Bra & Sister Size Calculator
Enter your snug underbust and standing bust in inches or centimeters. Get an estimated US starting size, two sister sizes, and fit notes. Check the fit before buying.
Developer self tests
Not sure whether to change your band or your cup size? Enter your measurements in the bra size calculator above for a US starting size and the closest sister sizes in both directions. For example, 32DDD/F and 34DD/E have approximately the same cup volume, but different band lengths. The result is a starting point for trying on bras, not a guarantee that every style will fit.
How to measure for your true size
- Measure your snug underbust: Wrap a flexible tape around your rib cage directly beneath your breasts. Breathe out normally and keep the tape level. It should feel snug without hurting or digging in.
- Measure your standing bust: Without a padded bra, wrap the tape around the fullest part of your bust. Keep it level and avoid compressing breast tissue.
- Check your starting size: Select the correct unit, enter both measurements, and choose Find my size. Copy the results if you want to compare sizes when shopping.
How sister sizes actually work
Cup letters don’t represent one fixed volume. A 32D cup is smaller than a 38D cup. Sister sizing adjusts the band and cup letter in opposite directions to keep approximately the same cup capacity.
- Tighter band, up a cup: A 34C becomes a 32D. Try this if the 34 band rides up but the cups otherwise fit.
- Looser band, down a cup: A 34C becomes a 36B. Try this if the cups fit but the 34 band itself feels uncomfortably tight.
Band stretch varies between brands and even between styles from the same brand. Check the manufacturer’s size chart and fit notes rather than assuming that every bra from one country fits the same way.
Even the same bra in two colors may feel a little different. If you’re between sizes, compare the actual band tension instead of relying on the size label alone.
Common US sister-size examples
For a quick check: 30DDD/F, 32DD/E, and 34D are sister sizes. Another group is 30G, 32DDD/F, and 34DD/E. US cup names vary by brand, so DD/E and DDD/F in this calculator are alternative labels for the same step in its US size sequence, not a UK conversion.
What the calculator can and cannot tell you
This tool rounds your snug underbust to a supported even-numbered US band size and estimates the cup from the difference between your bust and underbust measurements. It covers bands 26 through 48 and cup labels AA through L. It does not assess breast shape, projection, wire width, or how much your rib cage compresses. Measurements outside its available size range may be limited to an endpoint, so check a brand’s own chart rather than relying on an extreme result.
If the suggested size seems far off, remeasure and compare it with a different fitting method. A Bra That Fits uses six measurements, including leaning and lying bust measurements, which can be useful when one standing measurement doesn’t capture your shape well. This BluFashion calculator estimates US-style labels only. For UK or EU bra shopping, use the retailer’s bra-specific conversion chart rather than assuming clothing-size conversions apply.
Fixing common fit issues
- Cups gaping at the top: First adjust the straps and settle your breast tissue into the cups. Then try a different cup shape or a smaller cup on the same band. Changing to a sister size alone does not reduce cup volume.
- Spilling out at the center: Try a larger cup on the same band or a style with more room near the center. A looser-band sister size keeps similar cup volume, so it may not fix the spilling.
- Band rides up: The band may be too loose. Try one band size down and one cup step up, then check that the band stays level without pinching.
- Straps digging into shoulders: Check whether the band provides enough support and whether the straps are overtightened. Adjust them first, then reassess the band and cup fit.
Frequently Asked Questions
Is 34DD the same as 32DDD?
They are sister sizes in common US labeling, with approximately the same cup volume. The 32DDD/F band is shorter, while the 34DD/E band is longer. Actual fit still varies by brand and style.
What is the sister size for 32DDD?
In the US cup sequence used by this tool, 30G is the tighter-band sister size and 34DD/E is the looser-band sister size. Check the brand’s cup lettering before ordering.
How tight should a bra band be?
It should feel snug and stay level around your rib cage without hurting. A new bra typically starts on the loosest hook, leaving room to tighten it as the band relaxes. Finger-count rules are not reliable for every body.
How do I know if my bra fits correctly?
The band stays level, the cups contain breast tissue without gaping or spilling, and the straps do not dig in. In a wired bra, the wires should follow the breast root and the center should usually rest comfortably against the chest.
Do all lingerie brands use the same sizing?
No. Band stretch, cup construction, and sizing labels vary by brand. Start with the calculator result, check the retailer’s size chart, and adjust the cup or band according to what actually feels wrong.
Are these US, UK, or EU bra sizes?
The calculator uses a US-style band and cup sequence. UK cup letters differ above DD, and EU bands use different numbers. Verify conversions on a bra-specific brand chart before purchasing.
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