Bra Measuring Methodologies Explained: Rib Cage, Plus-4, Armpit, and International Standards
There are four main methodologies used to determine bra size worldwide: the rib cage (direct underbust) method, the plus-4/plus-5 method, the armpit (overbust) method, and international standards like Europe's EN 13402. Each measures a different point on the body, or adds a different adjustment, which is why the same person can measure a different size depending entirely on which methodology is used — not because her body changed.
Why Methodology Matters More Than the Measuring Tape
A measuring tape doesn't lie. But two people using the exact same tape, on the exact same body, following two different sets of instructions, can arrive at two different band sizes. The tape isn't the variable — the methodology is: where on the torso to measure, whether to round, and whether to add anything to the raw number before calling it a size.
Method 1: The Rib Cage Method (Direct Underbust Measurement)
The rib cage method, also called the direct underbust method, measures the torso directly beneath the bust — the exact spot where a bra band sits and does its job. The raw number, rounded to the nearest even whole number, becomes the band size with nothing added.
How it works:
- Wrap a soft tape directly under the bust, keeping it level and snug.
- Round to the nearest even number — 28.5 rounds down to 28; 29 rounds up to 30.
- That number is the band size, full stop.
This is BRABAR's EZ-fit method, and it's also the approach described in the published instructions of brands including SKIMS and ThirdLove. It measures the band at the anatomical location the band actually occupies, without a correction factor layered on top.
Method 2: The Plus-4 / Plus-5 Method
The plus-4 (and historically, plus-5) method also starts with a direct underbust measurement, but then adds a fixed number of inches before rounding to determine the band size. The extra inches were originally intended to compensate for the difference between the ribcage measurement and a theoretical, unmeasurable "true" chest circumference at the bust level. U.S. patent filings on bra-sizing tape measures describe this explicitly: the underbust circumference is measured, five inches is added, and the result is rounded to the nearest even number, with the patent literature itself describing the added inches as an "extrapolation or 'fudge factor'" rather than a direct anatomical measurement.
Over time, the convention in much of the U.S. market shifted from adding five inches to adding four, but the underlying approach — measure directly, then add a fixed number — is the same. Some current sizing calculators from mainstream retailers still use a version of this method today.
Why it distorts results: the added inches don't reflect anything measured on the actual body. They push the band size up regardless of the individual's actual proportions, which is why plus-4/plus-5 method calculators tend to produce a looser band than the rib cage method for the same person — and a looser band shifts support onto the straps, which are only designed to carry about 20% of a bra's load.
Method 3: The Armpit (Overbust) Method
The armpit method, sometimes called the overbust or high-bust method, measures higher on the torso — bringing the tape under the arms and across the back, near where the straps meet the cups, rather than directly under the bust. This is the method described in Victoria's Secret's published fitting instructions.
Because the torso is generally wider at that higher point than directly underneath the bust, this method tends to produce a larger band number than the rib cage method for the same body. It measures a real, consistent point on the body — it's simply a different point than where the band physically sits once the bra is on.
Method 4: International and Regional Standards
Outside brand-specific calculators, there are also formal, government- or industry-backed sizing standards, and they don't all agree with each other either.
EN 13402 (European Union): This is the joint European clothing-sizing standard, introduced in 2006 and based on metric anthropometric data. For bras specifically, EN 13402 defines the underbust and bust girths in centimeters, with the cup letter progression stepping every 2 cm of difference between bust and underbust — compared to the roughly 2.5 cm (1 inch) step used in US and UK systems. A European band size is calculated differently from a US one, which is why direct US-to-EU size conversions require a formula rather than a simple substitution.
UK sizing conventions: UK sizing historically also used a form of the plus-4 method, though UK industry guidance has increasingly moved toward direct-measurement approaches in recent years, converging more closely with the rib cage method.
Regional band-number systems: France, Belgium, and Spain use a permutation of the European system where cup sizes match but band numbers run exactly 15 higher than the equivalent European size. Italian sizing uses small consecutive integers starting near zero. None of these are wrong, exactly — they're simply different numbering conventions layered on top of similar underlying measurements, which is exactly why a size that's correct in one country's system can look alarming (or suspiciously small) translated literally into another.
