Finding a bra that actually fits is a nightmare. Honestly, most of us are walking around in underwires that dig, straps that slip, or cups that have more gaps than a picket fence. You've probably heard the statistic that 80% of women are wearing the wrong size. It’s an old stat—often attributed to a 1980s study by K.H. Plum and later popularized by brands like Victoria’s Secret—but even if the exact number is debated by modern fit experts, the sentiment holds true. Most people have no clue how to work out bra size because the industry makes it unnecessarily complicated.
The math is weird. Every brand seems to have its own secret "vanity sizing" logic. And your body? It changes. Weight fluctuations, hormonal cycles, or just getting older can shift your measurements in a matter of months. If you’re still wearing the same size you wore three years ago, there’s a massive chance you’re uncomfortable and don't even realize it's the bra's fault.
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The Measurement Myth and Why Your Math Might Be Wrong
Before we grab the measuring tape, we have to talk about the "Plus Four" method. You’ve seen it. It’s the traditional way many department stores teach you how to work out bra size. You measure your underbust, add four inches, and that’s your band size.
Stop. Just stop.
This method is a relic from the early 20th century when bra fabrics weren't stretchy. Back then, you needed those extra inches so you could actually breathe. Modern bras use high-tech elastics and Lycra. If you add four inches to your measurement today, you’ll end up in a band that is way too big. When the band is too big, it slides up your back. When it slides up, the front tips down. Suddenly, your shoulders are doing all the heavy lifting, leading to that chronic neck pain you’ve been blaming on your desk chair.
Measuring Your Underbust (The Anchor)
Grab a soft measuring tape. Stand in front of a mirror. You want to be shirtless or in a very thin, non-padded bra. Wrap the tape around your ribcage, directly under your breasts where the band usually sits.
Pull it tight. No, tighter. It should feel like a firm hug. You want the tape to be level all the way around—if it's drooping in the back, your numbers will be useless. Write down that number. If it’s an even number, that is usually your starting band size. If it’s an odd number, like 33, you’ll likely need to try both 32 and 34 to see which feels more secure.
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Finding the Bust Volume
Now for the "fullest part" measurement. Wrap the tape around the widest part of your chest. Don’t pull this one tight. You aren’t trying to squish anything here; you’re just skimming the surface.
Pro tip: Lean forward slightly. Gravity is your friend here. By leaning over, you ensure you're measuring all the breast tissue. This is especially important if your breasts are "relaxed" or have a teardrop shape. If you measure standing straight up, you might underestimate your cup size, leading to the dreaded "quadra-boob" effect where the top of the cup cuts into your tissue.
How to Work Out Bra Size Using Modern Calculations
Once you have your two numbers—Underbust and Full Bust—it’s time for some simple subtraction.
- Full Bust - Underbust = Cup Size
- If the difference is 1 inch, you’re an A.
- 2 inches? You're a B.
- 3 inches? C.
- 4 inches? D.
- 5 inches? DD (or E in some UK brands).
It seems easy, right? It rarely is. Let’s say your underbust is 32 and your bust is 37. That’s a 5-inch difference, making you a 32DD. But wait. If you go to a store like ThirdLove or Cuup, their specific construction might mean you need a 32E or even a 32D depending on the style.
The shape of your breasts matters just as much as the volume. Some people have "shallow" breasts where the tissue is spread out over a wide area. Others are "projected." You could have the exact same measurements as your best friend but require a completely different bra style because your tissue is distributed differently. This is why "sister sizing" is a concept you absolutely have to master.
The Secret Language of Sister Sizes
Sister sizes are groups of bra sizes that have the same cup volume even though the band and cup letters are different. This is the "Aha!" moment for most people learning how to work out bra size properly.
Think about it this way: a 34C cup holds the same amount of volume as a 32D and a 36B.
If you find a bra where the 34C band feels perfect but the cups are just a tiny bit too small, you don't go to a 36C. If you go to a 36C, you’re increasing the band and the cup. Instead, you stay in the 34 band and move up to a 34D. Conversely, if the cups of a 34C fit perfectly but the band is so tight you can't breathe, you move to your sister size "up": the 36B. You get a bigger band, but you keep the cup volume that worked for you.
Why the "Pinch Test" is Better Than Any Tape Measure
Measurements are just a starting point. They get you into the dressing room, but the "fit" happens in the mirror.
Check the gore first. The gore is that little piece of fabric between the cups. In a perfect world, it should sit flat against your sternum. If it’s hovering in mid-air, your cups are too small. Your breasts are literally pushing the bra away from your body because they can't fit inside.
Then, look at the band. It should be level. If it arches up toward your shoulder blades, the band is too big. You’re getting zero support. 80% of a bra's support should come from the band, not the straps. If you take the straps off and the bra falls down, the band is failing you.
Do the scoop and swoop. This sounds silly, but it’s mandatory. Lean forward, reach into the side of the cup near your armpit, and pull all the tissue forward into the cup. Most people realize they are actually a cup size larger once they stop leaving half their breast tissue tucked under the wire near their ribs.
The Problem With Different Brands
A 34D at Panache is not the same as a 34D at La Senza. European, UK, and US sizing systems are all over the map once you get past a D cup. UK brands (like Freya or Fantasie) use a double-letter system (DD, E, F, FF, G, GG). US brands often go (DD, DDD/F, G, H).
If you are buying a bra from a UK-based company, you must use their sizing chart. Don't assume your US size will translate 1:1. Generally, UK brands are more consistent for larger bust sizes because they’ve been designing for a wider range of shapes for decades.
Common Red Flags You’re Ignoring
- The "Muffin Top": If your breast tissue is spilling over the top or sides, go up a cup size.
- The Gape: If there’s space at the top of the cup, it might be too big, but it’s more likely the wrong shape. You might need a balconette style rather than a full-coverage plunge.
- The Slip: Straps falling down? First, tighten them. If they still fall, your band is likely too wide, causing the straps to sit too far out on your shoulders.
- The Dig: Underwires should never sit on your breast tissue. They should encircle it. If that wire is stabbing your side, the cup is too narrow.
Real-World Fit: The "Two-Finger" Rule
When you've got the bra on, try to slide two fingers under the band at the back. You should be able to do it, but with some resistance. If you can pull the band more than two inches away from your spine, it’s too loose.
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Also, always start a new bra on the loosest hook. Bras are made of elastic. They will stretch out over time. You want to be able to tighten the band as the bra ages to prolong its life. If you start on the tightest hook, you have nowhere to go once the fabric relaxes.
Practical Steps to Get Your Best Fit
Knowing how to work out bra size is a skill, not a one-time event. You should re-evaluate every six months.
- Measure tonight. Use the snug underbust and the leaning full-bust method.
- Check your current "favorite" bra. Does the gore touch your chest? Is the band level? If not, use your measurements to find your new starting point.
- Shop by brand reputation. If you have a large bust, look at Elomi or Curvy Kate. If you are smaller-chested but wide-set, look at Natori.
- Try three sizes. When ordering online, order your "measured" size, one cup size down, and one cup size up. Returns are a hassle, but it's the only way to find the "the one."
- Wash with care. Never, ever put your bras in the dryer. Heat destroys the Lycra that provides the support you just spent all this time measuring for. Hand wash or use a delicate cycle with a mesh bag, then air dry.
Ultimately, the number on the tag doesn't matter. Nobody is going to check your label. If a 36G makes you feel supported, pain-free, and confident, then that is your size—regardless of what the tape measure or the "Plus Four" method tried to tell you. Forget the rules and listen to how your body feels in the fabric.