Wearing the wrong band size is the single biggest reason bras feel wrong, not your body, the number on the tag.
Accurate, private, and always free- no signup, and nothing you enter is stored.
How This Calculator Works
Enter your underbust (measured snug, right under your bust, no padding) and your bust (measured at the fullest point). The calculator uses the modern band method (rounding your underbust up to the nearest even number) instead of the older “add 4 inches” method most fitters have moved away from, because bands today are built with more support and don’t need the extra padding built into the number.
The Exact Measuring Technique
Wear a well-fitting, unpadded bra or none at all. Wrap the tape measure snugly around your ribcage, directly under your bust.
Parallel to the floor, not riding up in the back. “Snug” means you can slide one finger under it, not that it digs in.
For the bust measurement, wrap the tape around the fullest part of your chest, standing naturally with your arms relaxed at your sides, not pulled tight.
Measure twice. It’s common to get a slightly different number the second time, especially on the bust measurement. Take the average if the two readings are within half an inch of each other, and re-measure a third time if they’re further apart than that.
Why the Band Matters More Than People Think
Most of a bra’s support comes from the band, not the straps. Roughly 80% by most fitters’ estimates.
That’s why a band that’s too loose causes shoulder pain and strap-digging even when the cup size is technically correct: the straps are compensating for support the band should be providing. Getting the band right first is the single most impactful fix most people can make.
If Your Result Looks Unusual
If the calculator gives you a band or cup combination you’ve never seen before, that’s not an error. It usually means your previous bras were the wrong size, not that this one is.
Very small bands with fuller cups, or larger bands with smaller cups, are less commonly stocked in stores but are completely normal measurements. Try the result before assuming it’s wrong.
How Often to Re-Measure
Re-measure after any noticeable weight change, after pregnancy or breastfeeding, after starting or stopping a new exercise routine, and at least once a year even if nothing has obviously changed. Ribcage and tissue shift gradually in ways that are easy to miss day to day.
Common Measuring Mistakes
The three mistakes that throw off results most often: measuring over a padded or heavily underwired bra (adds false bulk to the bust number), pulling the tape too tight out of habit (shrinks the band number artificially), and measuring the underbust at an angle instead of parallel to the floor (skews the band reading higher or lower than it should be).
What Happens After You Get Your Result
Your calculated size is a strong starting point, not a guaranteed universal fit. The next step is trying it in an actual bra and checking the specific fit signs (flat center gore, snug-but-comfortable band, no cup gaping) covered in How to Measure, since construction varies enough between brands that even a correct calculation sometimes needs a half-size adjustment for a specific style.
How This Compares to a Professional Fitting
A professional in-store fitting adds the benefit of trying on multiple brands and styles in person, which this calculator can’t replace. But the underlying math (the same modern band-and-cup method most professional fitters use) is identical, so the number itself should match what a good fitter would tell you from the same two measurements.