How to measure

Measure in a soft, unpadded, unlined bra, or with nothing on top at all. A moulded or push up cup moves tissue and adds thickness, which corrupts the second measurement by a centimetre or more. Do not measure over a jumper, and do not measure over a sports bra, which compresses in a way that is designed to distort exactly this number.

1. Stand upright in front of a mirror, feet together, shoulders relaxed and arms down at the sides.
2. Run the tape round the ribcage directly beneath the breast tissue, where the band of a bra sits.
3. Check in the mirror that the tape is level all the way round. It will want to ride up at the back. Push it down until it is horizontal.
4. Pull it snug: firm enough that it stays put when the arms drop, loose enough that a normal breath is comfortable.
5. Breathe out and read the number in centimetres. Do it three times and take the middle reading.

That figure is the band. In continental European sizing it rounds to the nearest five centimetres, so a measurement between 73 and 77 becomes a 75. In the United Kingdom and the United States it converts to the nearest even inch, so the same body wears a 32 band. French sizing uses the European number plus fifteen, so that 75 becomes a 90. Nothing is added to the measurement in any of these systems.

1. Keep the same posture. Run the tape round the fullest part of the bust, usually across the nipple line.
2. Keep the tape level front to back. Sagging at the back is the classic error here and adds volume that is not there.
3. Rest the tape on the skin without pulling. Any tension compresses tissue and loses a cup size.
4. Read the number in centimetres.

If the bust is soft or full enough that standing upright gives a very different result from leaning forward, take a third reading bent at the waist with the bust hanging free. Where the standing and leaning numbers differ a lot, the truth is usually between them, and the leaning figure is the better guide to the volume a cup has to contain.

The cup comes from the difference between the two measurements, not from the overbust figure alone. Roughly one inch of difference per cup letter: one inch is an A, two a B, three a C, four a D, five a DD in British sizing and an E in European. Because European cups step in two centimetre increments rather than 2.54, the two systems drift apart slightly in the larger sizes, which is one reason a converted size is always a starting point.

The calculator handles all of this and prints the full conversion table below the form, so it still works with scripting disabled.

Adding inches to the underbust

The rule about adding four or five inches belongs to an era of non stretch band fabric. Applied to a modern band it produces a garment that rides up and cannot support anything.

Measuring over a padded bra

Adds between one and three centimetres to the overbust figure, which is a whole cup letter of error.

A tape that slopes

The single most common physical mistake. It is invisible from the front, which is why a mirror is not optional.

Holding a breath in

Measure on a normal exhale. A held breath expands the ribcage and produces a band that will feel tight all day.

Measuring once

Take each figure three times. If the readings vary by more than a centimetre, the tape is moving and the technique needs fixing before the numbers mean anything.

Once a year is sensible for most people, and again after any noticeable change in weight, after pregnancy and feeding, and whenever a familiar size suddenly stops behaving. Bodies change faster than drawers do. Fit through change covers what shifts and on what timescale.