Find Your Real Bra Size
Use the slideshow to the left to find your
Five Measurements and enter them below.

1. Snug Underbust: inches
2. Tight Underbust: inches
3. Standing Bust: inches
4. Leaning Bust: inches
5. Lying Bust: inches
Estimated Bra Size
Nearest Sister Sizes
This calculator finds your UK bra size. If you would like to convert to another size, please see the charts below.

After discovering the community A Bra That Fits over at reddit, user irisflame realized that their sizing guide could be made into a calculator. The members of the community frequently complained that there was no calculator online that correctly estimated their bra size, probably because most use the +4 sizing method or something equally unreliable. New people were frequently coming into the forums and posting for help finding their size. The math involved in calculating the bra sizes is sometimes confusing for new people, tedious for others, and mistakes are easy to make. Many of the experts could quickly deduce a bra size based off the Five Measurements given, but this wasn't as feasible for the everyday browser.

Irisflame, known as Treasure Linton offline, wanted to help. Although she had moderate experience with HTML and CSS, she knew next to nothing about JavaScript - the coding language necessary for making such a calculator. No problem! She had been wanting to learn anyway. So she spent one Saturday night (March 15, 2014 to be exact) reading JavaScript tutorials online, contacted a programming friend in case she needed help, and the next day she began writing the code for the calculator. By Monday night, March 17, 2014, the first version of the calculator went live and was posted onto ABTF. It only used three measurements at the time and there was very little focus on design.

This first iteration garnered a lot of attention, and also a lot of critique. Many suggestions came in from the community for ways to improve the calculator. By the next night (March 18), the second version of the calculator was released. This time, all five measurements were incorporated and the accuracy went up dramatically. A few other adjustments were made over the course of the next few days to further increase its accuracy. By March 24, the creator decided it was time to take a break.

Plans had been made to improve the overall design of the site and to include more information. It wasn't until late April, however, that work began on the new design. Finally, in the wee morning hours of May 1, the third version of the site went live. Although there were no changes made to the calculator's function, the site itself was vastly revamped with a more user-friendly design. There are still further plans to add more features to the calculator which can be found under the Updates section of the site.

How It Works

This calculator follows the sizing guide made by . It uses JavaScript functions to calculate the measurements that you provide and return to you an estimated UK bra size as well as the nearest sister sizes.

Here are the steps the calculator takes to estimate your bra size.

Step 1: Determining your band size
  • By taking your snug underbust and tight underbust measurements, the calculator can find your band size.
  • Normally, it takes your snug underbust measurement and rounds it to the nearest even whole number to find the band size.
  • If the difference between your tight and snug measurements is significant (indicating a lot of "squish") your results will show a bandsize lower than what your snug measurement may indicate. This is to provide you with better support.
  • If the difference between your tight and snug measurements is minimal (indicating little to no "squish") and your have a small frame, you will be reommended to "Sister Size Up" but your results will not show an increase in band size because some people prefer the tighter band while others find it uncomfortable.
Step 2: Determining your bust size
  • By taking your standing bust, leaning bust, and lying bust measurements, the calculator can find your bust size.
  • Usually, it will take your leaning bust measurement.
  • If the difference between your leaning bust and standing bust is significant, however, it will average your standing bust and leaning bust instead. This average will give a good overall estimation of your bust size.
  • If the difference between your lying bust and your leaning bust is significant, it will now average all three bust measurements to get an overall estimation of your bust size. This is because pendulous, projected or soft breasts will often cause an overestimate in size when using just the leaning bust, while shallow breasts can cause the calculator to underestimate when using just the lying bust.
Step 3: Determining your cup size
  • The calculator will use your snug underbust and chosen bust measurements to find your cup size.
  • The difference between these two measurements corresponds to a cup letter. 1 inch is an A, 2 inches is a B, 3 inches is a C, and so on and so forth, all the way up to an 18 inch difference.
  • If the difference happens to be inbetween two whole numbers, your results will suggest two cup size. Either of these may work.
  • If the difference is very significant (G+ cup), you will be alerted that your results may not be entirely accurate and that you should only use the estimate as a starting point.
Step 4: Determining your bra size
  • By combining your band size and cup size, the calculator can give you an estimated bra size based off the measurements you entered.
Step 5: Determining your sister sizes
  • Using your results, the calculator will also find the two nearest sister sizes.
  • These other bra sizes hold the same volume of breast tissue as your result, and might be good options for you to try as well.

Differences from the Original Guide

This calculator is not perfect and is meant to be used only as an estimation for your size. After working out a few kinks, there are now very few differences between this calculator and the guide. Below are the current differences that still exist.
  • This calculator cannot take your breast shape into account, yet.
    • It does, however, consider the difference between bust (lying) and bust (leaning), which is usually a high number in projected or pendulous breasts


This website was coded by Treasure Linton, user irisflame of reddit, with the help and guidance of Joe Saltsman, of reddit.

The material on this website was written by Treasure Linton.

The images on this website were drawn by Treasure Linton.

The content of this website is based off of the guides at reddit community A Bra That Fits. It is continuously reviewed and improved by the lovely contributors of ABTF. Without them, this website would not be possible.