Bra Size Calculator Guide: Underbust, Bust, Band & Cup Size
Calculator-style bra sizing guide that explains how to turn underbust and bust measurements into band and cup size.
Interactive tool
Bra size calculator
Enter snug underbust and full bust measurements in inches to estimate band, cup and sister-size direction.
This uses a common calculator-style estimate. Brand methods vary, so confirm the result with band tension, wire placement, cup depth and sister sizes.
Estimated size
34DD / E
Band
34
Difference
5.0 in
UK / EU cup
DD / E
Bra band size chart
Start with underbust to estimate band size, then combine it with cup difference.
| Snug underbust | US band | UK band | EU band | Fit note |
|---|---|---|---|---|
| 26-27 in / 66-69 cm | 30 | 30 | 65 | Try 28 only if the brand runs stretchy |
| 28-29 in / 71-74 cm | 32 | 32 | 70 | Common small-band range |
| 30-31 in / 76-79 cm | 34 | 34 | 75 | Common medium-band range |
| 32-33 in / 81-84 cm | 36 | 36 | 80 | Check band tension on the loosest hook |
| 34-35 in / 86-89 cm | 38 | 38 | 85 | Sister size down if the band rides up |
| 36-37 in / 91-94 cm | 40 | 40 | 90 | Extended band range |
| 38-39 in / 97-99 cm | 42 | 42 | 95 | Extended band range |
| 40-41 in / 102-104 cm | 44 | 44 | 100 | Extended band range |
Band methods vary by calculator and brand. A band should sit level and firm on the loosest hook when new.
Cup size chart by bust minus band difference
Subtract band/underbust reference from full bust to estimate cup letter. Cup labels vary by country.
| Difference | US cup | UK cup | EU cup | Fit note |
|---|---|---|---|---|
| 1 in / 2.5 cm | A | A | A | Small difference between bust and band |
| 2 in / 5 cm | B | B | B | Common starter cup difference |
| 3 in / 7.5 cm | C | C | C | Medium cup difference |
| 4 in / 10 cm | D | D | D | Check gore and wire placement |
| 5 in / 12.5 cm | DD / E | DD | E | Labels diverge by region |
| 6 in / 15 cm | DDD / F | E | F | US and UK labels may not match |
| 7 in / 18 cm | G | F | G | Confirm brand cup progression |
| 8 in / 20 cm | H | FF | H | Often needs brand-specific fit notes |
| 9 in / 23 cm | I | G | I | Extended cup range |
| 10 in / 25 cm | J | GG | J | Extended cup range |
Cup letters are not absolute volume. The same cup letter changes volume when the band size changes.
Bra sister size examples
Sister sizes keep similar cup volume while changing band tension.
| Starting size | Tighter band sister size | Looser band sister size | When to use it |
|---|---|---|---|
| 32C | 30D | 34B | Use when cup volume is close but the band fit is wrong |
| 34D | 32DD / 32E | 36C | Down a band usually means up a cup letter |
| 36DD | 34DDD / 34E | 38D | Useful for trying nearby stock sizes |
| 38G | 36H or UK 36FF | 40DDD / 40F | Always verify US vs UK labels |
Use sister sizes for small adjustments, not as a replacement for checking wire width, cup depth, and strap placement.
How to measure before choosing a size
Related size charts
More in Bra Fit
Bra Size Chart
Bra size searches usually need a measurement workflow: underbust for band, bust difference for cup, then fit checks for band tension and cup volume.
Cup Size Chart
Cup size users want to decode letters such as DD, DDD, E and F and understand that cup letters change with band size.
Bust Size Chart
Bust size searchers often need measurement instructions and a bridge from full bust to dresses, jackets, shirts and bra cups.
FAQ
Why did a calculator give me a smaller band?
Many modern bra fitting methods use a snug underbust as the band starting point instead of adding several inches.
Should I round my underbust up or down?
Use the nearest even band as a starting point, then adjust for comfort, fabric stretch and brand fit.
Can I calculate sports bra size the same way?
Use the same measurements as a start, but high-impact sports bras may use compression sizing and separate brand charts.