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 bra size in both UK and US sizes as well as the nearest sister sizes.

Due to recent theft of this calculator, I have taken down the step by step process of the JavaScript function it uses. If you would like to know more about how your size was calculated, you can contact /u/irisflame on Reddit or post a size check in /r/abrathatfits.

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

Update Log

3/24/2014 01:30 PM EST Removed the US bra size because it was confusing many people. Added a UK to US converstion chart under the "How to Use" section.
3/20/2014 04:45 PM EST Changed hosts and URLs. We are now our own website! Also, cleaned up and organized the code in the process.
3/20/2014 02:30 AM EST Changed the method of choosing bust size so that if the difference between standing and leaning is greater than 2, it finds the average (instead of being greater than or equal to 2). Now gives you two cup sizes if the difference between your underbust and bust falls between two numbers.
3/19/2014 02:00 AM EST Fixed the method of sister sizing down for significant "squish."
3/18/2014 01:00 AM EST - Added underbust (tight) and bust (lying) measurements and integrated them into the calculation function as suggested in the original thread on Reddit. Changed the layout slightly. Updated the About section. Changed to Version 2. Posted to r/abrathatfits.
3/17/2014 08:30 PM EST - Changed "How to Measure" section to "How to Use." Added subsections "What Your Results Mean" and "What Are Sister Sizes?" Added a sister size chart for reference. Added "Update Log" and "Future Plans" to the "About" section.
3/17/2014 05:50 PM EST - Added KK to the UK Cup size list, as it was mistakenly left out.
3/17/2014 04:30 AM EST - Changed the function that automatically sizes up band sizes of 32 or below so that it now prompts you to consider sister sizing up for comfort. Changed the function that automatically sizes down band sizes of more than 36 so that it now prompts you to consider sizing down for support.
3/17/2014 03:45 AM EST - Bra Size Calculator goes live and is shared on r/abrathatfits.

Future Plans

Here are some things that will be incorporated into the calculator later on.
  • Use of breast shape
  • Guides for how breast shape and other factors affect your bra size
  • Suggested bra brands for your size and shape
  • Other EUR bra sizes
  • Update the layout and design of the page so that is super awesome and looks like an expert made it, and not bland like a 12 year old made it.


This calculator was coded by T. Linton () with the help and guidance of Joe Saltsman ().

Bra Size Calculator v2.4

JavaScript must be enabled on your browser for this calculator to work.

underbust (snug): inches
underbust (tight): inches
bust (standing): inches
bust (leaning): inches
bust (lying): inches

Estimated Bra Size Nearest Sister Sizes

/r/ABraThatFits uses UK bra sizes, and we recommend that you do as well. If you would like to convert your size because you are looking at buying a US brand bra, please see "What Your Results Mean" in the "How to Use" section to your right.

All sizes given are merely estimates and should not be considered definite. Certain breast shapes and size ranges may require slightly different methods of calculating bra size; if you would like a more personalized calculation, consider posting a Measurement Check post on . In addition, there can be slight variations between brands or even colors. You may need to try on a variety of bras surrounding this estimated size before you find one that fits you perfectly. Do not get discouraged! Once you find that perfect size, your body will thank you! :] Enjoy!

How to Use

Before You Begin

You should not be wearing a bra when you take these measurements. You should also not distort your breasts in any way while you are taking these measurements. Let them fall naturally.

The calculator takes measurements in inches. It will accept decimals as well, but not fractions.

What You Need

  1. A soft/flexible measuring tape
    • If you do not have a soft measuring tape, you can use a piece of string and a ruler/yardstick instead.
  2. Something to record your measurements with or on.
    • You will enter all of these into the calculator.


