Cup Bra Measurement
A cup bra measurement is the pair of tape readings behind every cup letter: the bust at its fullest and the underbust at its root. The difference between them is your cup. Nothing less, nothing more — and both readings take about thirty seconds each.
This page focuses purely on the cup half of a bra size: how to take the two readings cleanly, how the difference becomes a letter in each system, and how to turn your cup bra measurement into a complete size.
Quick answer
To take a cup bra measurement, wrap a soft tape loosely around the fullest part of your bust and snugly around your underbust, keeping both level. Subtract the underbust from the bust: about 1 inch per letter. A 3-inch difference is a C cup; 5 inches is DD in the UK or E in continental sizing.
6
sizing systems (US, UK, EU, FR/ES, AU/NZ, JP)
460+
charted sizes with bust ranges
2
measurements needed (underbust + bust)
The two readings behind a cup bra measurement
Every cup bra measurement starts on the ribcage. Wrap the tape snugly under the root of the bust — firm, level, one finger of slack — and record the underbust. Then move the tape to the fullest part of the bust, let it rest on the tissue without pressing, and record the bust. Two numbers, one cup letter waiting to happen.
From readings to letter
Subtract the underbust from the bust and the cup bra measurement becomes a difference. Each inch of difference is roughly one letter: one inch A, two B, three C, four D, and the systems fork from there — UK DD, continental E, and so on. Enter both readings below and the calculator resolves the full size — band plus cup — in six systems at once.
If you would rather read than compute, the cup size chart lists every letter with its difference range per system, and the breast size chart extends the view across all six dialects in one table.
Getting the bust reading right
The bust half of a cup bra measurement fails in three familiar ways: a tape that rides up at the back (shortens the number), a padded bra (lengthens it), and a compressing grip (shortens it again). Keep the tape parallel to the floor — check in a mirror — and let it rest on the fullest point without shaping it. The fuller side wins if the two differ.
Breathe normally and stand tall but not rigid. The bust reading feeds the cup letter directly, so an inch of error here is a full letter of error downstream. The dedicated bust measuring page covers this reading in full detail. Protect the bust number and the cup bra measurement protects its letter.
Getting the underbust reading right
The underbust half anchors everything: it becomes the band, and it is the baseline the difference is measured from. A too-tight underbust reading inflates your cup letter; a too-loose one deflates the band. Aim for snug — the tape stays put, you can slide one finger underneath, and you are breathing out normally, not squeezed.
Position matters too: directly under the root of the bust, where the wire of a well-fitted bra would sit. Higher or lower by a couple of centimetres changes the reading measurably. Consistency across days is your best evidence that the cup bra measurement is honest.
Common cup measuring mistakes
Five errors account for most wrong cup letters:
- Measuring the bust over a padded or moulded bra.
- Letting the tape tilt — up at the back, down at the front, or spiralling.
- Pulling the underbust tape tight enough to compress the ribcage.
- Rounding before subtracting, instead of after.
- Mixing units between the two readings — one unit for both, always.
- Rounding mid-way — a cup bra measurement needs raw numbers until the end.
- Skipping the mirror — a level tape is the whole cup bra measurement.
From cup to complete size
A cup bra measurement becomes a label only beside its band. Pair your letter with the band your underbust resolves to and you have a starting size; test it with the four fit checks from the bra fit guide and adjust along sister sizes when a label is nearly right. Letter by letter, the specific cup pages — A, B, C, D and E — show what each step means on real bands. Band plus cup bra measurement equals the label you shop with.
Cup bra measurement: quick recap
Condensed to four lines, a cup bra measurement is: underbust snug under the root, bust loose at the fullest point, both tapes level, subtract before rounding. The difference, not either number alone, is the cup bra measurement — and one inch of it is one letter on every ladder on this site.
From there the machinery takes over: the calculator pairs your letter with the right band, the charts let you cross-check by eye, and the fit checklist confirms the result on your body. Take the cup bra measurement once, carefully, and every page on this site becomes usable; re-take the cup bra measurement whenever a fit starts drifting, because letters follow inputs. Re-take the cup bra measurement and the ladder re-ranks itself.
Frequently asked questions
How do you take a cup bra measurement?
Measure the bust loosely at the fullest point and the underbust snugly beneath the root, both with a level tape. The difference between the two readings is your cup bra measurement — roughly one letter per inch. Level tapes in, one letter out — that is a cup bra measurement.
How do you measure cup size correctly?
Keep the tape level, wear an unpadded bra or thin top, breathe normally, and measure the fuller side if your breasts differ. Subtract underbust from bust and read the letter from the cup size chart.
What cup size is a 2-inch difference?
A B cup — each inch of difference is about one letter. The cup bra measurement pages for each letter cover the bands those letters pair with. A two-inch cup bra measurement is a B on every band.
Do I need my band size to know my cup size?
Strictly, you need the underbust reading as the baseline: the cup is a difference, so the same bust gives different letters over different ribcages. The band label itself follows from the same reading. The cup bra measurement and the band share one tape session.
Why does my cup letter change with the band?
Because the letter tracks the difference, not absolute volume. Sister sizes exploit this: one band down, one letter up keeps the volume nearly constant. The letter travels because a cup bra measurement is a difference.
How accurate is a home cup bra measurement?
Very accurate for translating measurements to labels; the remaining variance is brand patterning and breast shape. Measure twice and fit-test to close the loop. A careful cup bra measurement is right far more often than not.
Why does my cup bra measurement give a different letter than my old bra?
Because the cup bra measurement reflects your current difference, not the label you bought years ago. Re-measure, recompute, and trust the cup bra measurement over the tag.
Sources & standards
Every size on this site is cross-checked against these published standards and chart families before it ships.
- 2017
ISO 8559-1:2017 — Size designation of clothes — Part 1: Anthropometric definitions for body measurement
Defines the body measurement points (chest/bust girth, underbust girth) this site measures.
- 2017
EN 13402 (label family) — Size designation of clothes — European size labelling
Basis of the continental EU band ladder (65, 70, 75 …) in 5 cm steps.
- 2023
JIS L 4006:2023 — Sizing systems for foundation garments
Japanese foundation-garment sizing; the label family behind JP cup-first sizes such as C75.
- 2019
KS K 9404 — Sizing systems for foundation garments (Korea)
Korean foundation-garment sizing; referenced while the KR system awaits full verification.
- 2025
Multi-source retail consensus — US, UK, FR/ES and AU/NZ bra size charts (cross-brand average)
Inch-based band rounding, UK double-letter cup ladder, FR/ES +15 offset and AU/NZ dress-size bands.
By Bra Size Calculator ·