Bra Size Calculator

A bra size calculator is an online tool that helps you determine your band and cup size using two simple measurements: your underbust and your bust. Enter both measurements below to get an estimated size, plus one or two sister sizes to try if the first doesn't fit quite right.

Calculate Your Size

Select Unit: Underbust (Band) Measurement: Bust Measurement:

Your bra size will appear here.

DD/E

DDD/F

H+

Where Your Measurement Falls

Why the Right Bra Size Matters

Wearing the right bra size is not just about appearance — it's about comfort, support, and confidence. Industry surveys are widely cited as finding that a large majority of women wear the wrong bra size, which often leads to unnecessary discomfort. The right bra can improve posture, prevent back pain, and even make your clothes fit better.

Imagine walking all day in shoes that are too small or too loose — your feet would ache, your posture would be affected, and you'd feel uncomfortable. The same logic applies to bras. A poorly fitted bra can dig into your skin, slip off your shoulders, or fail to provide proper support. A well-fitted bra, by contrast, feels almost invisible while giving you freedom of movement and support.

Beyond comfort, the right bra size also affects how your clothes sit. When your bra fits correctly, your outfits look smoother and your silhouette appears more balanced.

How to Measure for a Bra Size

Underbust (band): Wrap a soft tape measure snugly around your ribcage, directly under your bust, keeping it level and parallel to the floor.
Bust: Wrap the tape around the fullest part of your chest, keeping it relaxed rather than tight.

Measure in your usual bra, or unsupported, but be consistent — mixing the two will skew your result.

How Bra Sizing Works (Band + Cup)

Bra sizing might seem confusing at first, but it comes down to two components: band size and cup size.

Band Size: The number part of your bra size (e.g., 32, 34, 36). This calculator rounds your direct underbust measurement to the nearest even number to get it.
Cup Size: The letter part of your bra size (e.g., A, B, C, D). It's based on the difference between your bust measurement and your underbust measurement — each inch of difference generally moves you up one cup letter.
For example: if your underbust measures 34 inches and your bust measures 37 inches, the difference is 3 inches, which lands on a C cup — making your estimated size 34C.

Chart showing a 3-inch bust-underbust difference landing in the C-cup zone, giving an estimated size of 34C

Bra sizes are also relative, not absolute — a 34C and a 36B don't look identical on the body even though they're considered similar cup volumes. That relationship is called sister sizing, covered below.

Modern Method vs. the Older "Plus Four" Method

This calculator uses the direct-measurement method that most professional fitters currently recommend: your underbust measurement, rounded to the nearest even number, is your band size.

Older sizing guidance commonly added 4 or 5 inches to the underbust measurement before rounding — a method that dates back to less elastic bra fabrics. With today's stretchier materials, that extra allowance usually results in an oversized, less supportive band. Because of this, decades-old size charts, hand-me-down bras, or some department-store fittings may label the same body with a noticeably larger band number than this calculator does.

Understanding Sister Sizes

A "sister size" shares roughly the same cup volume as your calculated size but reaches it with a different band-and-cup combination. Going up one band size while going down one cup letter (or the reverse) generally keeps the cup volume similar. If your calculated size runs tight in the cup but loose in the band, or vice versa, a sister size is usually the first thing worth trying — this calculator lists them automatically alongside your main result.

Signs Your Bra Doesn't Fit

The band rides up in the back instead of sitting level with the front.
Straps dig into your shoulders or slip down repeatedly.
The cup gapes, wrinkles, or your tissue spills over the top or sides.
The center gore (the fabric between the cups) doesn't sit flat against your sternum.

A Note on International Sizing

US, UK, EU, and French sizing all describe the same measurements differently. Cup letters can shift by one letter between systems at the same measurements, and EU/French band numbers are based on centimeters rather than inches, using an entirely different numbering scale. Because of this, converting a size across systems is only approximate — checking a specific brand's own size chart is more reliable than converting between systems.

This calculator gives an estimated starting size based on your measurements. Actual fit varies by brand, style, and body shape, so trying on a few options — including your calculated size and its sister sizes — is the best way to confirm a comfortable fit.

Frequently Asked Questions

What is a Bra Size Calculator?

A Bra Size Calculator is an online tool that helps you estimate your band and cup size using two measurements: your underbust (band) and your bust at its fullest point.

How do I use the Bra Size Calculator?

Measure your underbust and your bust, enter both values, and click Calculate. You'll get your estimated size plus one or two sister sizes with a similar cup volume.

Why is it important to know my correct bra size?

Wearing the right size improves comfort and support, reduces shoulder and back strain from an ill-fitting band or straps, and helps clothes sit better.

Why might this calculator give a different size than an old bra I own?

This calculator uses the current recommended method, which rounds your direct underbust measurement to the nearest even number. Older sizing commonly added 4-5 inches to the underbust before rounding, a method now considered outdated for today's stretchier fabrics. That older method typically produced a larger band number than the direct-measurement method used here.

What are sister sizes?

Sister sizes are different band-and-cup combinations that share roughly the same cup volume. Going up one band size while going down one cup letter (or the reverse) usually keeps the cup volume similar, which is useful when a brand runs differently than expected.

Is the Bra Size Calculator accurate?

It gives a reliable starting point based on your measurements, but actual fit varies by brand, style, and body shape. Treat the result as a size to try first, then adjust using sister sizes or in-person fitting if needed.

At The End

Bra sizing is both a science and an art. Knowing your measurements gives you a reliable starting point, but trying on different styles is often necessary since every brand has slight variations.