Bra Size Calculator
Find your accurate bra size in US, UK, EU, AU & FR in under 30 seconds. Two measurements, one trusted result.
Understand bra sizes, end to end
The calculator is the start. These guides explain how bra sizes work, how they translate worldwide, and how to fix a fit that's off — so the number actually means something.
How Bra Sizes Work
Band, cup, and the one-inch-per-cup rule — the logic behind every size.
Read the guide →Cup Sizes Explained
What each cup letter means, and why it changes with the band.
See cup sizes →International Bra Sizes
US, UK, EU, FR & AU — how the systems differ and convert.
Compare systems →Sister Sizes
Same cup volume, different band — the fix when the band is off.
Find sisters →Common Fit Problems
Gaping, riding up, spillage, slipping straps — and the fix for each.
Diagnose fit →How to Measure
Two measurements, done right, with the mistakes that skew results.
Measuring guide →How to measure your bra size
Two measurements, 30 seconds, one soft tape. Stand relaxed in front of a mirror, ideally bare or in a thin non-padded bra.
- Measure the underbust. Wrap the tape snug and level around your ribcage, directly under the bust. Round to the nearest even number — that's your band.
- Measure the bust. Loosely measure around the fullest point, tape parallel to the floor, no compression.
- Find the cup. Subtract band from bust. Each inch of difference is one cup size.
- Try the sister sizes. Use your result as the centre of a small range and try the sister sizes in store.
Measure once, measure correctly
Padding inflates the bust measurement. Measure bare or in a thin, unlined bra.
If the tape tilts or sags at the back, the band reads too large and the cup too small.
The old “+4” rule was made for older fabrics. Modern fitters measure snug and round to the nearest even number.
Pulling the tape too tight shrinks the cup result. Let the tape rest lightly at the fullest point.
Interactive bra size chart
Every band (28–44) mapped against every cup (AA–DDD) in US sizing. Filter to highlight your size, or read across to see the measurements each size expects.
The band column shows your underbust range; each cell is the size at that band and cup. Highlighted cells match your filter.
Sister size visualizer
A sister size has the same cup volume on a different band. Pick a size to see how the volume stays constant while the band changes — the fix when cups fit but the band doesn't.
International bra size conversions
A US 34C is written differently in London, Paris, and Sydney. Bands shift to centimetre systems and cup letters diverge above D.
Showing your calculated size from the calculator above.
Popular brands & their sizing systems
Full-bust specialist with firmer bands and dependable everyday support.
Fashion-forward designs with lighter support and a comfortable fit.
Structured support with premium construction for everyday comfort.
Mainstream American sizing with familiar fits and broad availability.
Curated international brands across multiple sizing systems and fits.
Comfort-focused fit with reliable support and a broad size range.
Common bra fit problems
Select everything you notice in your current size. Most fit problems map to a band or cup change — often a sister size. Tap all that apply.
Select a symptom above to see fit recommendations.
Match bras to your breast shape
Two people in the same size can need different bras. Shape — where fullness sits — decides which styles actually fit.
Bra sizing, answered
The questions people ask most about measuring, converting, and fitting bras.
What is a bra size?
A bra size is a band size and a cup size combined — for example 34C — describing your ribcage circumference and your breast volume.
How it works
The number is the band, taken from your underbust; the letter is the cup, taken from the difference between your bust and band.
Together they describe two different things: how the bra wraps your body, and how much it needs to hold.
Why it matters
Because the cup is measured relative to the band, the same letter is a different volume on a different band — a 30C, 34C and 38C are not equal. The same size can be written in US, UK, EU, FR and AU systems.
A bra size is never just a letter — band and cup must always be read together.
What is a bra size calculator?
A bra size calculator is a tool that converts two measurements — your underbust and bust — into your bra size across US, UK, EU, FR and AU.
How it works
Enter the snug measurement around your ribcage and the looser measurement across the fullest point. It returns your band, cup, full size and sister sizes, using the modern snug-measure method rather than the outdated “add four inches” rule.
Calculator vs chart
A calculator computes a personalized result from your numbers; a bra size chart is a fixed table you read manually.
A calculator gives a personalized starting size — confirm it by trying that size and its sister sizes on.
How is bra size calculated?
Bra size is calculated by rounding the underbust to the nearest even number (the band) and counting one cup per inch of the bust-minus-band difference.
The steps
Measure the underbust snug and level, and round to the nearest even number — that is your band.
