Finding a bra that truly fits can feel surprisingly difficult. Many women wear the wrong size for years without realizing it, tolerating straps that dig in, bands that ride up, or cups that gap. The good news is that bra sizing follows a simple, learnable system. With just a soft measuring tape and a few minutes, you can calculate your bra size at home with confidence.
This Bra Size Calculator and guide walk you through the entire process. You will learn how to measure your underbust and bust correctly, how band and cup sizes relate to each other, what sister sizes are, and how US and UK sizing systems differ. Whether you are shopping online or simply curious about your true size, this guide gives you the knowledge to get a better fit.
What Is a Bra Size and How Does It Work?
A bra size consists of two parts: a band size and a cup size. The band size is the number, such as 34, and it represents the measurement around your ribcage just under your bust. The cup size is the letter, such as C, and it represents the difference between your bust measurement and your band size. Together they form sizes like 34C or 36D.
The band does most of the support work. In fact, a well-fitted band provides roughly 80 percent of a bra’s support, while the straps provide the rest. That is why getting the band right matters more than most people expect. The cups, meanwhile, must fully contain the breast tissue without spilling over the top or sides and without gaping.
How to Measure Your Bra Size
All you need is a soft, flexible measuring tape and a mirror. Wear an unpadded bra or a thin top so the tape sits flat against your body. Stand up straight, breathe normally, and keep the tape level all the way around your torso. Take each measurement twice to make sure it is consistent, and round to the nearest half inch.
Measuring Your Underbust
Wrap the tape around your ribcage directly beneath your breasts, where the bra band would sit. Pull the tape snug but not tight; you should be able to fit one finger underneath it. Keep the tape parallel to the floor, checking in the mirror that it has not slipped down your back. Record this number in inches. Standard bra sizing rounds this measurement to the nearest even number, rounding odd numbers up, to get the band size.
Measuring Your Bust
Now wrap the tape around the fullest part of your bust, usually across the nipples. Keep the tape level but do not pull it tight; it should rest gently on your body without compressing the tissue. Stand normally without pushing your chest out or slouching. Record this number in inches. The difference between this bust measurement and your band size determines your cup size, with each inch of difference corresponding to one cup letter.
Understanding Band and Cup Sizes
Band and cup sizes only have meaning together. A 34C and a 36C do not have the same cup volume; the cup letter is relative to the band. Understanding this relationship is the key to finding your real size and to using sister sizes effectively when a particular band feels too tight or too loose.
What the Band Number Means
The band number is your underbust measurement rounded to the nearest even number. Because bands are made in even sizes like 32, 34, and 36, an underbust of 31 inches rounds up to a 32 band, while an underbust of 33 inches rounds up to a 34 band. The band should feel snug on the loosest hook when new, since it will stretch slightly with wear.
How Cup Sizes Are Calculated
Cup size comes from the difference between your bust measurement and your band size. A difference of about half an inch or less is an AA cup, one inch is an A cup, two inches a B cup, three inches a C cup, four inches a D cup, five inches a DD cup, and so on through DDD, G, H, I, and J. Larger differences continue up the alphabet with double letters and beyond.
Relative Cup Sizing Explained
Cup letters are not absolute volumes. A D cup on a 32 band holds less volume than a D cup on a 38 band, because the cup scales with the band. This is why two people can both wear a D cup yet need completely different bras. It also explains sister sizes: a 34C, a 36B, and a 32D all hold roughly the same cup volume on different band lengths.
What Are Sister Sizes and Why Do They Matter?
Sister sizes are bra sizes that share the same cup volume but have different band lengths. If your calculated size is 34C, your sister sizes are 36B (band up, cup down) and 32D (band down, cup up). The cup letter changes to keep the volume constant as the band gets longer or shorter.
Sister sizes are useful when your true size is not available or when a brand runs tight or loose in the band. If a 34 band feels too snug but the cups fit well, try a 36B. If the band feels loose but the cups are right, try a 32D. Keep in mind that sister sizing works best for a one-size shift; going two sizes away changes the fit more noticeably, especially in the band’s support.
US vs UK Sizing Systems
Bra sizing is not universal. The US and UK systems share band numbers and the early cup letters, but they diverge after DD. In US sizing, the progression above D typically goes DD, DDD, G, H, I, J. In UK sizing, the same range is usually labeled DD, E, F, FF, G, GG, H. This means a US DDD is roughly equivalent to a UK E, and a US G is roughly equivalent to a UK F.
When shopping internationally, always check the brand’s size chart rather than converting in your head. Many European brands use their own numbering entirely. If you know your US size, use a conversion chart to find the matching UK size before ordering, and read customer reviews to learn whether a brand runs small or large.
Common Bra Fit Problems and How to Fix Them
Even with the right size on paper, fit issues can appear because bodies and brands vary. Most problems trace back to the band or the cups. Learning to diagnose the four most common issues will save you from keeping bras that never quite work.
Band Riding Up
If the band creeps up your back during the day, it is too loose. The band should sit level and parallel to the floor. Try fastening on a tighter hook first; if it still rides up, go down a band size and up a cup size (a sister size) to keep the same cup volume.
Cup Spillage
Breast tissue bulging over the top or sides of the cups, sometimes called quad-boob, means the cups are too small. Go up one cup size while keeping the same band. If spillage happens on only one side, fit the larger side and adjust the smaller side with the strap.
Gaping Cups
Wrinkling or empty space at the top of the cups usually means the cups are too large, the style is wrong for your shape, or the straps are too loose. Try going down a cup size, tightening the straps slightly, or switching to a style with less coverage on top, such as a balconette.
