Bra Size Calculator v1.0

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

Estimated Bra Size Nearest Sister Sizes

Click Here for the Most Recent Version


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.

Here are the steps the calculator takes to estimate your bra size.
  1. Takes your underbust (snug) measurement and rounds it to the nearest EVEN whole number to get your band size
    • if that number is less than or equal to 32, it will prompt you to consider sister sizing up, since this is commonly done for comfort
    • if that number is more than 36, it will prompt you to consider sister sizing down, since this is commonly done for support
  2. Takes your bust (standing) and bust (leaning) measurements to calculate your bust size
    • if the difference between your bust (leaning) and bust (standing) is greater than or equal to 2, it averages the measurements
    • if the difference is less than 2, it takes the bust (leaning) measurement
  3. Subtracts your underbust (snug) measurement from the bust size determined in step 2.
    • If it is not a whole number, it is rounded down (ex: 7.25, 7.5 and 7.75 all become 7)
  4. Equates the result of step 3 with a letter in both US and UK sizes
    • Each number equals a letter. <1 = AA, 1 = A, 2 = B and so on
  5. Combines the band size from step 1 with the cup letter from step 4 to create your bra size
  6. Calculates the immediate sister sizes for your bra size (up means an increase in band, down means a decrease)

Differences from the Original Guide

This calculator is not perfect and is meant to be used only as an estimation for your size. A few things have been slightly altered from the original guide linked above.
  • The difference between bust (standing) and bust (leaning) must be greater than or equal to 2 to find the average, as opposed to just being greater than 2
  • If the difference between the underbust and bust is not a whole number, it is rounded down. The guide recommends looking at both cup sizes that correspond to the nearest whole numbers.
    • A difference of 6.5 is rounded down to 6 and corresponds to DDD/F (US) and E (UK), whereas the guide would recommend both DDD & G (US) and E & F
  • Does not take the underbust (tight) or bust (laying) measurements into account
    • The underbust (tight) measurement is used when you have a GG+ cup size or a difference of 2 or more between your underbust (snug) and underbust (tight) measurements.
    • Round your underbust (tight) measurement to the nearest whole number. However many band sizes you went down, you need to go an equal amount of cup sizes up. So if you started as a 36G and went down to a 32, you would need to go up to an H cup.
  • This calculator cannot take your breast shape into account

Update Log

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 underbust (tight) measurement
  • Use of bust (lying) measurement
  • Two sizes for those with a difference between their bust and underbust that is not a whole number
  • Use of breast shape
  • Guides for how breast shape and other factors affect your bra size
  • Suggested bra brands for your size and shape


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

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 3/5 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. This measurement is not needed for this calculator.
  3. Bust (Standing): Measure your bust 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. This measurement is not needed for this calculator.

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'll 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.

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