π Bra Size Calculator
Find your US bra size from two simple measurements, plus sister sizes.
Your estimated US bra size
A starting estimate. Fit varies by brand and body shape; try sister sizes when trying on.
How to Measure Your Bra Size
You need a soft measuring tape and a non-padded, unlined bra (or no bra if you prefer). Stand straight, breathe normally, and keep the tape level all the way around. Take each measurement twice and use the average.
Measure your underbust: wrap the tape snugly around your ribcage directly under your bust, where the band will sit. Round to the nearest half inch.
Measure your bust: wrap the tape around the fullest part of your bust, usually across the nipples. Keep it level in the back, not too tight.
Enter both numbers above. The calculator finds your band size from the underbust and your cup from the difference between bust and band.
How Bra Sizing Actually Works
Band size is your underbust rounded to the nearest even number: that is the number in 34D. Cup size is the difference between your bust and band measurements: each inch is one cup letter. A 1-inch difference is an A cup, 2 inches a B, 3 a C, 4 a D, 5 a DD/E, and so on. So a 34-inch underbust with a 38-inch bust is a 34D.
Here is what most sizing guides skip: cup size is relative to band size. A 32D and a 38D have very different cup volumes. That is why sister sizes work. If the band feels tight but the cups fit, go up a band and down a cup. If the band is fine but you are spilling out, go up a cup on the same band.
Signs Your Current Bra Does Not Fit
Studies suggest most people wear the wrong bra size, usually a band too big and cups too small. Warning signs: the band rides up your back (band too big), the center gore does not touch your sternum (cups too small), spillage over the top or sides (cups too small), gaping at the top of the cups (cups too big or wrong shape), and straps digging in (the band is not doing its job; straps should carry only about 10 percent of the support).
The band provides roughly 80 percent of support, not the straps. A new bra should feel snug on the loosest hook; as the elastic relaxes you move to tighter hooks. If you start on the tightest hook, the band is already too big.
Sister Sizes Explained
Sister sizes share the same cup volume across band sizes. Your sister sizes are one band up with one cup down, and one band down with one cup up. For a 34D, the sisters are 36C and 32DD. Use them when your size is sold out, when a brand runs tight or loose in the band, or when you are between band sizes. Some people deliberately sister-size for comfort: a looser band for long days, a firmer band for support.
When to Get Professionally Fitted
A calculator gets you close; a good fitter gets you perfect. Consider an in-person fitting if you are pregnant or nursing, post-surgery, experiencing pain, or consistently between sizes. Look for independent lingerie boutiques over department stores: their fitters usually have deeper training. Bring the calculator's result as your starting point and try the size plus one sister up and down.
Frequently Asked Questions
How do I measure for bra size at home?
You need a soft tape measure and a non-padded bra. Measure snugly around your ribcage directly under your bust for the underbust, keeping the tape level. Then measure around the fullest part of your bust, standing straight and breathing normally. Enter both numbers above.
What is a sister size?
Sister sizes have the same cup volume on a different band. Go up a band and down a cup (34D becomes 36C) or down a band and up a cup (34D becomes 32DD/E). Sister sizes are lifesavers when your exact size is sold out, or when one brand's band runs tight.
Why does my measured size feel wrong?
Tape measurements estimate a starting point, not a final answer. Breast shape, fullness distribution, and brand differences all matter. If the calculator says 34D but it feels off, try the sister sizes 32DD and 36C. Fit issues like quad-boob (cups too small) or gaping (cups too big) tell you which direction to adjust.
How often should I re-measure?
Re-measure every 6 to 12 months, and after significant weight change, pregnancy, or breastfeeding. Most people wear the wrong size simply because their body changed and their size did not. If your bras are uncomfortable, re-measure before buying more of the same size.
Do US and UK sizes differ?
Yes. US and UK band sizes match, but cup letters diverge after DD: US goes DDD, G, H while UK goes E, F, FF, G. This calculator uses US sizing. If you buy UK brands like Freya or Panache, convert using the brand's chart or our sister-size logic as a starting point.