Understanding Bra Sizing
What Is a Bra Size Calculator?
A bra size calculator uses your underbust (ribcage) and overbust (fullest point) measurements to determine your recommended bra size. The two numbers that make up a bra size — the band number and the cup letter — correspond to these two measurements.
How Sizing Works
The band size is derived from your underbust measurement. In the direct method, it is simply the rounded underbust in inches. In the +4 method (traditional), 4 or 5 inches are added to the underbust to get the band size. The cup size is determined by the difference between the overbust and the band size — each inch of difference corresponds roughly to one cup letter.
Why International Conversion Matters
Bras are manufactured around the world using different sizing standards. A 32C in the US is a 32C in the UK but a 70C in the EU, an 85C in France, and an AU10C in Australia. This calculator converts your measurements into all seven major sizing systems simultaneously.
Understanding Sister Sizes
Sister sizes are sizes that have approximately the same cup volume but different band sizes. For example, 32C and 34B and 30D are sister sizes. If your preferred size is unavailable, trying a sister size can help you find a comfortable fit.
How to Measure for Your Correct Bra Size
Getting an accurate bra size starts with proper measurement technique. You need two measurements: the underbust (band) measurement taken snugly around your ribcage directly under your breasts, and the overbust (bust) measurement taken at the fullest point of your breasts, usually across the nipples. For the most accurate results, measure while wearing an unlined or lightly lined bra that fits well — never measure over clothing or with a padded bra.
Stand upright with your arms relaxed at your sides. Use a soft measuring tape and ensure it sits parallel to the floor all the way around your body. The tape should be snug but not tight for the underbust measurement, and loose enough to slide a finger underneath for the overbust measurement. Take each measurement two or three times and use the average for the best accuracy.
Understanding Band Size
The band provides approximately 80% of the bra's support, making it the most critical component of fit. A properly fitted band should be snug enough that you can only slide two fingers underneath it. If the band rides up your back during wear, it's too loose — try going down one band size. If it leaves deep red marks or causes discomfort, it may be too tight.
Band sizes are typically even numbers in the US and UK systems (28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48). The EU system uses centimeters rounded to the nearest 5 (60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110). France adds 15 to the EU band, creating their system (75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125).
The Cup Size Calculation
Cup size is determined by the difference between your overbust and band measurements. Each inch of difference corresponds to approximately one cup size increment. A 1-inch difference is an A cup, 2 inches is a B cup, 3 inches is a C cup, and so on. A 0-inch difference (bust equals band) is an AA cup, representing the smallest standard cup size.
It's important to note that cup size is not absolute — it's relative to band size. A 34C and a 36C have different absolute volumes. The cup of a 36C is actually the same volume as a 34D (this is the principle behind sister sizes). This is why you cannot simply say "I'm a C cup" without also specifying your band size.
Direct Method vs +4 Method
There are two common approaches to calculating band size from measurements. The direct method rounds the underbust measurement to the nearest even number — if you measure 33 inches, your band size is 34. The +4 method adds 4 inches to an even underbust or 5 inches to an odd underbust before rounding. A 29-inch underbust becomes a 34 band with +4 method (29+5=34).
The +4 method originated in the 1930s-1950s when bra materials were much less stretchy than modern fabrics. Most fitting experts today recommend the direct method for modern bras with elastic bands. However, some US retailers still use the +4 method, so we provide both options in our calculator.
Common Fitting Problems and Solutions
Studies consistently show that 70-85% of women wear the wrong bra size. The most common error is wearing a band that's too large combined with a cup that's too small. Signs of poor fit include: straps digging into shoulders (band too loose, relying on straps for support), band riding up in the back (band too loose), breast tissue spilling over cups (cup too small), wrinkled or gaping cups (cup too large), and the center gore not lying flat against the sternum (cup too small or wrong style).
If your bra fits poorly, try adjusting before buying a new one. Start with the loosest hook when new (the band will stretch over time). Ensure straps are adjusted to provide lift without bearing all the weight. The band should sit level all the way around your body — lower in the back indicates a too-loose band.