Getting the right adjustable strap bra size means measuring your underbust snugly and your full bust loosely, then mapping those two numbers to the brand’s own chart.
Most bad bra fits start with a tape measure, not a size chart. A band that rides up in back or a gore that floats off your chest usually traces back to one tilted measurement or a chart from the wrong brand.
Two numbers do most of the work: your underbust, which sets the band, and your full bust, which sets the cup. Below is the order that works, the fit checks that confirm it, and where adjustable straps actually help.
How Do You Measure Your Underbust and Bust Correctly?
Measure your underbust directly under the bust with the tape snug and level, then measure the fullest part of your bust with the tape level across your back and loose enough that it doesn’t compress tissue.
Start by wearing a lightly lined or unpadded bra — not a thick padded one, which distorts both numbers. The National Breast Cancer Foundation’s bra fit guide walks through the same sequence.
- Underbust: wrap the tape right under the bust, keep it level all the way around, and pull it snug against the rib cage. This number sets your band.
- Full bust: wrap the tape across the fullest part of the bust and around your back at the same height. Keep it level and don’t cinch it.
- Cup math: subtract the band from the bust. Each inch of difference typically steps up one cup letter — 0 for AA, 1 for A, 2 for B, 3 for C, 4 for D, 5 for DD, and onward through F, FF, G, GG, H, HH, and J.
Some charts use a shortcut for the band itself: if the underbust measures odd, add 5; if even, add 4. Triumph and Sloggi both publish fitting guides with their own variations — the method is standard, the final numbers are not.
What Changes the Answer From Brand to Brand?
Brand charts don’t agree, because there is no universal US standard for bra sizing. The same body can land in two different sizes depending on which chart you use.
Bands and cups shift between manufacturers, and so does the conversion for letter-based sizes. One adjustable-bra chart maps XS through XL to band-and-cup combinations rather than plain letters: XS covers 30C, 30D, 32A, and 32B, while M covers 34C, 34D, 36A, and 36B. A 34C isn’t one fixed size across every brand.
| Letter Size | Band/Cup Combinations |
|---|---|
| XS | 30C, 30D, 32A, 32B |
| S | 32C, 32D, 34A, 34B |
| M | 34C, 34D, 36A, 36B |
| L | 36C, 36D, 38A, 38B |
| XL | 38C, 38D, 40A, 40B |
If you land between two rows, take the smaller band and the larger cup for a closer fit, then rely on the straps and hook positions to fine-tune. Adjustable straps help here, but they can’t fix a band or cup that’s genuinely wrong.
Once you know your numbers, comparing styles gets easier — our roundup of the best adjustable strap bra options filters by these exact band-and-cup combinations.
What Fit Checks Confirm the Size Is Right?
A correct fit has a band that stays parallel to the floor, a center front that lies flat against your chest, and straps snug enough to stay put without digging in. Put the bra on and run these checks in order:
- Band test: raise your arms and move around. If the band rides up in back, it’s too loose — drop down a band size and step up a cup.
- Center front: the gore between the cups should sit flat on your chest. Gapping there usually means the cups are too small, not too big.
- Cup coverage: breast tissue should sit entirely inside the cup with no spill over the top or sides, and the underwire should sit against the rib cage, not on top of breast tissue.
- Straps: tighten to comfortably snug — supportive without leaving marks. This is the one place adjustable straps matter, and even they won’t rescue a mismatched band.
Common mistakes that throw the numbers off: measuring over a thick padded bra, letting the tape tilt or sag, and pulling the bust tape tight enough to compress the chest. Any one can push you a full cup size off. Measure again, level and loose on top, and compare against the specific brand’s chart before you buy. The FDA notes that fit and sizing vary by manufacturer, so recheck each new brand’s guide rather than trusting a saved size. The National Breast Cancer Foundation’s bra fit guide covers the same measurement order in detail.
FAQs
Should I measure with or without a bra on?
Wear a lightly lined or unpadded bra, or none at all, depending on the brand’s guide. A padded bra throws both the underbust and bust readings off. When in doubt, skip the bra and measure against bare skin.
Do adjustable straps fix a bra that doesn’t fit?
No — they only fine-tune lift and comfort within a size that already works. If your band rides up or the center gore gaps, the band or cup is wrong and tightening the straps just masks it. Fix the band and cup first; the straps handle the last small adjustments.
Why do I get a different size at different brands?
Because there’s no universal US sizing standard, so every brand builds its own chart. The same measurements can map to different band-and-cup combinations. Always check the new brand’s specific chart before ordering.
References & Sources
- National Breast Cancer Foundation. “Bra Fit Guide.” Measurement order and fit-check steps.
- Triumph. “Bra Fitting Guide.” Band and cup sizing variations by brand.
- Sloggi. “Bra Fitting Guide.” Underbust and bust measuring method.