How to measure your bra size at home (the modern method, not the +4 rule)

Eighty percent of women wear the wrong bra size — a stat cited in every fitting guide for the last 20 years. The cause is usually one of two things: never having measured, or having measured with the obsolete +4/+5 rule that gives consistently loose bands. Two measurements done right gets you to the modern correct size in three minutes.

What you need

• A fabric tape measure (paper works in a pinch).
• An unpadded soft bra or sports bra (not a push-up).
• A mirror, ideally — keeps the tape level.

Measure 1: band (underbust)

Wrap the tape around your ribcage directly under your bust, parallel to the floor. Pull snug— the tape shouldn’t dig in, but it shouldn’t have any give either. Exhale fully. Read the measurement.

Round to the nearest even inch. That’s your band size. (Most US/UK band sizes are even numbers. If your measurement lands on an odd number, both the immediately smaller and immediately larger even number are worth trying — band fit varies dramatically between brands.)

Measure 2: bust (full)

Same tape, same level (parallel to the floor), now around the fullest part of your bust. Pull straight, no compression. Lean slightly forward if your bust is fuller at the bottom than the top — the goal is to capture full volume.

Derive the cup

Subtract band from bust in inches. The difference is your US cup:

• 0 inches = AA
• 1 inch = A
• 2 inches = B
• 3 inches = C
• 4 inches = D
• 5 inches = DD
• 6 inches = E (or DDD in US-only crosswalk)
• 7 inches = F

Beyond 7 inches the lettering varies by region — see our bra size calculator for the full US/UK/EU crosswalk.

Sister sizes — what to try if the math feels off

Sister sizes have the same cup volume but different band size: 34D, 32DD, and 36C are all roughly the same cup volume. Go up a band, down a cup; or down a band, up a cup. Useful when:

• The band feels loose: try the smaller-band sister size.
• The band rides up at the back: smaller band, larger cup.
• The cups wrinkle: smaller cup, larger band.
• The cups overflow: larger cup, smaller band.

The pragmatic fitting flow

1. Measure as above. This is your starting size.
2. Order or try on that size in a brand you trust.
3. If the fit is off, try the sister size in the direction matching the problem.
4. For online ordering: pick stores with free returns, order 3 sizes (your computed + sister-up + sister-down), keep what fits.
5. Once you find a brand + size combo that works, that’s your size in that brand. Other brands will vary.

When a professional fitting helps most

If your underbust-to-bust difference is more than 7 inches (US DD+), if you find anything that fits but it’s always uncomfortable, or if you’re shopping for specialty needs (post-mastectomy, sports for high impact, nursing), an in-person fitting at a specialty store (Bravissimo, Soma in the US, Rigby & Peller in the UK) is worth the appointment. Department-store fittings are often no better than measuring yourself with our calculator.