Bra Size Calculator
Band and cup from two measurements, UK and US.
Calculate bra size from underbust and bust measurements, in UK and US cup lettering, with both the modern and plus-four band methods.
Snug around the ribcage directly under the bust.
Loosely around the fullest part.
UK size
32D
same in US sizing
The band carries most of the support. If it rides up at the back it is too loose — go down a band and up a cup to hold the same volume.
How the Bra Size Calculator works
Two things make bra sizing harder than it needs to be. The band can be worked out by two different methods that disagree by up to four inches, and the cup letters stop agreeing between the UK and US above a D. Both are handled here rather than quietly assumed, because a calculator that picks one convention silently is how people end up two sizes out.
Also known as: cup size calculator · bust measurement calculator · bra fitting calculator
How the number is derived
A bra size is two independent measurements wearing one label. The band comes from the ribcage directly under the bust; the cup comes from the difference between the bust measurement and the band, at roughly one inch per letter. A four-inch difference is a D, a six-inch difference is a UK E, and so on up the scale.
The band is where the two methods in circulation disagree. The modern method rounds the underbust measurement to the nearest even number and uses that. The older plus-four method adds four inches first, which produces a band up to two sizes larger. Both are still in use — many high street chains fit on plus-four — so a size derived one way can be two bands out against a chart built the other way.
The cup is where the countries disagree. UK and US lettering agree up to and including DD, then diverge: the UK inserts E, F, FF, G, GG while the US repeats D as DD and DDD before jumping to G. A UK 32E and a US 32DDD are the same garment.
A worked example
An underbust of 32 inches and a bust of 38. On the modern method the band is 32. The difference is six inches, which is a UK E cup and a US DDD, giving 32E in the UK and 32DDD in the US.
On the plus-four method the same person measures a 36 band, and the six-inch difference is now taken against 36 rather than 32 — which is why the two methods do not simply differ by a constant. The volume implied by the cup changes with the band it is attached to, and that is the same relationship that makes sister sizing work.
Where the answer misleads
A calculated size is a starting point for trying on, not an answer. Cup shape varies enormously between makers and styles — a balconette and a full cup in the same nominal size hold differently — and the band stretches with wear, so a bra that fitted on the loosest hook a year ago should now be fastening on the tightest.
The most common fit fault is a band that is too loose rather than a cup that is wrong. The band carries most of the support, so if it rides up at the back, the fix is to go down a band and up a cup, holding the volume constant. Going up a cup alone makes the problem worse.
Measurements taken over a padded bra, or pulled tight at the bust, will both read high and produce a cup that is too large. The underbust should be snug and the bust loose — the two measurements are taken with deliberately different tension, which is easy to miss.
Frequently asked questions
How do I measure for a bra?
Measure snugly around the ribcage directly under the bust for the band, then loosely around the fullest part of the bust. Both while standing, in a non-padded bra or none. The cup comes from the difference between the two, at roughly one inch per letter.
Should I add four inches to my underbust?
That is the older plus-four method and it is still used by many high street shops. The modern method rounds the underbust to the nearest even number instead. They can differ by two band sizes, so it is worth knowing which one a retailer's chart assumes.
Why do UK and US cup letters differ?
They agree up to D and then diverge. The UK inserts DD, E, F, FF, G, GG; the US repeats D as DD and DDD before continuing. A UK E is a US DDD — the same cup with two different labels.
My band rides up at the back. What is wrong?
Almost always a band that is too loose. The band carries most of the support, so if it rides up, go down a band size and up a cup to keep the same volume — the sister size calculator works that out.
How often should I remeasure?
Whenever weight, pregnancy or a new brand is involved, and otherwise once a year. Ribcage measurements shift more than people expect, and a band that has stretched out reads as a fit problem when it is simply worn.