Measure the bust loosely at the fullest point, then subtract the band. Each inch of difference is one cup: 1″ = A, 2″ = B, 3″ = C, 4″ = D, then DD and beyond. A 37″ bust over a 34″ underbust is a 34C.
The method
This is the modern snug-measure method. The older rule that added 4–5″ to the underbust was built for 1930s non-stretch fabrics and usually gives a band too loose and a cup too small.
Band comes from the underbust; cup comes from the bust-minus-band difference.
What is the difference between band size and cup size?
Band size is the number in a bra size (from the underbust); cup size is the letter (from the bust-minus-band difference).
How it works
The band sets how the bra fits around your ribcage and provides roughly 80% of a bra’s support. The cup reflects breast volume at one inch per cup.
Why it matters
Crucially, the cup is always measured relative to the band, so a 30D, 34D and 38D are all “D cups” but each holds progressively more.
Band and cup measure different things and only make sense read together.
How do bra cup sizes work?
Each cup size equals one inch of difference between your bust and band, so cups are a ratio — not a fixed volume.
How it works
Because the cup is measured against the band, the same letter means different volumes on different bands: a 32D holds less than a 38D.
Above a D cup
Above a D cup, lettering diverges by region — the US doubles letters (DD, DDD), the UK continues the alphabet (DD, E, F, FF, G), and EU and FR run a single sequence (E, F, G).
A cup letter only has meaning next to its band.
What are sister sizes?
Sister sizes are different bra sizes that share the same cup volume — for example 34C, 32D and 36B.
How it works
Going down one band and up one cup (34C → 32D) keeps the same cups while tightening the band. Going up one band and down one cup (34C → 36B) keeps the same cups while loosening the band.
When to use them
Reach for a sister size when the cups fit but the band feels loose or tight, when a brand runs small or large, or when your usual size is out of stock.
If the cups fit but the band doesn’t, you need a sister size — not a different cup.
What is the correct way to measure band size?
Wrap a soft tape snug and level around your ribcage directly under the bust, and round to the nearest even number.
The method
Measure in inches, standing relaxed, bare or in a thin non-padded bra. Keep the tape firm with no slack and perfectly level front to back.
If you're between bands
If your number is odd, you sit between two bands — try the snugger one first and consider its sister size. Don’t let the tape sag at the back, which inflates the band and shrinks the cup.
A snug, level underbust rounded to the nearest even number is your band.
What is the correct way to measure bust size?
Wrap the tape loosely around the fullest part of your breasts, level and parallel to the floor, without compressing the tissue.
The method
Unlike the band, the bust is measured loose — compressing it understates your cup. Stand upright and relaxed, and measure bare or in a thin non-padded bra, since padding inflates the number.
Why it matters
Your bust measurement alone is not a cup size: the cup comes from the difference between your bust and band. Measuring twice and averaging improves accuracy.
Measure the bust loose and level — the cup comes from bust minus band.
What is the most accurate way to measure bra size?
The most accurate method is the modern snug-measure approach: a snug underbust rounded to the nearest even number, plus a loose bust, at one cup per inch of difference.
The method
Use a soft tape, stand relaxed, and measure bare or in a thin non-padded bra. Keep the tape parallel to the floor.
For best accuracy
Take each measurement twice and average them, and re-measure on a different day if you’re between sizes, since measurements shift with your cycle and the time of day.
Measured carefully, a calculated size is an accurate starting point — confirm it in person.
What is the difference between a bra size calculator and a bra size chart?
A bra size calculator computes your size from your measurements; a bra size chart is a fixed reference table you read manually.
How they differ
The calculator is interactive — enter your underbust and bust and it returns your band, cup, full size and sister sizes across regions. A chart maps measurements or sizes to one another, such as converting a US size to UK and EU.
When to use each
In practice they work together: use the calculator to find your size, then the chart to convert it between regions or check the measurement ranges each size covers.
Use the calculator to find your size, and the chart to convert and cross-check it.
Are bra sizes the same in the US, UK, EU, AU and FR?
No — bra sizes are written differently across regions, which is why international sizing and conversions matter.
US vs UK
The US and UK share band numbers and agree on cups up to D, but diverge above D: the US doubles letters (DD, DDD) while the UK continues the alphabet (DD, E, F).
EU, FR and AU
EU and FR use centimetre bands — EU ≈ US × 2.5 − 10, and FR adds 15 to the EU number; AU is roughly US − 22. So a US 34C is a UK 34C, EU 75C, FR 90C and AU 12C.
Always convert deliberately — your home-country size doesn’t carry over abroad.
Why does my bra size change between brands?
