Reddit Bra Size Calculator (ABTF Method)
The most accurate, community-driven method for finding your true bra size.
Wrap the tape loosely around your ribcage, just under your breasts.
Wrap the tape snugly. It should be as tight as you’d want a comfortable bra band to be.
Wrap the tape as tight as possible, breaking the tape tight, after exhaling fully.
Measure around the fullest part of your bust while standing straight.
Lean forward 90 degrees and measure around the fullest part of your bust.
Lie on your back and measure around the fullest part of your bust.
Visual Comparison of Your Measurements
| Go Down a Band (Tighter) | Your Starting Size | Go Up a Band (Looser) |
|---|---|---|
| — | — | — |
What is the Reddit Bra Size Calculator?
The Reddit Bra Size Calculator refers to the sizing method popularized by the Reddit community r/ABraThatFits. It’s an alternative to outdated “plus-four” methods that often lead to poor fits. The A Bra That Fits (ABTF) method uses six precise measurements to get a much more accurate picture of your true size, accounting for variations in breast shape and volume.
This calculator is for anyone frustrated with uncomfortable bras, gaping cups, or slipping bands. It rejects the idea that a small selection of sizes can fit everyone and instead provides a starting point based on your unique body. By taking more detailed measurements—especially the leaning bust, which accounts for breast projection—it delivers a size that is often a smaller band and a larger cup than what most people are used to, but provides significantly better support and comfort.
Reddit Bra Size Calculator Formula and Explanation
The core logic of the ABTF method is to use your underbust measurement directly for the band size and calculate the cup size based on the difference between your bust and band. This contrasts with methods that add inches to the underbust.
- Band Size Calculation: The band size is typically determined by your snug underbust measurement. This calculator rounds your snug underbust measurement to the nearest even number to find a firm, supportive band size.
- Bust Measurement Calculation: To account for breast shape and tissue distribution, the calculator averages your three bust measurements (standing, leaning, and lying). For some breast shapes, the leaning measurement alone is the most accurate, but an average provides a reliable starting point for most.
- Cup Size Calculation: The cup volume is found by subtracting the final band size from the average bust measurement. Each inch of difference corresponds to one cup size in the UK system.
Formula: Cup Size = Average Bust Measurement - Band Size
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Snug Underbust | The primary measurement for determining band size. | Inches or Centimeters | 24 – 50 in |
| Average Bust | The average of standing, leaning, and lying bust measurements. | Inches or Centimeters | 28 – 65 in |
| Cup Difference | The difference in inches that determines the cup letter (A, B, C…). | Inches | 1 – 20 in |
Practical Examples
Example 1: A Common Misfit
A person currently wears a 36C but experiences a loose band and gaping cups. Their measurements are:
- Inputs: Loose 31″, Snug 30″, Tight 29″, Standing 36″, Leaning 38″, Lying 37″
- Band Calculation: The snug 30″ underbust suggests a 30 band.
- Bust Calculation: (36 + 38 + 37) / 3 = 37″
- Cup Calculation: 37″ (Avg. Bust) – 30″ (Band) = 7″ difference.
- Result: A 7″ difference is an F cup in UK sizing. The calculator’s starting size is 30F. This is a huge change from 36C but will provide much better support. For more details on sizing, see our bra size charts.
Example 2: Larger Band and Cup
Someone looking for a supportive plus-size bra enters their measurements:
- Inputs: Loose 42.5″, Snug 42″, Tight 40″, Standing 50″, Leaning 53″, Lying 51″
- Band Calculation: The snug 42″ underbust points directly to a 42 band.
- Bust Calculation: (50 + 53 + 51) / 3 = 51.33″
- Cup Calculation: 51.33″ (Avg. Bust) – 42″ (Band) = 9.33″ difference, which rounds to 9″.
- Result: A 9″ difference is a G cup in UK sizing. The starting size recommendation is 42G.
How to Use This Reddit Bra Size Calculator
- Select Your Units: Choose between Inches (in) or Centimeters (cm). Inches are standard for UK/US sizing, but the calculator will convert cm automatically.
- Take Six Measurements: Following the helper text for each input, measure your underbust (loose, snug, tight) and bust (standing, leaning, lying) without a padded bra. Be precise.
- Enter Your Numbers: Input each measurement into its corresponding field. The calculator updates in real-time.
- Read Your Starting Size: The primary result is your recommended UK bra size. Note that this is a starting point.
- Check Sister Sizes: The table shows you alternative sizes. If a 32F band feels too tight, you could try its sister size up, 34E, which has the same cup volume on a looser band. See our guide on how to find your perfect fit for more info.
- Interpret the Results: Use this size to start trying on bras. Pay attention to how the band, cups, and gore (the center part) fit. Shape is as important as size!
Key Factors That Affect Reddit Bra Size Calculator Results
- Breast Shape: Whether your breasts are projected (stick out more) vs. shallow (spread over a wider area) can affect which styles fit best, even in your correct size. The leaning measurement helps account for projection.
- Breast Fullness: Being more full on the top or bottom of your breasts will determine which cup styles (e.g., balconette vs. full cup) will prevent gaping or spillage.
- Measurement Accuracy: A loose or crooked tape can throw off results. Ensure the tape is parallel to the floor for all measurements.
- Time of the Month: Hormonal fluctuations can cause breast size to change. It can be helpful to measure at different points in your cycle.
- Brand Variations: Just like clothing, bra sizing is not perfectly standardized between brands. You might be a 30F in one brand and a 30FF in another. Our bra style guide can help navigate these differences.
- Rounding Method: This calculator uses a common rounding method, but some people are more comfortable in a band size slightly above or below their snug measurement. The tight underbust measurement helps determine how much “squish” you have and if you can comfortably size down in the band.
Frequently Asked Questions (FAQ)
1. Why is the recommended size so different from what I wear?
Most stores use outdated methods that put people in a very limited size range, usually with bands that are too large and cups that are too small. The “sticker shock” of seeing a smaller band and larger cup letter is normal but often leads to a much better fit.
2. What’s the difference between UK and US sizing?
UK and US sizing are the same up to a D cup. After that, the UK system uses double letters (DD, E, F, FF, G) while the US system is less consistent (DD/E, DDD/F, G). This calculator uses the more standardized UK system.
3. What is a sister size?
A sister size maintains the same cup volume on a different band length. To go up a band size, you go down a cup size (e.g., 32F -> 34E). To go down a band size, you go up a cup size (e.g., 32F -> 30FF).
4. My leaning bust measurement is much larger. Is that normal?
Yes, a significant difference between your standing and leaning bust measurements often indicates projected breasts. This is important information for finding the right bra shape.
5. The calculator gave me a starting point. Now what?
The next step is to try on bras in and around your recommended size. Pay attention to our bra fit guide to assess whether the band is snug, the cups are filled, and the wires sit correctly.
6. Can I use this calculator if I was assigned male at birth (AMAB)?
Yes. The six-measurement method is effective for all bodies. It captures your specific frame and tissue volume to recommend a supportive size.
7. How often should I re-measure myself?
It’s a good idea to re-measure every 6-12 months, or after significant weight changes, pregnancy, or hormonal shifts, as these can all affect your bra size.
8. Does the calculator work for sports bras?
This calculator gives you your standard bra size. Sports bra sizing can vary (e.g., S/M/L), but knowing your true size is the best starting point for finding the right level of support. Check the brand’s specific size chart with your ABTF size in mind.