Measuring your own bra size takes about two minutes and requires nothing more than a soft tape measure. Yet most women have never done it — relying instead on guesswork, fitting room experiences that varied wildly, or a size they were told years ago that may no longer be accurate.
This guide walks you through every step, explains exactly where to place the tape, and tells you the most common mistakes that lead to inaccurate measurements. By the end, you'll have two numbers — your underbust and your bust — and you can enter them directly into our calculator to get your size across every international system.
Before You Measure
Two things make a meaningful difference to accuracy before you take a single measurement:
What to wear: Wear a well-fitting, non-padded bra — an unlined underwire or soft-cup style works well. If you don't have one, measure without a bra. Avoid padded, push-up, or moulded cups; they add volume to your bust measurement and give an inaccurate result. A fitted t-shirt over a non-padded bra is fine.
When to measure: If your size tends to fluctuate with your menstrual cycle, measure in the week after your period ends — this is when breast volume is most stable. Measuring in the week before your period, when many women retain fluid, can give a reading that's larger than your "average" size.
The 4 Steps to Measuring Your Size
Find your underbust measurement
Stand upright and wrap the tape measure snugly around your ribcage, directly underneath your bust — not over the breast tissue. The tape should feel firm against your skin but not tight enough to dig in. Breathe out normally (not sucked in) and note the measurement. Take it twice and average the results if they differ slightly. This measurement in inches gives you your band size — if it's an odd number, round up to the next even number.
Find your bust measurement
Keep the tape measure parallel to the floor and wrap it around the fullest part of your chest — typically across the nipple line. Let the tape rest gently against your skin; it shouldn't pull tight or dig in. Lean forward slightly to let your breasts hang naturally, then take the measurement. This is your bust measurement.
Calculate the difference
Subtract your underbust from your bust. The result — in inches — tells you your cup size. A difference of 1 inch = A cup, 2 inches = B cup, 3 inches = C cup, 4 inches = D cup, 5 inches = DD, and so on. See the full table below.
Enter your measurements into the calculator
You don't have to do the maths yourself. Enter your underbust and bust into our free calculator — it works in both inches and centimetres — and it gives you your size in US, UK, EU, AU, IT and JP sizing instantly.
Cup Size Reference Table
If you'd rather work it out manually, use this table. Take your bust measurement minus your underbust measurement (in inches):
| Difference | US Cup | UK Cup | EU Cup |
|---|---|---|---|
| 1 inch | A | A | A |
| 2 inches | B | B | B |
| 3 inches | C | C | C |
| 4 inches | D | D | D |
| 5 inches | DD | DD | E |
| 6 inches | DDD / F | E | F |
| 7 inches | G | F | G |
| 8 inches | H | FF | H |
Have your measurements? Enter them into our free calculator for your size in every international system — no maths needed.
Use the Free Calculator →5 Common Measurement Mistakes
Padded cups add volume to your bust measurement. Always measure in a non-padded bra or no bra at all.
A tight underbust reading leads to a band size that's too small. The tape should be snug — firm enough that you can't slide more than a finger underneath, but not digging in.
The tape should stay parallel to the floor all the way around. If it dips lower at the back than the front, your bust reading will be too large.
Breathing out normally gives a more accurate underbust reading than measuring while inhaling. A sucked-in breath artificially reduces the measurement.
Small variations in how you hold the tape are normal. Measure each point twice and average the results if they're more than half an inch apart.
What If the Calculated Size Doesn't Feel Right?
Calculated sizes are a starting point, not a guaranteed perfect fit. Manufacturing differences between brands mean the same size can fit differently. If your calculated size feels slightly off, try sister sizing — adjusting the band by one size and the cup by one letter in the opposite direction. Our sister size chart makes it easy to find your alternatives.
Also check our bra fitting guide for the exact signs to look for that indicate the band is too tight, the cup is too small, or the straps are carrying too much load.