Bra size calculator

What is a bra size calculator?

A bra size calculator turns two simple body measurements — your band (underbust) and your bust (the fullest part of your chest) — into a standard bra size such as 34C. A bra size is made of two parts: a band number that describes the snug measurement around your ribcage just under the bust, and a cup letter that reflects how much larger your bust is than your band.

Because women’s lingerie is sold under several regional standards, the same body can be labelled differently in the United States, the United Kingdom, and continental Europe. This tool removes the guesswork: enter two numbers, pick your sizing system, and read off the matching size.

Why band and cup are measured separately

The band carries most of a bra’s support, so it must fit firmly around the ribcage. The cup, by contrast, depends on the difference between your bust and band — not on the bust alone. Two people with an identical 36-inch bust can wear very different sizes if their underbust measurements differ, which is exactly why a single “chest size” is not enough.

This separation also explains the idea of sister sizes: as the band goes up, the cup that holds the same volume goes down a letter, and vice versa.

How does the calculator work?

1. Measure your band. Wrap a tape measure snugly around your ribcage, directly under your bust, keeping it level.
2. Measure your bust. Measure around the fullest part of your chest, with the tape level and relaxed.
3. Find the band size. In the US/UK systems the underbust measurement (in inches) is rounded to the nearest even number. In the EU system the underbust (in centimetres) is rounded to the nearest 5.
4. Find the cup. Subtract the band from the bust. Each unit of difference moves you up one cup letter.

Formula

Step 1 — Band size

For US and UK sizing (inches):

$$\text{band} = \text{round}\!\left(\frac{\text{underbust}_{in}}{2}\right) \times 2$$

For EU sizing (centimetres):

$$\text{band}_{EU} = \text{round}\!\left(\frac{\text{underbust}_{cm}}{5}\right) \times 5$$

Step 2 — Cup difference

$$\Delta_{in} = \text{bust}_{in} - \text{band}$$ $$\Delta_{cm} = \text{bust}_{cm} - \text{band}_{EU}$$

Step 3 — Cup letter

In the US system the inch difference maps to the cup as: 1 → A, 2 → B, 3 → C, 4 → D, 5 → DD, 6 → DDD, and so on. The UK system shares A–DD but continues E, F, FF, G. The EU system uses a centimetre difference: 12–13 cm → A, 14–15 cm → B, 16–17 cm → C, 18–19 cm → D, 20–21 cm → E.

Examples

Example 1: US size from inches

Underbust = 32 in, bust = 36 in.

$$\text{band} = \text{round}\!\left(\frac{32}{2}\right) \times 2 = 32$$ $$\Delta_{in} = 36 - 32 = 4 \Rightarrow \text{D}$$

Result: 32D.

Example 2: UK size with a larger cup

Underbust = 30 in, bust = 35 in.

$$\text{band} = 30, \quad \Delta_{in} = 35 - 30 = 5 \Rightarrow \text{DD (UK)}$$

Result: 30DD.

Example 3: EU size from centimetres

Underbust = 70 cm, bust = 85 cm.

$$\text{band}_{EU} = \text{round}\!\left(\frac{70}{5}\right) \times 5 = 70$$ $$\Delta_{cm} = 85 - 70 = 15 \Rightarrow \text{B}$$

Result: 70B.

Practical notes

• Measure at the end of the day and over a non-padded bra (or bare skin) for the most representative numbers.
• Sizing is an approximation: brands cut bras differently, so treat the result as a confident starting point and try sister sizes if the fit is off.
• If the band rides up at the back, go down a band size and up a cup; if it digs in, do the opposite.
• A correctly fitting band should sit horizontal and snug, while the cups fully contain the bust without gaps or spillage.

Frequently Asked Questions

How do I measure for a bra at home?

Use a soft tape measure. Measure snugly around your ribcage just under the bust for the band, then around the fullest part of the chest for the bust. Keep the tape level in both cases and stand relaxed.

What does the cup letter actually mean?

The cup reflects the difference between your bust and band, not the absolute bust size. A one-unit difference is roughly an A cup, two units a B, three a C, and so on, which is why band and cup must be read together.

Why is my US size different from my EU size?

Each region rounds the band differently (even inches in the US/UK, nearest 5 cm in the EU) and labels cups on its own scale. The underlying body is the same — only the printed label changes.

What are sister sizes?

Sister sizes hold the same cup volume with a different band. Going up one band size and down one cup letter (or the reverse) gives a near-equivalent fit, useful when your exact size is out of stock.

For related conversions, see the clothing size calculator, the dress size calculator, and the shoe size converter.