Underbust - this is the line immediately below where your breast tissue attaches to your chest (called the inframammary fold)
Bust - this is the line across your breasts at their fullest point (usually at or near the nipple)


  1. Underbust (Snug): Measure your underbust with the tape "comfortably snug" (not too tight but not loose). Keep the tape parallel to the ground.
  2. Underbust (Tight): Measure your underbust with the tape as tight as possible without breaking the tape (exhale for this measurement). Keep the tape parallel to the ground.
  3. Bust (Standing): Measure your bust at its fullest point while standing. Keep the tape parallel to the ground.
  4. Bust (Leaning): Measure your bust while leaning over at a 90 degree angle so that you are looking at the ground. Keep the tape perpendicular to the ground.
  5. Bust (Lying): Measure your bust while lying on your back. Do not manipulate your breasts. Keep the tape perpendicular to the ground.

What Your Results Mean

The calculator is not perfect, but it is fairly accurate. The first bra size it gives you is your estimated bra size. The other two sizes are the nearest sister sizes for your estimated bra size.

If you have a 32 inch or less underbust, you might want to consider sister sizing up for comfort. If your breasts are rather large (GG+) you might need to keep the band size lower for added support. If you have more than a 36 inch underbust, you will also want to consider sister sizing down for support, since the band sizes greater than 36 tend to be stretchier.

You may need to try on surrounding band and cup sizes in order to find the perfect fit. Remember, not all bras are made with the same cut, so a 34G in one brand might not fit the same as a 34G of another brand. Breast shape also plays a role in determining your bra size. More on shapes will be added at a later time.

If you are looking to buy a US brand bra in your size, here is a conversion chart for cup sizes. Please note that US brands tend to be less consistent than UK brands, and you may have difficulty finding US brands with cups larger than a DDD.


What are Sister Sizes?

Cup size is entirely dependent on band size. Sister sizes hold the same volume of cup (amount of boob, if you will), but may fit better or worse depending on your preferred band measurement. For every band size you go up, you should go down an equal number of cup sizes, and vice versa.

For the purposes of this guide/calculator, the term "sister size up" means to go up in band size and down in cup size (32G > 34FF) and the term "sister size down" means to go down in band size and up in cup size (32G > 30GG).

Below is a chart with several sister sizes. Each row is a family - all the sizes listed on that row (same color) are sister sizes and have the same cup volume. Note that the further away you go from your estimated bra size, the less likely it is that the bra will fit. This chart is written with the UK sizing method, since it is more consistent.

26 28 30 32 34 36 38 40 42 44 46
26A 28AA
26B 28A 30AA
26C 28B 30A 32AA
26D 28C 30B 32A 34AA
26DD 28D 30C 32B 34A 36AA
26E 28DD 30D 32C 34B 36A 38AA
26F 28E 30DD 32D 34C 36B 38A 40AA
26FF 28F 30E 32DD 34D 36C 38B 40A 42AA
26G 28FF 30F 32E 34DD 36D 38C 40B 42A 44AA
26GG 28G 30FF 32F 34E 36DD 38D 40C 42B 44A 46AA
26H 28GG 30G 32FF 34F 36E 38DD 40D 42C 44B 46A
26HH 38H 30GG 32G 34FF 36F 38E 40DD 42D 44C 46B
26J 28HH 30H 32GG 34G 36FF 38F 40E 42DD 44D 46C
26JJ 28J 30HH 32H 34GG 36G 38FF 40F 42E 44DD 46D
26K 28JJ 30J 32HH 34H 36GG 38G 40FF 42F 44E 46DD
26KK 28K 30JJ 32J 34HH 36H 38GG 40G 42FF 44F 46E
28KK 30K 32JJ 34J 36HH 38H 40GG 42G 44FF 46F
30KK 32K 34JJ 36J 38HH 40H 42GG 44G 46FF
32KK 34K 36JJ 38J 40HH 42H 44GG 44G
34KK 36K 38JJ 40J 42HH 44H 46GG
36KK 38K 40JJ 42J 44HH 46H
38KK 40K 42JJ 44J 46HH
40KK 42K 44JJ 46J
42KK 44K 46JJ
44KK 46K

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