Bra Size Calculator
Estimate your bra size in US, UK or EU sizing from two tape measurements: a snug underbust circumference and a full bust circumference. Enter both in cm or inches and get band, cup and sister-size guidance.
Estimate Your Bra Size from Two Measurements
Bra Size Formula
Where:
- \(UB\) — snug underbust circumference, measured around the ribcage directly under the bust
- \(FB\) — full bust circumference, measured at the fullest part of the bust
- \(B\) — band size: the nearest even number of inches (US/UK) or the nearest multiple of 5 cm (EU)
- \(k\) — cup index, mapped to a cup letter from the cup table of the chosen system
The band is derived from the underbust measurement alone, while the cup letter comes from how much larger the full bust is than that band. The two parts of the size are therefore found independently, which is why the same bust can need different cup letters on different bands.
Bra Size – Calculation Example
Example: a snug underbust of 76 cm and a full bust of 90 cm, using the US sizing system (the calculator's default values).
\[ UB_{\text{in}} = \frac{76}{2.54} = 29.9 \;\Rightarrow\; B = 30 \quad \text{(nearest even band)} \]
\[ FB_{\text{in}} = \frac{90}{2.54} = 35.4, \qquad k = \mathrm{round}(35.4 - 30) = 5 \;\Rightarrow\; \text{cup DD} \]
Result: an estimated bra size of 30DD (US), with a bust-minus-underbust difference of 14.0 cm (5.5 in).
How to use the calculator
- Measure snugly around your ribcage, directly under the bust, with the tape parallel to the floor — firm, but not indenting the skin. Enter this value in Snug underbust circumference and pick cm or in as the unit.
- Measure around the fullest part of the bust without compressing, wearing your best-fitting non-padded bra or a thin layer. Enter this value in Full bust circumference and choose its unit.
- Select the Sizing system: US (e.g. 32DD), UK (e.g. 32DD) or EU / International (e.g. 70E).
- Read the results: your band size, cup size, estimated bra size and the bust minus underbust difference, followed by a step-by-step explanation of how the size was derived, along with sister-size suggestions.
What bra size means and how to interpret it
A bra size has two independent parts. The band (the number) describes your ribcage frame and provides nearly all of the support — a well-fitted band sits level all around, stays put on the loosest hook, and you should be able to slide no more than a finger or two under it. The cup (the letter) does not describe an absolute breast volume: it describes how much larger your bust is than your band. That is why a 30DD and a 36DD hold very different volumes.
This is also the logic behind sister sizes. If a band feels too tight but the cups fit, go up one band and down one cup letter (30DD → 32D); if the band rides up but the cups fit, go down one band and up one cup (30DD → 28E/DDD depending on the brand). The cup volume stays the same — only the band changes. Trying two or three sister sizes is the fastest way to dial in a fit.
Interpret the calculator's output as a starting point for shopping, not a verdict. Manufacturer sizing varies considerably, and breast shape (projected vs. shallow, wide vs. narrow roots) affects which bra cut will suit you even when the size is technically correct. A good practical check: lean forward, scoop all breast tissue into the cups, then judge the fit by the band and the wire — the wire should trace your root without digging into breast tissue and the gore should lie flat against your sternum.
US, UK and EU cup letters compared
The three systems agree below a D cup and then diverge. The table below shows the approximate equivalences used by this calculator, indexed by the difference between the full bust and the band:
| Difference (in) | Difference (cm, EU) | US | UK | EU |
|---|---|---|---|---|
| 0 | 10–12 | AA | AA | AA |
| 1 | 12–14 | A | A | A |
| 2 | 14–16 | B | B | B |
| 3 | 16–18 | C | C | C |
| 4 | 18–20 | D | D | D |
| 5 | 20–22 | DD | DD | E |
| 6 | 22–24 | DDD | E | F |
| 7 | 24–26 | G | F | G |
| 8 | 26–28 | H | FF | H |
A common example: US 32DD = UK 32DD = EU 70E. Note that US and UK share band numbers but differ in letters above D, while the EU band is expressed in 5 cm steps (70, 75, 80…) with its own letter sequence.
