Bra Size Calculator: Measure Your True Size
Most of us are wearing the wrong bra size — and we have no idea.
I’ve styled women for years, and the single most common fitting problem I see isn’t body shape or budget — it’s a bra that’s quietly doing nothing. A band that rides up, straps carrying all the weight, cups that gape or spill. The fix starts with two numbers from a tape measure, not a dressing-room guessing game. Type them in below and this calculator will give you your true size, your sister sizes, and a quick fit checklist — the same method a professional fitter would use.
Calculate Your Bra Size
Your calculated size, from :
Try a sister size if the band feels too tight or too loose — same cup volume, different band. Explained below.
Quick fit check — your new size should pass all four
- The band stays level. It should sit horizontally around your torso. If it rides up your back, the band is too big — that's the #1 sign.
- Two fingers, not four. You should be able to slide two fingers under the band on the loosest hook. More room than that means size down in the band.
- Cups contain, not overflow. No spilling over the top or sides ("quad-boob"), and no gaping or wrinkling at the top of the cup.
- The center sits flat. The middle piece between the cups (the gore) should rest against your breastbone, and underwires should sit on your ribcage — never on breast tissue.
How to Use This Calculator
- Grab a soft tape measure and follow the measurement guide below — two minutes, over the bra you’d normally wear (non-padded gives the truest reading).
- Pick your unit — inches or centimeters — and type in your underbust and bust numbers.
- Hit “Find My Size.” You’ll get your true size, plus two sister sizes to try if the band feels too snug or too loose.
- Do the 4-point fit check that appears with your result. The number is a starting point; the fit check is the verdict.
How to Measure Correctly
A calculator is only as honest as the numbers you feed it. Here’s the two-minute method — stand in front of a mirror so you can keep the tape level all the way around:
- Underbust: wrap the tape snugly around your ribcage, directly underneath your bust where a bra band would sit. Breathe out normally — a relaxed exhale, not a sucked-in breath — and keep the tape firm but not digging in. Write down the number.
- Bust: wrap the tape around the fullest point of your bust, usually across the nipples. Wear a non-padded bra (or none at all) — a padded or push-up bra will inflate the number and give you a cup that’s too big. Keep the tape snug without compressing, and parallel to the floor. Breathe normally.
- Measure twice. Tape measures slip. Take each measurement a second time — if the two differ, use the average.
Sister Sizes, Explained Simply
This is the part that changes everything once you understand it. A 34C, a 32D, and a 36B all hold roughly the same cup volume — they’re “sister sizes” of each other. When you go up a band size, you go down a cup letter to keep the volume the same; go down a band, up a cup. The calculator shows both of yours automatically.
- Band feels too tight but cups fit? Try the sister size with the bigger band and one cup letter smaller (for a 34D, that’s 36C).
- Band rides up or feels loose but cups fit? Try the sister size with the smaller band and one cup letter bigger (for a 34D, that’s 32DD).
- Between brands: sizing runs slightly differently by brand, so sister sizes are also your backup plan when your exact size is sold out or a brand cuts small.
5 Signs Your Current Bra Doesn’t Fit
Run through this list against the bra you’re wearing right now. If two or more sound familiar, it’s time to re-measure:
- The band rides up your back. The band does about 80% of the support work — if it climbs, it’s too loose, and your straps are doing a job they were never meant to do.
- Your straps dig in. Straps should rest lightly, not carve grooves into your shoulders. Digging straps usually mean the band isn’t supporting enough.
- Spillage — top or sides. If breast tissue bulges over the top of the cup or out toward your armpits, the cup is too small. (Bras should never create a second set of curves up there.)
- Gaping or wrinkling cups. Empty space at the top of the cup, or fabric that wrinkles and folds, means the cup is too big — or the style is wrong for your shape.
- The underwire sits on breast tissue. Underwires should rest flat on your ribcage, framing the breast — not pressing into it. If it pokes or sits on top of tissue, the cup is too small or the band too loose.
What Changes After 40
This is the part nobody told us, and it matters more than any formula:
- Breast tissue changes over time. Hormonal shifts, pregnancies, weight changes, and simply the passing years all change density and position. The size that fit perfectly at 32 is very rarely the size that fits at 45 — and that’s normal, not a flaw.
- Most women are wearing the wrong size. Because so few of us ever re-measure, women commonly wear a band that’s too big and a cup that’s too small — the exact combination that causes riding bands and digging straps.
- Re-measure once a year. It takes two minutes (see the guide above). Think of it like checking your blood pressure — a small habit that prevents years of quiet discomfort.
- Bands stretch out. Even a perfect bra loses elasticity over 6–12 months of regular wear. That’s why you start on the loosest hook — so you can tighten as the band relaxes, instead of buying a new bra.
Knowing your numbers matters — you can also calculate your waist to hip ratio in seconds.
Frequently Asked Questions
How often should I re-measure my bra size?
Once a year is the honest answer — and after any significant weight change (up or down), pregnancy, or breastfeeding. Breast tissue and ribcage measurements shift gradually, so a yearly two-minute check keeps you from slowly drifting into the wrong size without noticing.
Do sister sizes really fit the same?
Very nearly — the cup volume is the same, which is why they work as backups when the band feels off. They aren’t identical garments, though: the band length, strap placement, and wire width differ slightly, so always try both and keep the one that passes the 4-point fit check above.
What if my measurements fall between sizes?
Round the band to the nearest even number (the calculator does this for you) and try the cup on the larger side first — a slightly roomy cup with a snug band fits better than a tight cup with a loose band. If you’re truly between, order both sizes; returns exist for exactly this reason.
Can I measure without a soft tape measure?
You can approximate: use a piece of string or a phone-charging cable, mark the length, then measure it against a ruler. It works in a pinch, but a soft tape measure costs almost nothing and is far more accurate — worth having if you’re going to do the yearly re-measure habit.
My two breasts are different sizes. What do I do?
Fit the larger one — this is extremely common and completely normal. Measure the bust at the fullest side, buy for that side, and let the smaller side have a little extra room. A bra that fits the larger side comfortably always beats one that compresses it.
Does weight gain or loss change my bra size?
Often, yes — both the band and the cup can shift, since weight distributes across the ribcage and breast tissue. After a noticeable change in either direction, re-measure before assuming your old size still works. It’s one of the most common reasons a previously perfect bra starts misbehaving.
The Bottom Line
You’re probably not the size you were at 30 — and that’s completely fine. What matters is wearing the size you are now, measured this year, not remembered from a decade ago. Know your size, then discover the style personality it was always meant to serve.