Bra Size Calculator

What is a bra size calculator?

A bra size calculator estimates your US bra size from two measurements: your underbust (band) and your bust. Enter both numbers in inches in the tool above and it instantly returns your band number, your cup letter, and the combined size such as 32C or 34D, along with sister sizes to try.

It answers the question most people are really asking — “what is my bra size?” — without you having to remember a chart. Because fit varies by brand and style, the result is a well-grounded starting point rather than a single fixed answer, which is why the tool also points you toward sister sizes.

How does the bra size calculator work?

The calculator uses the standard US sizing method built from your two measurements. The band is your underbust measurement rounded to the nearest even inch. The cup comes from the difference between your bust and your band: each whole inch of difference equals one cup size.

The exact method the widget uses is:

band = underbust rounded to the nearest even inch
difference = bust − band
cup: 0 = AA, 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, 6 = DDD (and up)
bra size = band + cup

A few definitions and units:

• Underbust — the snug measurement around your ribcage directly under your bust, in inches. Rounded to the nearest even inch, this becomes your band number.
• Bust — the measurement around the fullest part of your chest, in inches.
• Band — the number part of your size (for example, the 34 in 34D). It carries the bra’s support.
• Cup — the letter part, derived from how many inches your bust is larger than your band.
• Sister sizes — sizes with the same cup volume but a different band, useful for fine-tuning fit.

Because each inch of difference maps to exactly one cup, the cup letters step up predictably: 0 inches is AA, then A, B, C, D, DD, DDD, and larger sizes continue from there.

Examples

Each example below matches the tool’s output exactly.

Example 1 — underbust 32 in, bust 35 in

• Band: 32 (underbust 32 already rounds to the nearest even inch)
• Difference: 35 − 32 = 3 inches
• Cup: 3 inches is a C cup
• Bra size: 32C

Example 2 — underbust 34 in, bust 38 in

• Band: 34 (underbust 34 rounds to the nearest even inch)
• Difference: 38 − 34 = 4 inches
• Cup: 4 inches is a D cup
• Bra size: 34D

Example 3 — underbust 30 in, bust 35 in

• Band: 30 (underbust 30 rounds to the nearest even inch)
• Difference: 35 − 30 = 5 inches
• Cup: 5 inches is a DD cup
• Bra size: 30DD

Notice that the cup letter depends only on the inch difference, while the band tracks your underbust. Two people with the same difference can wear different cup letters at different bands, which is exactly why sister sizes exist.

Cup size by bust-to-band difference

This table shows how each inch of difference between bust and band maps to a US cup letter, straight from the method above.

Bust − band (inches)	Cup size
0	AA
1	A
2	B
3	C
4	D
5	DD
6	DDD

Combine the cup letter with your band number to get the full size. For example, a band of 34 with a 4-inch difference is 34D, and a band of 32 with a 3-inch difference is 32C.

What can you use a bra size calculator for?

The same estimate helps in several practical ways:

• Shopping online — getting a size to start from when you cannot try bras on in a store.
• Re-checking your size — bodies change with weight, fitness, age and pregnancy, so a quick remeasure can catch a size you have outgrown.
• Understanding sister sizes — learning which neighbouring sizes share your cup volume so a near-miss fit still works.
• Comparing brands — using a consistent number-and-letter baseline when one label runs small or large.
• Planning for body changes — remeasuring across pregnancy and nursing, when band and cup can shift noticeably over a few months.

Tips and common mistakes

A few points help you read the result correctly:

• Measure the band snugly, the bust loosely. The underbust tape should be firm against your ribs; the bust tape rests gently over the fullest part without compressing.
• Round the underbust, not the bust. The band is the underbust rounded to the nearest even inch; the cup uses the raw difference.
• Do not skip sister sizes. If your calculated band feels too tight, go down a band and up a cup (for example 34C to 32D); too loose, do the reverse.
• Measure in a non-padded bra or none at all. Padding inflates the bust number and pushes the cup too large.
• Remember brands differ. A “perfect” 34D in one label may fit like a 34C or 36C in another, so treat the calculator as a starting point, not a verdict.

Limitations and notes

This tool uses one widely taught US sizing method and rounds the band to the nearest even inch, so it cannot capture every fitting nuance. Real fit depends on bra style, fabric stretch, wire shape, your torso, and how a bra has worn in over time, which is why two bras of the same labelled size can feel different. The result is an estimate and a starting point, not a guaranteed fit, and checking sister sizes plus trying a bra on remains the best confirmation. Sizing conventions also differ between regions, so a US estimate will not map one-to-one to UK or EU labels.

Your measurements stay in your browser. The calculation runs entirely on your device — nothing you type is uploaded or stored, so your numbers remain private.

For more tools as your body changes, pair this with the due date calculator to project a due date and the ovulation calculator to plan your fertile window, or browse all related tools on the pregnancy & women page.