Your bra size varies between brands because there is no enforced, universal bra-sizing standard.
Why it happens
Each brand drafts its own cups, gores, wires and band stretch for its target customer, so a 34C in one label can fit like a 34B or 34D in another. This is normal.
What to do
The fix is to treat your calculated size as a reference point: be ready to move a band or cup when you switch brands, use sister sizes to fine-tune, and read each style’s fit notes.
Treat your size as a reference point and expect to adjust by a size between brands.
How do I know if my bra fits correctly?
Bra fit describes how well a bra’s band, cups, gore and straps match your body.
What to check
The band should sit firm and level with room for two fingers — it carries most of the support. The cups should fully contain the breast with no gaping or spillage, the centre gore should lie flat against the sternum, and the straps should stay put without digging.
The quick test
A quick test: if you’re tightening the straps to feel supported, the band is too big. Fit also depends on breast shape, and a sister size often fixes a band or cup problem.
Most support comes from the band — if you’re tightening straps to feel held, the band is too big.
Does breast shape affect bra fit?
Yes — breast shape is how breast tissue is distributed, and it affects fit as much as size does.
Why it matters
Two people in the same calculated size can need different styles depending on whether fullness sits at the top, bottom, sides or centre.
Common shapes
Common shapes include round, teardrop, bell, east-west, athletic, side-set, relaxed and asymmetrical — and each pairs best with particular bra styles.
Same size, different shape — matching shape to style is what makes a size comfortable.
How often should I measure my bra size?
Re-measure your bra size at least once a year, and sooner after any change that affects your body.
When to re-measure
Weight change, pregnancy and breastfeeding, hormonal shifts, menopause and a new exercise routine can all change both your band and cup.
Worn bands
Bands also wear out: if a once-snug band sits on its tightest hook and still feels loose, re-measure and likely size the band down. Measure on a normal day, since measurements fluctuate with your cycle.
Re-measure yearly and after any body change to avoid drifting into the wrong size.
How accurate is an online bra size calculator?
An online bra size calculator is accurate as a starting point — typically within one cup or band — but it can’t replace trying a bra on.
Where it's limited
The math is reliable, but a calculator can’t account for breast shape, tissue softness and projection, the cut of a specific style, or the lack of a universal standard between brands.
How to use it
Use it to find your size and sister sizes, then try that range on and judge it against the band, cup, gore and strap checks — testing both sizes if you’re between.
Use the calculator to get close fast, then fine-tune the fit in person.
How this calculator works
No guesswork and no secret formula — just the method professional fitters use, applied consistently and shown in full.
Band, then cup
Your band comes from the underbust, rounded to the nearest even number. Your cup comes from the bust-minus-band difference — one inch per cup. Read together, they describe both ribcage fit and breast volume.
Because the cup is relative to the band, the same letter is a different volume on a different band — the principle behind sister sizes and international conversions.
Quick answer
What bra size am I? Measure snugly under your bust for the band and around the fullest point for the bust. Round the underbust to the nearest even number; subtract it from the bust, and each inch of difference is one cup. A 37″ bust over a 34″ underbust is about a 34C.
How bra sizes work — at a glance and in depth
A band number + cup letter (e.g. 34C) describing ribcage circumference and breast volume.
The number is the band, from your underbust. The letter is the cup, from bust minus band — at one inch per cup.
Cups are relative, not fixed: a 30C and 36C differ in volume. That's why band and cup are read together and why sister sizes exist.
Sizing logic follows the modern snug-measure method used by professional bra fitters.
Educational resource — not medical advice.
Glossary
Band
The horizontal part that wraps the ribcage, sized from the underbust; provides most support.
Key takeaway: A properly fitted band provides around 80% of a bra’s support.
Cup
The part holding each breast, sized from bust minus band — one inch per cup.
Key takeaway: The same cup letter holds different volumes on different bands.
Sister size
A different band/cup combination with identical cup volume.
Key takeaway: If the cups fit but the band doesn’t, your sister size is the fix.
Gore
The centre panel between the cups; a good fit has it flat against the sternum.
Key takeaway: A gore that floats off the sternum usually means the cups are too small.
Strap
The adjustable shoulder support — it fine-tunes fit but shouldn't carry the bra's weight.
Key takeaway: Straps that dig in usually signal a loose band, not tight straps.
Underwire
The curved wire under each cup, following the natural breast root for shape and support.
Key takeaway: Wire resting on breast tissue means the cup is too small.
Bra Size Calculator
The most accurate bra size calculator. Two measurements. One trusted result.
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