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Bra Size Calculator

Find your true fit (US, UK & EU)
Measure tight around ribcage, just under bust.
Measure loosely around fullest part of bust.
Your US/UK Size 30D
European (EU) -
Sister Size Up -
Australia (AU) -
Sister Size Down -

Why 80% of Women Wear the Wrong Size

Finding the perfect bra is notoriously difficult. Studies suggest that 8 out of 10 women are wearing the wrong size, usually choosing a Band that is too big and a Cup that is too small.

This happens because of the "Add 4" myth. In the 1930s, bra materials were stiff, so retailers told women to add 4 inches to their rib measurement for comfort. Modern elastic materials mean this rule is outdated, yet many stores still use it.

Our calculator uses the modern method: Your rib cage measurement is your band size. This ensures the band takes 80% of the weight, relieving pressure on your shoulders.

How to Measure Correctly

To get an accurate result, you need a soft measuring tape and a non-padded bra (or no bra).

1. The Band (Ribcage)

Wrap the tape measure around your ribcage, directly under your bust. Make sure it is snug and level (parallel to the floor). Exhale to get the smallest measurement.

Example: If you measure 29 inches, round to the nearest even number (30). Your band is 30.

2. The Bust (Cup)

Measure loosely around the fullest part of your bust. Ensure the tape is straight across your back.

Example: If you measure 34 inches.

The Math: Calculating Cup Size

Cup size is simply the difference between your Bust and your Band.

Formula: Bust - Band = Cup

  • 1 inch diff = A Cup
  • 2 inch diff = B Cup
  • 3 inch diff = C Cup
  • 4 inch diff = D Cup

In our example: 34 (Bust) - 30 (Band) = 4 inches. The size is 30D.

The Magic of "Sister Sizes"

If your calculated size fits in the cup but feels too tight in the band (or vice versa), you need a Sister Size.

Cup volume is relative to band size. A 34C cup holds the same volume as a 32D and a 36B.

The Rule:
• If you go UP a Band size, you must go DOWN a Cup size to keep the same volume.
• If you go DOWN a Band size, you must go UP a Cup size.

Signs of a Poor Fit

How do you know if you need to remeasure?

  • Spillage: If breasts spill over the top/sides, the cup is too small. Try a Sister Size (Up Cup, Down Band).
  • Riding Up: If the back band rides up your back, the band is too big. Size down the band.
  • Gaping: If there is empty space in the cup, it is too big or the wrong shape for your body.
  • Shoulder Pain: Straps digging in usually means the band is too loose and not providing support.

Body Measurement Tools

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