Bra Size Calculator & Fitting Guide
Enter your underbust and fullbust measurements to see your band size, cup letter and sister sizes.
Measure just below the bust, snug but loose enough to breathe.
Measure at the fullest point of the bust, keeping the tape parallel to the floor.
Results are based on EU/TR standard sizing. Fit varies by brand and style, so check the brand's own size guide before you buy.
How to Measure Your Bra Size at Home
You need two measurements: underbust circumference (sets the band size) and fullbust circumference (sets the cup size). The difference between them gives the cup letter. Use a soft dressmaker's tape measure for both.
Step 1: Measuring Your Underbust (Band Size)
Wrap the tape snugly around your torso just below your bust, parallel to the floor. It should be firm but not dig in. The calculator rounds the result up to the next even number in cm to get your band size. Example: 77 cm → 78, 73 cm → 74, 80 cm → 80.
Band Size = Underbust (cm) → round up to the next even number
Step 2: Measuring Your Bust (Cup Size)
Wrap the tape around the fullest part of your bust, parallel to the floor. Don't pull it tight; let it rest snugly. Leaning slightly forward as you measure lets your breasts fall into their natural position, which gives a more accurate reading.
How Is Cup Size Calculated?
Cup size is a letter based on the difference in centimeters between your fullbust measurement and your band size. In this calculator, each letter after A covers a 2 cm range. The larger the difference, the later the letter in the alphabet.
The Difference Between Bust and Underbust Measurements
Cup Difference = Fullbust − Band Size (cm)
Example: 79 cm underbust → 80 band; 87 cm fullbust − 80 cm band = 7 cm difference → D cup → 80D
Understanding A, B, C, D, DD, and Larger Cup Sizes
| Cup Letter | Difference (cm) | Description |
|---|---|---|
| AA | under 1 cm | Very small volume |
| A | 1–2 cm | Small volume |
| B | 2–4 cm | Small-medium volume |
| C | 4–6 cm | Medium volume |
| D | 6–8 cm | Medium-large volume |
| DD | 8–10 cm | Large volume |
| E | 10–12 cm | Large volume |
| F | 12–14 cm | Full-figure volume |
| G | 14–16 cm | Full-figure volume |
| H | 16 cm and over | Extended sizing / specialty |
What Are Sister Sizes in Bras?
Sister sizes share the same cup volume but have different band sizes. Going up one band size means dropping one cup letter, and the reverse, so the cup volume stays the same. They help when your exact size is unavailable or you fall between sizes.
How to Use the Sister Size Chart (e.g., Switching from 34C to 36B or 32D)
Yes. The sister sizes of 34C are 36B (looser band, one cup smaller) and 32D (tighter band, one cup larger). Cup volume is the same across all three. If the band feels too tight, try 36B; if it's too loose, try 32D. The same rule works for every size.
Sister Size Conversion Table
| Smaller Band Sister | Base Size | Larger Band Sister |
|---|---|---|
| 70C | 75B | 80A |
| 75C | 80B | 85A |
| 75D | 80C | 85B |
| 80D | 85C | 90B |
| 80E | 85D | 90C |
| 85E | 90D | 95C |
Signs You Are Wearing the Wrong Bra Size
Slipping Straps, Riding Up Back Bands, and Cup Spillage (Double Boob Effect)
One or more of the following signs typically indicates an incorrect bra size:
Straps keep slipping off
Band too large → straps doing all the support work
→ Go down a band size, up a cup letter
Band rides up in back
Band too big → pulls upward instead of staying level
→ Go down a band size
Cup spillage / double boob
Cup too small
→ Go up a cup letter
Cups wrinkle or gap
Cup too large
→ Go down a cup letter
Underwire digs in
Band too tight or cup too small
→ Try a sister size (up band + down cup)
Center gore doesn't lie flat
Cup too large or wrong shape for your body
→ Go down a cup letter or try a sister size