A Bra That Fits Calculator (Reddit Method)

Discover your accurate starting size using the celebrated 6-measurement method from the /r/ABraThatFits community.

Underbust Measurements

Bust Measurements

What is the ‘A Bra That Fits Calculator Reddit’ Method?

The “A Bra That Fits Calculator Reddit” method refers to a highly-regarded bra sizing system developed and refined by the community of the /r/ABraThatFits subreddit. Unlike traditional methods that often incorrectly add inches to the underbust measurement (the “+4 method”), this system uses six precise measurements to get a much more accurate starting point for your true bra size. The goal is to account for the unique variations in body and breast tissue, leading to better support, comfort, and health.

This calculator is for anyone who is tired of uncomfortable bras, straps that dig in, bands that ride up, and cups that overflow or gap. If you suspect you’re wearing the wrong size (studies suggest over 80% of people are), this method is a revolutionary first step toward finding a bra that actually fits.

The A Bra That Fits Formula and Explanation

The calculator’s logic uses your six measurements to determine band and cup size independently before combining them. It prioritizes the snug underbust for band comfort and uses a combination of bust measurements to account for breast shape and volume.

1. Band Size: The band size is primarily determined by your underbust measurements. The snug underbust is a key indicator, but the loose and tight measurements help the calculator determine if you have more or less “squish” around your ribcage, which might influence whether you prefer a tighter or looser band. Generally, the band size will be close to your snug underbust measurement in inches.
2. Cup Volume: The cup volume is calculated from the difference between your bust measurement and your underbust measurement. The ABTF method uses the average of all three bust measurements (or sometimes just the leaning measurement if it’s significantly larger) to get the most accurate representation of breast volume.
3. Final Size: Each inch of difference between the determined bust measurement and your snug underbust corresponds to one cup size.

Variable Explanations

Variable	Meaning	Unit	Typical Range
Loose Underbust	Ribcage measurement with loose tape	in / cm	24 – 50 in
Snug Underbust	Firm ribcage measurement	in / cm	23 – 49 in
Tight Underbust	Maximum tight ribcage measurement	in / cm	22 – 48 in
Standing Bust	Bust measurement while standing	in / cm	28 – 65 in
Leaning Bust	Bust measurement while leaning 90°	in / cm	29 – 70 in
Lying Bust	Bust measurement while on your back	in / cm	28 – 68 in

Practical Examples

Example 1: A Common Size Profile

• Inputs: Loose 30″, Snug 29″, Tight 28″, Standing 35″, Leaning 37″, Lying 36″ (all in inches)
• Calculation: The band size will be around 30. The bust average is (35+37+36)/3 = 36″. The difference is 36″ – 29″ = 7″.
• Result: A 7″ difference corresponds to an F cup in UK sizing. The starting size would be 30F.

Example 2: A Larger Band and Cup Profile

• Inputs: Loose 38.5″, Snug 38″, Tight 36″, Standing 46″, Leaning 49″, Lying 47″ (all in inches)
• Calculation: The band size will be around 38. The bust average is (46+49+47)/3 = 47.33″. The difference is 47.33″ – 38″ = 9.33″.
• Result: A 9-10″ difference corresponds to a G/GG cup. The calculator would likely suggest 38G as a starting point. For help finding the right fit, see our bra shape guide.

How to Use This A Bra That Fits Calculator

1. Select Your Units: Choose between inches or centimeters. Inches are standard for UK and US sizing.
2. Take 6 Measurements: Use a soft measuring tape. For best results, measure without a bra on. Ensure the tape is parallel to the floor for all measurements.
3. Enter Your Numbers: Input each of the six measurements into the corresponding fields. The calculator will update in real-time.
4. Interpret the Result: The calculator provides a starting size. Be prepared for “sticker shock”—it’s common for the result to be a smaller band and larger cup than you expect. This is your first step, not the final answer.
5. Explore Sister Sizes: The generated table shows you sister sizes. These are sizes with the same cup volume on a different band. If a bra in your recommended size feels too tight in the band but the cups fit, you can try the “sister size up” (e.g., from 32F to 34E).

Key Factors That Affect Your Bra Fit

Your calculator size is just the beginning. Perfect fit is also about shape. Here are key factors to consider:

• Breast Shape (Fullness): Are you more full on the top, bottom, or even? This affects what style of cup (e.g., demi vs. full cup) will work for you.
• Breast Projection: Do your breasts project far from your chest (projected) or are they shallower and cover a wider area (shallow)? Some bras are made for projection while others are not.
• Root Width: This refers to how wide the base of your breast tissue is on your chest. Wires that are too narrow will dig in, and wires that are too wide won’t provide proper support.
• Root Height: Similar to width, this describes how far up your chest your breast tissue goes. It can affect whether you experience “quad-boob” in certain styles.
• Symmetry: Most people have one breast that is larger than the other. Always fit to the larger breast and consider using an insert on the smaller side if needed.
• Brand and Style Variation: Just like jeans, bra sizing is not perfectly consistent across brands. You may be a 30F in one brand and a 30FF in another. Learning about common bra fitting mistakes can be a huge help.

Frequently Asked Questions (FAQ)

Why do I need six measurements?

The six measurements give a 3D picture of your torso and breast volume. The three underbust measurements assess the flexibility of your ribcage, while the three bust measurements account for how gravity and position affect your breast tissue, giving a more accurate volume estimate than a single measurement.

The size seems wrong! Why is the band so small and the cup so big?

This is a very common reaction called “sticker shock”. Most mainstream stores use outdated sizing methods that put people in bands that are too large and cups that are too small. A properly fitted band should be snug and provide ~80% of the support. A larger cup is needed to fully contain your breast tissue without spilling over.

Should I use Inches or Centimeters?

UK and US bra sizing is based on inches, so we recommend using inches for the most straightforward result. If you use centimeters, the calculator will convert them for the final sizing calculation.

What is a sister size?

A sister size is a bra size that has the same cup volume as your recommended size, but on a different band size. When you go up in the band, you go down in the cup letter to maintain volume (e.g., 34F has the same cup volume as 36E). This is useful if a bra’s band fits you differently than expected.

Does this calculator work for UK or US sizing?

This calculator provides a UK size, which is the standard used by the /r/ABraThatFits community because of its consistency in larger cup sizes. After DD, UK sizing goes E, F, FF, G, GG, H, etc., while US sizing is less standardized (often using DDD, G, H, etc.). You can find conversion charts online or explore our guide to UK vs US sizing.

How often should I re-measure?

It’s a good idea to re-measure every 6-12 months, or after any significant changes in your body such as weight fluctuation, pregnancy, or starting a new exercise routine. Your bra size is not static.

Is this the same as the official A Bra That Fits calculator?

This calculator is based on the well-documented principles and formulas used by the Reddit A Bra That Fits community and its official calculator. It is designed to give you the same reliable starting size recommendation.

Can I measure myself over a bra?

For the most accurate results, you should measure yourself without a bra on. A bra, even a non-padded one, can slightly alter the position and shape of your breast tissue, which can throw off the measurements.