How to Measure Bra Size
Two measurements, taken properly, in about ninety seconds.
You need two numbers: a snug underbust and a full bust. Wear a thin, unpadded bra or nothing on top. Keep the tape level all the way round. Take the underbust firmly while breathing normally, and the full bust across the fullest point without compressing. That is the whole method – everything below is about doing those two things accurately.
What you need
- A soft dressmaker’s tape measure. A piece of string and a ruler works – mark the string, then lay it flat.
- A mirror, or someone to help. Level matters more than most people expect.
- A thin, unpadded, non-minimising bra, or bare skin. Not a sports bra, not a push-up.
Step one: snug underbust
Stand upright and relaxed. Bring the tape around your ribcage, directly under the breast tissue where a band would sit. Pull it firm – firmer than feels natural, but not so tight that it hurts or shifts your ribs.
- Check in the mirror that the tape is level front to back. Tape that rides up at the back is the single most common source of an inaccurate band.
- Breathe normally and read the number at the end of a normal exhale. Do not hold a deep breath in or out.
- Record it in centimetres. If you land between marks, note the decimal – 74.5 cm is a perfectly usable number.
Written equivalent of the underbust diagram: the tape forms a horizontal loop around the ribcage immediately below the breasts, parallel with the floor, sitting at the same height at the front, sides and back.
Step two: full bust
Keep standing upright with arms relaxed at your sides. Bring the tape around the fullest part of the bust – usually across the nipple line, though not always.
- Let the tape rest. Do not pull it tight; you are measuring the tissue where it naturally sits, not compressing it.
- Level again. Because breast tissue projects forward, the tape wants to slide down at the back. Check the mirror.
- If your breasts sit low or the fullest point is hard to reach standing, lean forward at about 45 degrees so the tissue falls into the tape, then read the number.
Written equivalent of the full bust diagram: the tape forms a horizontal loop around the fullest projection of the chest, resting without tension, level with the floor front and back.
Step three: put the numbers in
Two numbers is all the calculator needs. It returns a starting AU size, two sister sizes and the arithmetic it used.
Six things that skew a measurement
- A padded or moulded bra. Adds two to four centimetres to the full bust, which is a full cup or more.
- Tape not level at the back. Almost always makes the underbust read larger and the band too loose.
- Holding your breath. Inflates the ribcage by several centimetres.
- Measuring over clothing. Even a t-shirt adds enough to change the result.
- Adding 10 cm to the underbust. An outdated instruction. The calculator expects the raw snug measurement.
- Measuring once. Take each measurement twice. If they disagree by more than a centimetre, take a third.
If measuring yourself is difficult
Reaching around to check the tape at the back is genuinely hard alone, and some people cannot do it at all. Options that work:
- Stand with your back to a mirror and check over your shoulder, or use a phone camera on a timer.
- Ask someone you trust. If you are helping another person, ask before you touch, describe what you are doing, and let them hold the tape at the front themselves.
- Use a length of non-stretch ribbon: mark where it overlaps, take it off, and measure it flat on a table.
- A professional fitting is a reasonable alternative and does not require any measurement beforehand.
How often to re-measure
Body measurements change with weight, exercise, medication, pregnancy, breastfeeding, menopause and the ordinary passage of time. Re-measure whenever your bras stop feeling right, and otherwise every six to twelve months. There is nothing significant about the number changing.