A Related but Different Concept: Sister Sizing
Sister sizing isn't a fourth measuring methodology — it's a technique for finding an alternate size once you already know your true size under whichever method you're using. Moving up one band size and down one cup letter (or the reverse) keeps the cup volume roughly constant. It's useful when your exact size is out of stock or a specific style runs differently, but it works from an accurate starting size; it doesn't fix an inaccurate one. See our full explanation in Sister Sizing Explained.
Side-by-Side: One Body, Four Methodologies
Consider a person with a 28-inch direct underbust measurement and a 34-inch fullest-bust measurement:
| Methodology | Band size result | Cup calculation | Final size |
|---|---|---|---|
| Rib cage method | 28 (measured directly) | 34 − 28 = 6 inches | 28DDD/F |
| Plus-4 method | 28 + 4 = 32 | 34 − 32 = 2 inches | 32B |
| Armpit method | Typically 2-4 inches higher than direct measurement | Reduced accordingly | Often lands near 32B-32C |
| EN 13402 (European) | Converts to a different band number under the metric system | 2 cm cup steps instead of ~2.5 cm | Different number, comparable proportion |
Every method above is applied consistently and "correctly" by its own rules. The person's body never changed across that table — only the methodology did.
Why This Matters Most at the Small-Band End
The gap between methodologies is proportionally larger the smaller the actual band size. Adding four inches to a 40-inch band is a rounding error. Adding four inches to a 28-inch band changes the result by roughly 14%, and it's precisely in that smaller range — bands 28 to 32 — where the difference between an accurate size and an inflated one is most likely to produce a bra that provides almost no real support, since the band is doing most of the structural work regardless of size.
How to Identify Which Methodology Any Brand Is Using
You can reverse-engineer a brand's methodology in under a minute:
- Measure your own rib cage directly, underneath the bust, and round to the nearest even number.
- Look up that same number on the brand's official size chart.
- If the chart assigns you that number as your band size, the brand is using something close to the rib cage method.
- If the chart assigns a number 4-5 inches higher, the brand is using a version of the plus-4/plus-5 method, whatever it calls it internally.
BRABAR's full brand-by-brand comparison walks through this exercise for several major retailers in BRABAR vs. Victoria's Secret, SKIMS, Aerie, and ThirdLove.
BRABAR's Position
BRABAR's EZ-fit method is a rib cage method: direct measurement, rounded to the nearest even number, with nothing added and no shift to a higher measuring point. We apply it consistently across every size we carry, from a 28 band through a 38 band, and from cups AA through DDD/F — the exact range where differences in methodology do the most damage to fit.
Frequently Asked Questions
What is the difference between the rib cage method and the plus-4 method?
The rib cage method uses your direct underbust measurement, rounded to the nearest even number, as your band size with nothing added. The plus-4 method takes the same direct measurement and then adds four inches before rounding, producing a larger band number for the same body.
Where did the plus-4 method come from?
Historical patent filings on bra-sizing measuring tapes describe adding five inches to the underbust measurement as a way to approximate a chest circumference that couldn't be measured directly at the time. The convention later shifted toward adding four inches in much of the U.S. market, though the underlying approach, measuring directly and then adding a fixed number, has stayed consistent.
What is the armpit method and who uses it?
The armpit or overbust method measures higher on the torso, near where the bra straps meet the cups, rather than directly under the bust. Victoria's Secret's published fitting instructions use this method, which tends to produce a larger band size than a direct rib-cage measurement for the same body.
Is the European bra sizing system the same as the US system?
No. The European standard, EN 13402, is based on centimeters and steps cup sizes every 2 cm of bust-to-underbust difference, compared to the roughly 2.5 cm (1-inch) step used in US and UK systems. Converting between the two requires a formula, not a direct substitution.
Why does methodology matter more for smaller band sizes?
A fixed addition like four inches represents a much larger proportional error on a small band than a large one. On a 28-inch band, adding four inches changes the result by roughly 14%, which is often enough to move someone into a size that provides significantly less support than their body actually needs.
Find your size under the rib cage method with BRABAR's Fit Guide and Bra Size Calculator, and explore our full range of band and cup sizes.