Slipping or Digging Straps
Straps that slide off your shoulders are often a sign the band is too big, forcing the straps to do the support work. Straps that dig painful grooves mean the band is too loose and the straps are overcompensating, or the straps are simply overtightened. Fix the band first, then adjust the straps so you can slide two fingers underneath.
When to Re-Measure Your Bra Size
Your bra size is not permanent. Weight changes, pregnancy, breastfeeding, hormonal shifts, and even aging can change your measurements. As a rule, re-measure at least once a year, and sooner if your bras start fitting differently. Signs it is time include a band that no longer feels supportive on the tightest hook, new spillage or gaping, or general discomfort you did not feel before.
- After significant weight gain or loss (roughly 10 pounds or more).
- During and after pregnancy and breastfeeding.
- When starting or stopping hormonal medication.
- If your current bras are more than a year old and stretched out.
- Before ordering from a new brand with unfamiliar sizing.
How to Use This Bra Size Calculator
Using the calculator above is simple and takes less than a minute. Enter your underbust measurement in inches, then your bust measurement in inches, and choose your sizing system (US or UK). Press Calculate to see your band size, cup size, full bra size, and both sister sizes. Press Reset to clear the form and start over.
- Measure your underbust snugly, right under your bust.
- Measure your bust at its fullest point without compressing.
- Enter both numbers and select US or UK sizing.
- Press Calculate and note your size and sister sizes.
- Try your size first, then a sister size if the band feels off.
Frequently Asked Questions
How accurate is a bra size calculator?
A bra size calculator is highly accurate as a starting point when your measurements are taken correctly. It applies the same standard formula fitters use: band from the underbust and cup from the bust-minus-band difference. Brand variations, breast shape, and personal comfort preferences mean you may still need to try a sister size, but the calculator removes the guesswork and gets you very close.
Why does my measured size differ from the size I currently wear?
Most people wear the wrong bra size, often a band that is too large and a cup that is too small. Stores historically added inches to the underbust measurement, a practice called the plus-four method, which pushes customers into a limited stock range. Your measured size may feel unfamiliar at first, but it usually provides better support once you try it.
Should I round up or down when measuring?
Round each measurement to the nearest half inch for the most accurate result. For the band, standard sizing rounds the underbust to the nearest even number, rounding odd numbers up. Never pull the tape tighter than a comfortable snug fit to force a smaller number; the band needs to reflect your true ribcage measurement to support you properly.
What is the difference between US and UK bra sizing?
US and UK sizing share band numbers and cup letters up to DD, then diverge. US sizing typically continues DD, DDD, G, H, I, J, while UK sizing continues DD, E, F, FF, G, GG, H. Because of this, a US DDD roughly equals a UK E. Always check the specific brand’s chart when ordering across systems.
How do sister sizes work?
Sister sizes keep the same cup volume while changing the band length. From a 34C, going up to a 36 band means dropping one cup letter to 36B; going down to a 32 band means rising one cup letter to 32D. They are handy when your exact size is unavailable or when a brand’s band runs tight or loose, though they work best for a single-size shift.
When should I re-measure my bra size?
Re-measure at least once a year and after any major body change, such as weight gain or loss, pregnancy, breastfeeding, or hormonal shifts. Also re-measure if your bras start riding up, gaping, or digging in. Bodies change gradually, so a size that fit perfectly two years ago may no longer be right.
Why does the band ride up my back?
A riding band is almost always too loose. The band provides most of the support, so it must sit snugly and level around your torso. First try a tighter hook; if it still rides up, go down one band size and up one cup size to preserve cup volume. An old, stretched-out bra will also ride up and should be replaced.
What causes spillage over the top of the cups?
Spillage, often called quad-boob, means the cups are too small for your breast tissue. The fix is to go up one cup size on the same band. If it happens on only one side, which is very common since most people are asymmetrical, fit the larger breast and adjust the strap on the smaller side for a smooth look.
Do sports bras use the same sizing?
Many sports bras use standard band-and-cup sizing and your regular size is the right starting point. However, some use S/M/L sizing, in which case you should consult the brand’s chart with your underbust and bust measurements. For high-impact activities, a snugger band and encapsulation-style cups (separate cups rather than compression) usually give better support and comfort.
Can I wear a bra without an underwire in my size?
Yes. Wireless bras come in full band-and-cup sizing from many brands, and your calculated size applies the same way. Without an underwire, the band and the overall construction do even more of the support work, so a correct band fit matters most. Look for wide bands, supportive fabrics, and structured cups for the best wireless support.
How tight should the band be?
The band should feel snug and supportive on the loosest hook when the bra is new, allowing you to slide two fingers underneath but not much more. It should stay level around your body without riding up. As the elastic relaxes over months of wear, you move to tighter hooks, which is exactly why new bras are designed to start on the loosest one.
Why do my straps keep slipping off?
Slipping straps usually mean the band is too big, so the straps are being asked to do the band’s job. Tightening the straps is only a temporary fix. Check your band fit first and consider going down a band size with a sister-size cup adjustment. Narrow or sloped shoulders can also contribute, in which case racerback styles or bras with closer-set straps help.
What does it mean if the center gore does not sit flat?
The center gore is the small piece of fabric between the cups. In a well-fitting underwire bra it should rest flat against your sternum. If it floats away from your chest, the cups are usually too small or the band is too loose. Going up a cup size or down a band size typically solves it, though very close-set breasts may never get a fully flat gore.
Are bra sizes the same across all brands?
No. Sizing varies between brands and even between styles from the same brand, because each company uses its own fit model and grading. Your calculated size is your best starting point, but always try the bra on or check the return policy when ordering online. Reading reviews for notes like runs small in the band can save you an exchange.