Estimate a band and cup size from underbust and bust measurements.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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Finding the right bra size is one of the most impactful — and most commonly neglected — aspects of everyday comfort. Studies estimate that between 64% and 85% of women in the UK and US are wearing the wrong bra size, with the majority wearing a back (band) size that is too large and a cup size that is too small. This comprehensive guide explains how bra sizes work in the UK and US, how to measure yourself correctly, how to convert between international sizing systems, what sister sizing means, and how breast changes over time affect fit.
In the UK, bra sizes consist of a band size (in inches) and a cup size (a letter). The band size corresponds to the measurement around the ribcage just under the bust, rounded to the nearest even number. UK band sizes run 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, and beyond.
UK cup sizes: A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K. Note that UK sizing uses "DD" after "D," then continues with double letters (DD, E, F, FF, G, GG, H...) — this is different from the US system.
UK cup size is determined by the difference between the fullest bust measurement (across the fullest part of the breast) and the band measurement (under the bust):
| Difference (inches) | UK Cup Size |
|---|---|
| 0–0.5 inches | AA |
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD |
| 6 inches | E |
| 7 inches | F |
| 8 inches | FF |
| 9 inches | G |
US bra sizing uses a similar system — band size in inches and a cup letter — but the cup progression differs from the UK after D. In the US, sizes go: A, B, C, D, DD (or E), DDD (or F), DDDD (or G), and so on. US cup sizing also commonly uses double and triple D letters rather than the single-letter progression used in UK extended sizing.
US band sizes are the same numerical values as UK band sizes when both measured in inches. However, calculation methods can differ slightly between brands. The most common US method involves adding 4 inches (if the under-bust measurement is even) or 5 inches (if odd) to the ribcage measurement to get the band size — though many modern bra fitters use the measurement directly without adding inches.
| Difference (inches) | US Cup Size | UK Equivalent |
|---|---|---|
| 1 inch | A | A |
| 2 inches | B | B |
| 3 inches | C | C |
| 4 inches | D | D |
| 5 inches | DD or E | DD |
| 6 inches | DDD or F | E |
| 7 inches | DDDD or G | F |
Shopping internationally or from online retailers often requires converting between sizing systems. The table below covers the most common equivalences:
| US/UK Band (inches) | EU/FR Band (cm) | AU/NZ Band |
|---|---|---|
| 28 | 60 | 6 |
| 30 | 65 | 8 |
| 32 | 70 | 10 |
| 34 | 75 | 12 |
| 36 | 80 | 14 |
| 38 | 85 | 16 |
| 40 | 90 | 18 |
EU cup sizes follow the alphabet from A, with no double letters: A, B, C, D, E, F, G, H, I, J. This means EU D = UK D, EU E = UK DD, EU F = UK E, EU G = UK F, and so on (offset by 1 cup for every size above D). French sizing adds 10 to the EU band size for the band number.
Accurate measurements are the foundation of a correctly fitting bra:
Example: Underbust = 32 inches, fullest bust = 37 inches. Difference = 5 inches. UK size = 32DD. US size = 32DD (same for this cup size).
Sister sizes are alternative bra sizes that have the same cup volume but different band sizes. If your exact size is unavailable or you need a different fit around the band, sister sizes offer a solution. Moving up one band size means going down one cup letter (to maintain the same cup volume); moving down one band size means going up one cup letter.
| Your Size | Smaller Band Sister | Larger Band Sister |
|---|---|---|
| 34C | 32D | 36B |
| 36DD | 34E | 38D |
| 32F | 30FF | 34E |
Understanding fit problems helps identify whether your size is correct:
Major UK lingerie brands (Marks & Spencer, Freya, Fantasie, Panache, Curvy Kate, Bravissimo) are well known for their wide cup range and accurate sizing, particularly in fuller cup sizes (DD and above). UK brands are generally considered by bra fitting experts to produce more accurately sized bras with a wider range of sizes than many US mass-market brands.
US mass-market brands (Victoria's Secret, Maidenform, Bali) typically offer a narrower size range (often 32–40 band, A–DDD cup) and historically have been criticised for recommending band sizes that are too large and cup sizes that are too small. Specialist US brands (ThirdLove, True&Co, Savage X Fenty) have expanded size ranges significantly in recent years.
