Bra Size Calculator
Estimate a starting bra band and cup size from snug underbust and full-bust measurements for US or UK sizing.
Enter your values
Change any assumption and calculate again to compare results.
How the estimate is calculated
Bra sizing is not perfectly standardized between brands. This calculator uses a modern starting-point method: it rounds the snug underbust to a nearby even-inch band and estimates cup volume from the difference between the full-bust measurement and that band.
For US and UK sizing, the cup-letter sequence begins similarly and then diverges in larger sizes. The calculator uses separate label maps so the output is a useful fitting starting point rather than pretending that every brand follows one universal chart.
How to measure
Measure the underbust snugly around the rib cage while keeping the tape level. Measure the full bust around the fullest part without compressing breast tissue. Use the same units for both measurements.
A soft measuring tape and a non-padded bra or no bra usually produce a more useful measurement. If the band rides up, straps dig in or the centre gore does not sit as expected, try nearby sister sizes even if the calculated size appears reasonable.
Why brands can fit differently
Band stretch, wire width, cup shape, fabric and grading rules vary. A 34D from one brand may feel different from another, and cup letters are not absolute volumes because cup size changes with band size.
Treat the result as the first size to try. Comfort, support and tissue containment matter more than matching a calculated label.
Important information
This calculator is for general education and planning. It is not a diagnosis, medical device or substitute for individualized healthcare advice. Measurements and formula estimates can differ from clinical assessments.
If the result could affect treatment, pregnancy care, medication, driving, military status or another safety-sensitive decision, confirm it with the appropriate qualified professional or official process.
Frequently Asked Questions
Is bra sizing standardized?
No. Brand patterns and cup-letter sequences vary, especially across countries and larger cup sizes.
What are sister sizes?
Sister sizes use a different band and cup letter while keeping a similar cup volume, such as moving down a band and up a cup.
Should I add four inches to my underbust?
Many modern fitting methods do not use the old +4 method. This calculator starts close to the measured underbust and rounds to an even band.