# Bra Size Calculator

URL: https://speedycalculator.com/calculators/everyday-life/bra-size/
Last updated: 2026-10-25

> The bra size calculator works out your band and cup size in US, UK and European sizing from your underbust and bust measurements, with sister sizes.

## Inputs

- Underbust (in, cm)
- Bust (in, cm)

## Results

- US / UK band size
- Cup difference (in)
- US cup
- UK cup
- EU band
- EU cup
- Sister size down: band
- Sister size down: cup (US)
- Sister size up: band
- Sister size up: cup (US)

## Formula

```
US/UK band = underbust rounded to the nearest even inch
Cup = bust − band: 1 in = A, 2 = B, 3 = C, 4 = D, 5 = DD …
EU band ≈ 2.5 × US band − 10 (32 → 70); EU cups follow US letters but use E and F for DD and DDD
```

## Explanation

Most people wear the wrong bra size, usually with a band that is too loose and a cup too small. The band provides most of the support, so it should be snug on the loosest hook. Cup letters stand for the difference between bust and band, so a 34C and a 36C have different cup volumes – sister sizes like 36B and 32D share the same cup volume.

### How to use the Bra Size Calculator

- **Underbust**
- **Bust**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Measure snugly around your ribcage just under the bust (underbust) and loosely around the fullest part of the bust while wearing an unpadded bra.

### Tips

- Measure over thin clothing with a soft tape held snug but not tight, and keep the tape level all the way round.
- Take each measurement twice; if you are between sizes, the larger size is usually the safer choice.
- Size charts vary by brand and country – check the maker's own chart when it is available.
- For sewing projects, pre-wash fabric before cutting and buy 10% extra for shrinkage, pattern matching and mistakes.

### Common mistakes to avoid

- Mixing inches and centimetres between the tape and the size chart.
- Forgetting seam and hem allowances when cutting fabric.
- Measuring an old, stretched garment instead of your body.

### Inputs explained

FieldWhat to enterUnderbustUnits: in, cmBustUnits: in, cm

### What each result means

- **US / UK band size** = 2×round([Underbust]÷0.0254÷2)
- **Cup difference (in)** = max(round([Bust]÷0.0254 - [US ÷ UK band size]), 0)
- **US cup** = min([Cup difference (in)], 14)
- **UK cup** = min([Cup difference (in)], 14)
- **EU band** = 2.5×[US ÷ UK band size] - 10
- **EU cup** = min([Cup difference (in)], 14)
- **Sister size down: band** = [US ÷ UK band size] - 2
- **Sister size down: cup (US)** = min([Cup difference (in)] + 1, 14)
- **Sister size up: band** = [US ÷ UK band size] + 2
- **Sister size up: cup (US)** = max([Cup difference (in)] - 1, 0)

### Example values

How the main result changes when **Underbust** changes and everything else stays at the example values above:

Underbust (in)US / UK band size161624243232404048486464

## Worked example

Inputs — Underbust: 32 in; Bust: 36 in.

Results — **US / UK band size = 32**; Cup difference (in) = 4; US cup = D; UK cup = D; EU band = 70; EU cup = D; Sister size down: band = 30; Sister size down: cup (US) = DD (E); Sister size up: band = 34; Sister size up: cup (US) = C.

## FAQ

**How do I measure my bra size at home?**
Measure under the bust snugly for the band and around the fullest part loosely for the bust; the difference in inches gives the cup.

**What are sister sizes?**
Sizes with the same cup volume on a different band: go down a band size and up a cup (34B → 32C) or the reverse.

**Why are UK and US cups different above D?**
The US continues DD, DDD, G…, while the UK uses DD, E, F, FF, G, GG… so large cups differ by letter.

**What formula does the Bra Size Calculator use?**
US/UK band = underbust rounded to the nearest even inch.