We are not affiliated with A Bra That Fits. This is an independent explanation of a publicly documented method, written for UK shoppers.
What it is
A Bra That Fits began as a Reddit community where people help each other find their size. Its volunteers developed a measuring method and a free calculator based on what consistently worked across a very large number of fitting discussions.
The method differs from a typical size guide in two ways. It adds nothing to the underbust measurement. And it takes six measurements where most guides take two, so that it can account for how compressible your ribcage is and how soft your breast tissue is.
The six measurements
All six are taken without a bra, with the tape level around the body.
Three underbust measurements
| Measurement | How to take it | What it tells you |
|---|---|---|
| Loose | Tape resting on the skin, just short of sliding down | The largest your ribcage reads |
| Snug | As firm as you would want a bra band to feel | Your likely band |
| Tight | Pulled as tight as you can manage, breathing out fully | How far your ribcage will compress |
The gap between loose and tight is the useful part. A wide gap means there is soft tissue for a band to settle into, so a firmer band will be comfortable and stay put. A narrow gap means the tape is meeting bone, and a band at the tight figure would hurt.
Three bust measurements
| Measurement | How to take it | What it tells you |
|---|---|---|
| Standing | Upright, tape around the fullest part, not compressing | Your bust as it sits unsupported |
| Leaning | Bent forward 90 degrees, back flat, tape around the fullest part | Your full breast volume with gravity's help |
| Lying | Flat on your back, tape across the fullest part | A third reading with the tissue spread differently |
Breast tissue moves. A standing measurement can miss a good deal of volume if your tissue is soft or sits low without support. The leaning measurement gathers it all forward, and the lying one acts as a cross-check. Using all three gives a more reliable bust figure than the standing one alone.
How the measurements become a size
The principle is the same one any direct-measurement method uses:
- Band: taken from the underbust measurements with nothing added, weighted by how compressible your ribcage is.
- Cup: the difference between a bust figure, drawn from the three bust measurements, and the underbust, at one cup per inch.
The extra measurements matter most in the borderline cases. Two people with a 31-inch snug underbust can be steered to different bands, a 30 for the one with a 29-inch tight measurement and a 32 for the one who cannot get the tape below 30½.
Reading the result in UK sizes
If your result comes out as a DD cup or smaller, the label is the same in the UK and the US. Above that, be sure which system you are looking at.
| Inches of difference | UK cup | US cup |
|---|---|---|
| 5 | DD | DD |
| 6 | E | DDD / F |
| 7 | F | G |
| 8 | FF | H |
| 9 | G | I |
| 10 | GG | J |
Much of the community's discussion uses UK sizing even among American members, because the British full-bust brands they often recommend are sold in UK sizes. If you read a recommendation for a "32G", check which country's G is meant. A UK G is two cups larger than a US G. The UK to US converter will translate either way.
How it compares with a two-measurement calculator
For many people the two approaches agree. Our own calculator uses the snug underbust and the bust, with the same no-added-inches principle, and flags when you sit between two bands.
The six-measurement approach is worth the extra few minutes when:
- your underbust falls between two bands and you cannot decide,
- your breasts are soft, or have changed after pregnancy, breastfeeding or weight loss,
- your leaning bust measures more than an inch bigger than your standing bust, or
- two-measurement results have never matched the bras that actually fit you.
Its limits
Six measurements describe you better than two. They still describe circumference, and a size is only part of fit. Two people with the same result can need different bras, because one has narrow, projected breasts and the other wide, shallow ones. The community is clear about this, and treats the calculator's output as the size to start trying, followed by a fit check and advice on shape.
That is the right way to use any calculator. We go into what measurements can and cannot tell you in how accurate bra size calculators are, and the on-body checks are in how a bra should fit.
Frequently asked questions
Is the ABraThatFits calculator accurate?
It is widely regarded as one of the better measurement-based calculators, because six measurements capture more about your body than two. It is still an estimate. The community behind it describes the result as a starting point to be confirmed by trying bras on.
Does ABraThatFits use UK or US sizes?
The method is not tied to either. The underlying measurements are the same, and the result can be expressed in UK or US lettering. The two only differ above a DD cup, so check which one you are reading if your result is an E or larger.
Why does it ask for three underbust measurements?
To learn how much your ribcage compresses. Someone with soft tissue around the ribs has a big gap between their loose and tight measurements and can usually wear a firmer band. Someone with very little gap has a ribcage that will not compress, and needs a band closer to the snug figure.
Do I have to take all six measurements?
No. A snug underbust and a standing bust give a usable result for many people. The extra four are most helpful if you are between sizes, have soft breast tissue, or find that two-measurement results never match the bras that fit you.
Not measured recently? It takes two minutes.
Open the bra size calculator →