; Brava explains why (the band does 100% of the work) and links the move to sister sizing: going down a band often means going up a cup to keep the same volume (retrieved 2026-10-15). A band is two inches in US and UK sizing, so one band size down from 34 is 32. In the “firm” option the calculator keeps your usual size first.
| Usual size | Try first | Also try |
|---|---|---|
| 32B | 30C | 30B |
| 34B | 32C | 32B |
| 34C | 32D | 32C |
| 36C | 34D | 34C |
| 36D | 34DD/E | 34D |
| 38D | 36DD/E | 36D |
Worked example
Your usual size is 34C and you use the loosest hook of your everyday bra.
- One band down: 32. Same cup volume: C + 1 letter = D.
- Try 32D first. If the cup is too big, try 32C.
- Your usual size is 34C.
Signs the band is wrong
- The back rides up while the front dips down.
- You can pull the band more than a centimetre away from your back.
- The bra shifts when you raise your arms, or you pull it up more than once a day.
- If any of these apply, go down another band size (Brava).
Look for two or three hook-and-eye closures for adjustment, and hand-wash the bra so that silicone grippers last longer (Journelle). The result is general guidance, and a fitter can confirm it in the shop. For other sizing, see the Sister Size Calculator and Sports Bra Size Calculator.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- Journelle — how to find a strapless bra that actually stays up (go down one band size) — checked 2026-10-15
- Brava Lingerie — strapless bra guide for D cup and up (band, sister sizing) — checked 2026-10-15
- Wikipedia — Brassiere measurement (sister sizes) — checked 2026-10-15
Frequently asked questions
What size strapless bra should I get?
Journelle’s fitters recommend going down one band size from your usual size for a strapless style: a usual 34C tries a 32C or a 32D. The calculator gives the sister size first (band down, cup up, same volume) and the same cup on the smaller band as the second try.
Why does a strapless bra need a smaller band?
Without straps, the band does all the work. Brava explains that a band that feels fine in a regular bra often fails as a strapless, and that going down a band size stops it migrating down. Journelle adds that silicone grippers cannot make up for a loose band.
When should I keep my usual band?
If the band of your everyday bra is already firm and sits on the tightest hook, a strapless in the same size may work. If it slips on the day, go down a band. The “firm” option reflects this.
Do strapless bras work for large cups?
Brava’s fitters say yes, for D to K cups, when the band is right and the style is built for a fuller bust. See their guide in the sources below.
How do I find my usual size first?
Use the Cup Size Calculator or Bra Size Converter (US/UK/EU/AU/FR/JP) if you need to translate. See also Sister Size Calculator for the same volume on other bands.