Assumptions, limitations and practical tips
- Measurement quality matters most. Measure while standing relaxed, without a thick or padded bra, and keep the tape parallel to the floor. A 1 cm error in either measurement can shift the cup by a full letter.
- The band method matters. This calculator uses the snug underbust directly (rounded to an even inch or a 5 cm step). Some brands and older guides add 3–5 cm or 4 inches to the underbust first, which produces looser bands; expect such labels to differ by one or two band sizes from this estimate.
- Rounding introduces edge cases. A measurement exactly between two bands or two cup letters may be reported one step lower or higher than a different rounding convention would give. If you are near a boundary, try both neighbouring sizes.
- Brands are inconsistent. The same nominal size can fit differently across manufacturers, and sports bras, bralettes and moulded cups follow their own sizing logic. Use the estimate to narrow your search, then fit by feel.
- Size changes over time. Weight changes, hormonal cycles and fitness training can each shift your size. Re-measure every so often rather than relying on a size that fit a year ago — the calculator makes re-checking quick.
If you track body measurements as part of a fitness routine, this pairs well with the A Body Shape Index calculator for a broader picture of body composition, and with the Steps to Miles calculator for monitoring your daily activity alongside those measurements.
Frequently Asked Questions
How do I measure myself for a bra size without help?
Stand in front of a mirror in a non-padded bra or a thin top. Wrap the tape around your ribcage right under the bust, keep it parallel to the floor all the way around, and exhale before reading the number — the tape should be snug without digging in. For the bust, measure at the fullest point, usually across the nipples. If you don't have a soft tape, use a piece of string and measure it against a ruler.
What is the most common or average bra size?
There is no medically defined "normal" size, but surveys in the US and UK most often place the average around 34DD–36C, and it has crept upward over the decades as fitting methods improved and more women were measured correctly. Keep in mind these figures come from self-reported data and vary by country and age. Any size the calculator gives you is a valid one — the goal is fit, not matching an average.
Why do some guides tell me to add 4 inches to my underbust measurement?
The +4 method dates back to the 1930s, when bra bands were made of non-stretch materials and cut differently than today's elastic bands. Modern bras are designed to fit a snug underbust directly, so most contemporary fitters have dropped the adjustment. If you follow an old +4 guide, expect a band 2–4 sizes larger and a correspondingly larger cup letter than this calculator reports. Neither is wrong per se — they describe different construction eras.
My result is different from the size I've worn for years — which is right?
If your long-time size came from a store fitting that used the +4 method or measured over clothing, a smaller band with a larger cup is a very common correction — many women who wore 36C find 32DD or 34D fits better. Test the new size practically: the band should stay level, support on the loosest hook, and the gore should lie flat against your sternum. If those checks pass, the new size is likely the better one; if not, stay with what fits.
How do sister sizes work when the cups fit but the band doesn't?
Sister sizes trade band for cup while keeping the actual cup volume constant. If the cups are fine but the band digs in, go up one band and down one cup letter — for example 32D → 34C. If the band rides up, go down one band and up one cup: 32D → 30DD. Each step moves roughly one inch (about 2.5 cm) of band. Trying two or three sisters in the store is the fastest way to find your true fit.
Does this size work for sports bras and bralettes too?
Not directly. Sports bras usually use a compressed S/M/L scale or a looser version of band-and-cup sizing, and bralettes rarely come in cup sizes at all. Your calculator result still helps: use the band number as a guide for the ribcage size, and pick compression or encapsulation styles based on your cup volume and activity level. High-impact activities generally need firmer bands and more structure than the everyday size suggests.
Can I convert a US size to UK or EU sizing myself?
Yes, with a few rules. US and UK bands are identical (both in inches); only the cups diverge above D — a US DDD is a UK E, and the letter sequences then differ step by step. For EU, convert the band to centimetres and round to the nearest multiple of 5 (32 in ≈ 81 cm → 80), then shift the cup: US DD ≈ EU E, US DDD ≈ EU F. Because brands label inconsistently, always double-check the brand's own conversion chart before ordering.