Measure wearing a non-padded bra. Underbust: snug around the ribcage, just under the bust. Bust: around the fullest part.
Why Your Bra Size Matters More Than You Think
Most women are wearing the wrong bra size — and the majority have no idea. It usually is not anyone’s fault: many of us were fitted once as teenagers and never again, or we simply grabbed whatever felt “close enough” off the rack. Bodies change with weight shifts, pregnancy, exercise and age, while the bra in the drawer stays the same.
A bra size has two parts, and both matter. The band — the number, like 34 — comes from your underbust measurement and does roughly 80% of the supporting work. The cup — the letter, like C — is not a fixed volume. It is simply the difference between your bust and your band. That is why a 34C and a 36C hold very different amounts: the letter only makes sense next to its number.
This calculator takes your two measurements and works out both parts in seconds, then shows your sister sizes — nearby sizes with the same cup volume — so you have options when a brand runs snug or loose. Treat the result as a smart starting point: try it on, check the fit signs below, and adjust from there.
Bra Size Reference Tables
Cup size from bust-minus-band difference
| Difference (inches) | Cup |
|---|---|
| 0″ | AA |
| 1″ | A |
| 2″ | B |
| 3″ | C |
| 4″ | D |
| 5″ | DD / E |
| 6″ | DDD / F |
| 7″ | G |
| 8″ | H |
| 9″ | I |
| 10″ | J |
Sister sizes (bands 32–40, shown for a C cup)
Sister sizes hold the same cup volume on a different band. Go one band up and one cup down, or one band down and one cup up.
| Your size | Sister: band up | Sister: band down |
|---|---|---|
| 32C | 34B | 30D |
| 34C | 36B | 32D |
| 36C | 38B | 34D |
| 38C | 40B | 36D |
| 40C | 42B | 38D |
A note on UK vs US cups: up to D the letters match, but beyond DD the systems split — what the US calls DDD, the UK usually calls E, and the two alphabets continue differently from there. This calculator uses US-style cup labels. If you shop UK brands, check their conversion chart.
Why Bra Sizes Are So Confusing
There is no single worldwide authority for bra sizing, so every brand is free to interpret the numbers its own way. One label’s 36C can fit like another’s 34D — and both can be “correct” by their own charts. Add vanity sizing, different fit models, and stretchy fabrics, and the size on the tag becomes a rough hint rather than a promise.
The biggest misunderstanding is the cup letter. A “D cup” sounds like a fixed amount, but it is not: the letter only describes the gap between bust and band. A 32D holds far less than a 40D. Once you see the letter as relative, fitter conversations — and sister sizes — suddenly make sense.
Old measuring methods add to the muddle. The once-popular “add four inches to your underbust” rule dates from an era of non-stretch fabrics and routinely puts women in bands far too loose to support anything. Modern fitting starts from your actual underbust — which is exactly what this calculator does.
5 Signs Your Bra Doesn’t Fit
1. The band rides up your back. A band doing its job sits level all the way around. If it creeps upward, it is too loose — and your straps are carrying weight they were never meant to.
2. The centre gore floats. On an underwire bra, the bridge between the cups should rest flat against your sternum. If it hovers, the cups are too small.
3. Spillage over the top or sides. Tissue escaping the cups — the classic “quad-boob” — means the cup volume is not enough. Go up a cup, not a band.
4. Straps dig in or slip off. Digging straps usually mean the band is too big and the straps are overcompensating. Slipping straps can mean a band too wide for your frame.
5. Cups wrinkle or gape. Empty fabric at the top or sides means too much cup — try a smaller cup, or a shape suited to your fullness.
Frequently Asked Questions
How often should I re-measure my bra size?
About once a year, and any time your body changes noticeably — weight gain or loss, pregnancy or nursing, a new workout routine, or simply noticing your current bras fit differently. Breasts are mostly soft tissue, so the number moves more than people expect.
Should I measure wearing a padded bra?
No — measure in a non-padded, unlined bra (or none at all). Padding adds inches to the bust measurement that are not your body, which inflates the cup calculation and gives you a size that only fits while you are wearing that padded bra.
What exactly are sister sizes?
Sister sizes are different band-and-cup combinations that hold the same breast volume. If a 34C’s band feels tight but the cups are perfect, its sister 36B keeps the same cup volume on a roomier band. They are your best tool when you fall between sizes or a brand runs small.
Do sports bras use the same sizing?
Some do (band + cup), but many use S/M/L, which is far less precise. For high-impact activity, a proper band-and-cup sports bra — ideally an encapsulation style rather than simple compression — gives far better support and comfort than a stretchy S/M/L top.
Underwire or wireless — which is better?
Neither is objectively better; it is about shape preference and comfort. Underwire gives more lift and separation, wireless gives softness and ease. The fit rules are identical either way: the band must be snug and level, and the cups must fully contain without spillage or gaping.
What if I fall between band sizes?
Try both neighbouring bands and let comfort decide — a new bra should feel snug on the loosest hook, since bands stretch with wear and you will tighten it over time. If the smaller band’s cups then feel off, check its sister size before giving up on the style.
How to measure at home
- Underbust: wrap the tape snugly around your ribcage, directly under the bust where the band sits. Keep it level and breathe normally.
- Bust: wearing a non-padded bra, measure around the fullest part of your bust. Keep the tape level across your back — not too tight, not loose.
- Measure at the end of the day and take each measurement twice to be sure.
- Sizes vary between brands. Use your calculated size as a starting point and try sister sizes too.