CalcPro

Bra Size Calculator

Estimate US bra band and cup size from two measurements.

How it works

The US bra sizing system relies on two independent measurements: your underbust (the circumference directly beneath your bust) and your bust (the fullest part of your chest). The calculator uses these to determine both your band size and your cup size.

Band size is based on underbust measurement rounded to the nearest even number. Cup size is determined by the difference between your bust and underbust measurements—each inch of difference equals one cup size.

The formula

Band size = round underbust to nearest even number; Cup size = (bust − underbust) in inches, where 1" = A cup, 2" = B cup, 3" = C cup, 4" = D cup, 5" = DD cup, and so on

Worked example

Let's say you measure:

  • Underbust: 32.5 inches
  • Bust: 36 inches

Step 1: Calculate band size Round 32.5 to the nearest even number: 32 (your band size).

Step 2: Calculate the difference 36 − 32.5 = 3.5 inches

Step 3: Convert difference to cup size

  • 1 inch difference = A cup
  • 2 inch difference = B cup
  • 3 inch difference = C cup
  • 3.5 inch difference = between C and D cups

Round 3.5 to the nearest full cup: D cup.

Result: 32D

In this example, you'd start by trying 32D bras. If the band feels loose, try 30DD (moving up one cup size while dropping the band). If it feels tight, try 34C.

Common mistakes

Measuring over clothing is the most frequent error. Thick fabrics add inches and skew your result. Always measure against bare skin or a thin layer.

Pulling the tape too tight (or too loose) changes both numbers. The tape should sit snug but not compress your skin. A friend can help ensure consistency.

Forgetting that band sizes are even only. If you measure 33 inches underbust, you must round to either 32 or 34—there is no 33 band size in standard US sizing. Most fitters recommend rounding down for a snugger fit, since bands stretch with wear.

Ignoring cup size changes when band size changes. If a 34C feels loose in the band, don't just try 32C. Instead, try 32D—moving down one band size means moving up one cup size to keep the same volume. The cup volume in 34C equals the cup volume in 32D.

Not accounting for brand variation. A 32D from one manufacturer may fit very differently from another's 32D due to differences in wire placement, cup depth, and stretch. Use this calculator as a starting point, then try on multiple brands to find your true fit.

Frequently asked questions

What measurements do I need?

You need two measurements in inches: your underbust (measured snugly around your ribcage directly under your bust) and your bust (measured around the fullest part of your chest while wearing an unpadded bra or no bra).

How accurate is this calculator?

This gives a reliable starting point based on the standard US sizing formula. However, fit varies by brand and style. Always try on bras to confirm, as cup depth, wire width, and band stretch differ between manufacturers.

Why do my measurements give a half-cup size?

If the difference between bust and underbust is an odd number, you'll land between standard cup sizes. Round to the nearest full cup, or try both sizes—many brands make half-cup increments, and fit preferences vary.

Should I measure over or under my clothes?

Measure directly against bare skin or a thin layer like a t-shirt. Thick sweaters or padding will throw off your measurements. Stand straight and relax; don't pull the measuring tape tight.

What if my band size is odd?

Bra bands come in even sizes only (32, 34, 36, etc.). If you measure an odd number, round down for a snugger fit or up for more comfort. Many people prefer rounding down because bands stretch slightly with wear.

Can I use centimeters instead of inches?

This calculator uses inches. To convert, divide your cm measurements by 2.54, then enter the result.

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