Find Your Starting Bra Size
Use your underbust and full bust measurements to estimate a starting bra size in a few seconds.
How This Bra Cup Size Calculator Works
This calculator uses a common sizing method: your band size comes from your snug underbust measurement, and your cup size comes from the difference between your full bust and band size. The result is a practical starting point for shopping online or narrowing options before an in-store fitting.
Think of it as a quick map, not a final verdict. Bra fit can vary by style, fabric stretch, wire shape, and brand-specific sizing patterns.
The Core Formula
- Band size: rounded to the nearest even number (US method).
- Cup index: full bust minus band size (in inches), rounded to the nearest whole number.
- Final size: band + cup letter (for example, 34C).
How to Measure Correctly at Home
- Wear a non-padded bra or no bra for best accuracy.
- Stand upright and keep the tape measure level all around your body.
- Take measurements in front of a mirror to avoid tape tilt.
- Measure twice and use the average if readings differ.
Underbust Tips
Pull the tape snug, but not painfully tight. If it slides around easily, it is too loose. If it digs deeply into skin, it is too tight.
Full Bust Tips
Measure at the fullest point of your bust, keeping arms relaxed. Avoid pulling the tape tight—this should be comfortably level and natural.
Cup Difference Guide (Approximate)
These ranges are approximate and vary by manufacturer, but they help explain how cup letters are assigned:
| Difference (Bust - Band) | Approximate Cup |
|---|---|
| 0 in | AA |
| 1 in | A |
| 2 in | B |
| 3 in | C |
| 4 in | D |
| 5 in | DD/E |
| 6 in | DDD/F |
| 7 in | G |
| 8 in | H |
Understanding Sister Sizes
Sister sizes have similar cup volume with a different band fit. If your band feels too tight or too loose, sister sizes help you adjust without changing cup volume too much.
- Go down one band size (for example, 34 to 32) and go up one cup (C to D).
- Go up one band size (34 to 36) and go down one cup (C to B).
Example: 34C, 32D, and 36B are classic sister sizes.
Common Fit Problems and Quick Fixes
Band rides up in the back
The band is likely too loose. Try a smaller band and adjust cup size accordingly (sister sizing).
Straps dig into shoulders
Straps should not carry most of the weight. A better-fitting band usually solves this.
Cups gape or wrinkle
Try a smaller cup, different cup shape, or a style that matches your breast shape better (balconette, plunge, full cup, etc.).
Spillage at top or sides
You likely need a larger cup, a different wire width, or both.
US vs UK vs EU Sizing Notes
This calculator returns a US-style starting size. International conversions are not always one-to-one across brands. Always check each brand’s chart and fit reviews.
- US: often uses DD, DDD/F, G, H...
- UK: often uses DD, E, F, FF, G, GG...
- EU: band numbers and cup progression can differ.
Frequently Asked Questions
Is this calculator accurate?
It is accurate as a starting point, especially when measurements are precise. Final fit still depends on style and brand.
Should I measure in centimeters or inches?
Either works. The calculator converts internally and applies the same logic.
How often should I remeasure?
Every 6-12 months, or after major weight changes, pregnancy, postpartum changes, hormonal shifts, or changes in training routines.
Final Thoughts
A good bra size is about comfort, support, and confidence—not just a label. Use this bra cup size calculator to find a reliable baseline, then fine-tune with sister sizes and real-world try-ons. Small adjustments can make a big difference in all-day comfort.