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.

Worked example: a 32-inch underbust, 36-inch bust

Step by step:

1. Underbust measurement: 32 inches. Already an even number, so band size is 32. (If you measured 31, you’d try both 30 and 32; if 33, you’d try 32 and 34. The +4/+5 rule from older guides would have steered you to 36 or 37, both of which ride up at the back on modern stretch fabric.)
2. Full bust: 36 inches.
3. Difference: 36 − 32 = 4 inches → cup D (US).
4. Computed starting size: 32D US.

Cross-region equivalents for 32D US:

• UK: 32D (same)
• EU (per EN 13402-3): 70D — the EU band number is underbust in cm rounded down to the nearest 5: 32 in = 81 cm → 70 band? No: EU bands are 65/70/75/80/85, where 70 corresponds to underbust ~68-72 cm. 32 in ≈ 81 cm, which maps to EU 80. The lookup tables in our bra size calculator handle the cm-to-band conversion correctly.
• French/Spanish: band 95 (EU band + 15)
• Italian: cup ranges, typically “4” for D
• Australian: 12D (band number reduced by 20)

Sister sizes to try if 32D doesn’t fit perfectly:

• Band loose, cups fine: 30DD (smaller band, larger cup, same volume)
• Band fine, cups overflow: 32DD (same band, larger cup)
• Band tight, cups fine: 34C (larger band, smaller cup, same volume)
• Band tight, cups wrinkle: 34B (larger band, smaller cup, smaller volume)

Common mistakes that produce the wrong size

• Measuring over a padded or push-up bra. Padding adds 1-3 inches to the bust measurement, inflating cup size dramatically. McGhee & Steele (2010) found 71% of women in their fitting study had inflated bust measurements from this single error. Measure in an unpadded soft bra or sports bra only.
• Holding the tape too tight on the band measurement.Pulling the tape an extra ½ inch tight subtracts a whole band size from the result. Snug means “no slack, no compression.” If you can’t slip a finger between tape and skin, loosen.
• Letting the tape slope down the back. A tape that dips below horizontal at the back gives a looser, larger band measurement than the actual horizontal underbust. Use a mirror and adjust until the tape is parallel to the floor all the way around.
• Trusting the “Bratabase” or “A Bra That Fits” methodology uncritically.The Reddit “Bratabase” method computes cup from a four-measurement system (underbust, snug, loose, leaning). It produces consistently smaller bands and larger cups than the two-measurement method. Some women report better fit with it; many find the cups too large. Use the two-measurement method first; switch only if the result is consistently off.
• Ignoring weight or hormonal changes. A 5-lb weight change typically shifts band size by 1 step. Pregnancy, menstrual cycle, hormonal birth control, and menopause all change cup volume by 0.5-1.5 cup sizes. Re-measure every 6-12 months or after any significant body change.

When the at-home method does NOT apply

• Asymmetric breasts (volume difference of 1+ cup sizes). The two-measurement method captures one number; asymmetry needs an in-person fitter or a specialised retailer with asymmetric-cup options. Up to 25% of women have ≥1 cup difference between sides, though most are within ¼-½ cup.
• Post-mastectomy and lumpectomy fitting. Prosthesis-compatible bras have specific construction (front opening, internal pockets, no underwires near the surgical site). Standard sizing tables don’t map cleanly. Specialty fitters certified by the BCRF or affiliated organisations handle these fittings.
• Maternity and nursing. Cup size changes weekly during late pregnancy and through the first 4-6 weeks postpartum as milk supply establishes. A standard fitting captures the wrong moment. Buy stretchy nursing bras 1 cup larger than your pre-pregnancy size and re-measure at week 6 postpartum.
• High-impact sports. Running, HIIT, and horseback riding need compression bras that fit differently from encapsulation bras. The Research Group in Breast Health (University of Portsmouth) publishes impact-specific fitting protocols. Use sport-specific sizing from the bra brand rather than translating from your everyday size.

The four signs a bra fits correctly

Independent of any sizing system, four observable criteria distinguish a well-fitting bra from a poorly fitting one. The professional fitting checklist from the McGhee & Steele (2010) study:

1. The band sits horizontally.If the band rides up the back, it’s too loose; drop a band size and go up a cup. The band — not the straps — provides 80% of the support.
2. The centre gore lies flat against the sternum. If it floats away from the body, the cups are too small. If it digs in, the band may be too tight or the cups too large.
3. The cups fully contain the breast tissue without spillage or wrinkles. Spillage (over the top, under the arm) means cup too small. Wrinkles or gapping mean cup too large.
4. The straps don’t dig in or slip. Tightening straps to compensate for a loose band is the universal wrong fix; it causes shoulder pain and posture issues without actually adding support.

A bra that passes all four checks is the right size for you in that brand and style — independent of what the label says. Two bras labelled 32D from different brands can pass for one body and fail for another. The checklist beats the size number.

Cross-region size conversion at a glance

The same physical body gets a different size code in different markets. The systems overlap incompletely; small mismatches reflect genuine differences in band construction. Use this table to translate a known good-fit size from one region to another:

Cup-letter mapping is approximate because the underbust-to-bust difference per cup varies between systems. The US uses 1-inch-per-cup; the UK historically used the same but switched to a half-inch-per-cup increment above DD. EU systems use 2 cm per cup consistently. A US 32DD ≈ UK 32DD ≈ EU 70E ≈ AU 12DD, but a US 32G doesn’t map directly because the US sister-size taxonomy and the UK extended-cup taxonomy diverge above F.

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.