Sports bras use two sizing systems: traditional bra sizing (same as regular bras) for structured underwired sports bras, and S/M/L/XL sizing for non-wired crop-style sports bras. For high-impact activities (running, HIIT), a supportive underwired sports bra in your correct bra size provides the most protection against breast ligament damage and discomfort. Research from the University of Portsmouth (a leading global centre for bra research) has shown that unsupported breast movement during high-impact exercise can exceed 10 cm vertically, contributing to chronic tissue damage and breast pain (mastalgia). In the US, the ACSM recommends properly fitted sports bras as essential equipment for female athletes of all cup sizes.
Breast size changes significantly during pregnancy and breastfeeding. The NHS advises being professionally measured for a maternity bra from around 20 weeks gestation, and again in the final weeks of pregnancy for a nursing bra. Breast size can increase by 1–4 cup sizes during pregnancy, and the back measurement may increase by 1–3 band sizes due to rib expansion. Nursing bras are fitted on the assumption that breasts are slightly larger when producing milk, so the optimal fit may change as supply stabilises.
In the US, Nordstrom, Motherhood Maternity, and specialist boutiques offer professional bra fitting for maternity and nursing bras. In the UK, major department stores (John Lewis, Debenhams, M&S) and specialist maternity brands offer professional fitting services.
Measure your underbust (snugly, under the bust in inches) for your band size. Measure your fullest bust measurement. Subtract the band from the bust measurement — the difference in inches gives your cup size: 1 inch = A, 2 = B, 3 = C, 4 = D, 5 = DD (UK) or DD/E (US), 6 = E (UK) or DDD/F (US). Combine band and cup for your size, e.g., 34C or 32DD.
UK and US band sizes are the same number (both in inches). Cup sizes are identical up to and including D. Above D, they diverge: UK uses DD, E, F, FF, G, GG...; US uses DD, DDD/F, DDDD/G... A UK E corresponds to a US DDD, and the gap widens for larger cups. Always check a brand's specific size chart when shopping internationally.
Sister sizes are bra sizes with the same cup volume but different band sizes. For example, 34C, 32D, and 36B are all sister sizes — the cup volume is the same, but the band length differs. If a band is too tight but the cup fits, try the sister size with one larger band and one smaller cup. This is a useful trick when your exact size is out of stock.
To convert UK band to EU: multiply the UK inch size by approximately 2.54 and round to the nearest 5 (giving the EU size in cm). So 34 (UK) = 75 (EU). 36 (UK) = 80 (EU). For French sizes, add 15 to the EU size. For cup sizes up to D: UK A=EU A, UK B=EU B, UK C=EU C, UK D=EU D. Above D: UK DD=EU E, UK E=EU F, UK F=EU G, UK FF=EU H.
Breast size typically increases by 1–4 cup sizes during pregnancy, and the ribcage may expand by 1–3 band sizes. The NHS recommends professional remeasurement at around 20 weeks for a maternity bra and again in late pregnancy for a nursing bra. Avoid underwire bras in late pregnancy and when breastfeeding if they cause discomfort, though modern soft-underwire nursing bras are available for support.
Studies estimate 64–85% of women wear incorrectly sized bras. The most common errors are band too large and cup too small. This often traces to self-measurement errors, using outdated "+4 inch" calculation methods, or accepting an incorrect size from a poorly trained fitter. Using the direct underbust measurement (without adding inches) and following the cup-size table above provides a more accurate starting point than traditional methods.
There is no single "best" size — the goal is your correct size in a sports bra designed for your activity level. For high-impact activities (running, HIIT), a supportive structured sports bra in your correct size minimises breast movement and the risk of ligament damage (Cooper's ligament). Research from the University of Portsmouth (UK) shows unsupported breasts can move 10+ cm vertically during running — a properly fitted, high-impact sports bra reduces this to approximately 3–4 cm.
It is recommended to get re-measured whenever your weight changes significantly (±10 lbs / 5 kg), during and after pregnancy, after breastfeeding ends, and every 1–2 years as part of routine wardrobe review. Bra elastic stretches over time — if your band rides up consistently even on the tightest hook, the bra has reached the end of its life regardless of size.