Bra Size Converter: Band, Cup and Sister Sizes

How do you turn two tape measure readings into a bra size? The Bra Size Converter takes your underbust and bust measurements in inches and gives an estimated size.

It also shows EU, French, and Australian sizes, plus two sister sizes. It is made for shoppers who buy from stores in other countries. Note that it works from measurements, not from an existing size label.

Table of Contents

• What the Bra Size Converter Does
• How to Use It, Step by Step
• The Band and Cup Formula
• Worked Examples
• How to Read the Results
• Sister Sizes and Size Systems
• Limitations and Assumptions
• FAQs About the Bra Size Converter

What the Bra Size Converter Does

Two number boxes feed seven result cards and a text report.

• Underbust Ribcage (Inches): the snug measurement around your ribcage. It starts at 30.
• Bust Fullness (Inches): the measurement around the fullest part of your bust. It starts at 37.
• US Size and UK Size cards show a band number and cup letter.
• EU Size, French Size, and Australian Size cards show a band number with a cup letter.
• Sister Size (Down Band) and Sister Size (Up Band) cards show two related sizes.
• Bra Size and Sister Size Report: a text summary, with a Copy Report button.

How to Use It, Step by Step

1. Measure around your ribcage, just under the bust, and enter it as Underbust Ribcage (Inches).
2. Measure around the fullest part of the bust and enter it as Bust Fullness (Inches).
3. The cards update as you type or press Enter. Press Calculate Size to refresh them.
4. Read the seven cards, or press Copy Report to copy the full report.

The Band and Cup Formula

The band comes from your underbust. The cup comes from the gap between your bust and your underbust.

• Band = underbust rounded to the nearest even number. Odd halfway values round up, so 31 gives 32.
• Difference = bust - underbust, rounded to a whole inch. Halves round up.
• One cup step per inch. A difference of 0 is AA, 1 is A, 2 is B, 3 is C, and 4 is D.
• At 5, the UK letter is DD and the US is DD/E. At 6 it is E and DDD/F. At 7 it is F and DDDD/G.
• At 8, UK is FF and US is H. At 9, UK is G and US is I. At 10, UK is GG and US is J.
• The US list continues K to R. The UK list continues HH, J, JJ, K, KK, and L, up to 17 inches.
• EU band = 2.5 × band - 10. French band = EU band + 15. Australian band = band - 22.
• Sister (down) = band - 2 with the next cup letter. Sister (up) = band + 2 with the previous letter.

Worked Examples

Priya measures 32 inches under the bust and 36 inches across the bust. Her band is 32. The difference is 36 - 32 = 4, which is cup D.

The starting values, 30 and 37, give a difference of 7. The tool shows 30DDDD/G (US), 30F (UK), EU 65F, French 80F, and Australian 8F. The sisters read 28H US / 28FF UK and 32DDD/F US / 32E UK.

The band does not jump at tiny changes. An underbust of 30 or 30.1 with a bust of 35 both give band 30. The result is 30DD/E (US) and 30DD (UK).

How to Read the Results

The US Size and UK Size cards join the band and cup, such as 32D. Past D, the letters differ, so the two cards can disagree.

The EU, French, and Australian cards use the UK cup letters. The page note says so, because real cup lettering in those countries varies by brand.

The two sister cards trade band size against cup letter. A larger band pairs with a smaller cup letter, and the reverse. When US and UK letters differ, the card shows both. A card reads N/A when the sister falls outside the chart.

Sister Sizes and Size Systems

A sister size has a different band and cup letter, with a similar cup volume. That idea is common in bra sizing guides. Use it as a reference when a store lacks your first size.

Size systems do not match one for one. Published US and UK cup lists disagree after D. Large cups are hard to compare across makers. Brand charts decide the final label.

The band table follows a widely published chart: US 32 equals EU 70, French 85, and Australian 10. The EU band steps 5 for every 2 inches, which is the same as 2.5 per inch. Compare any number with the brand's chart.

For other sizes, try the clothing size converter or the shoe size converter. To convert your tape readings into centimeters, use the length converter or the unit converter. The unit and time converters hub lists more.

Limitations and Assumptions

• Sizes are approximate and vary by brand and country. Always check the brand's size chart.
• The tool does not convert an existing size from one system to another. It needs measurements.
• Underbust must be 27 to 56 inches. The bust may not be smaller than the underbust.
• The bust can be at most 17 inches above the underbust. Beyond that, the tool shows a message.
• Blank, zero, or text input shows N/A on every card and a message that names the box.
• Sister sizes show N/A outside bands 28 to 56 or past the ends of the cup lists.
• The nearest-even band is this tool's method. Some older charts add inches to the underbust instead, so other sources may give a different band.
• The cup letters after D are conventions that differ by chart, not a single standard.

FAQs About the Bra Size Converter

This calculator gives general estimates only and is not professional advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.