US Cup Size Calculator

Find Your Perfect Bra Fit

US Bra Cup Size Calculator

Accurately determine your US bra cup size by measuring your underbust and bust. This calculator helps estimate your size, but fit can vary by brand and bra style. For best results, always try on bras before purchasing.

Common US Bra Sizes & Measurements

Band Size (Inches)	Bust Measurement (Inches)	Estimated Cup Size	Approx. Cup Volume (cubic inches)
30	33	A	~23.3
30	34.5	B	~28.6
30	36	C	~34.0
30	37.5	D	~39.3
30	39	DD	~44.6
32	35	A	~26.5
32	36.5	B	~32.0
32	38	C	~37.6
32	39.5	D	~43.2
32	41	DD	~48.8
34	37	A	~29.8
34	38.5	B	~35.7
34	40	C	~41.6
34	41.5	D	~47.5
34	43	DD	~53.5
36	39	A	~33.2
36	40.5	B	~39.7
36	42	C	~46.1
36	43.5	D	~52.6
36	45	DD	~59.1
38	41	A	~36.7
38	42.5	B	~43.8
38	44	C	~50.9
38	45.5	D	~58.0
38	47	DD	~65.2

What is a US Cup Size?

Your US cup size is a crucial component of bra sizing, representing the volume of your breast relative to your underbust measurement (band size). It’s not an absolute measurement but a comparison. In the US system, cup sizes typically increment by letters (A, B, C, D, DD, etc.), with each letter representing approximately a 1-inch increase in the difference between your bust and underbust measurements, after the band size is determined. This system aims to provide a standardized way for women to find bras that offer both support and comfort. It’s important to note that cup size is only half of the equation; the band size is equally, if not more, critical for a proper fit. A well-fitting bra should feel snug around the ribcage (band) and lift the bust comfortably without gapping or spilling (cup).

Who should use it: Anyone seeking to understand their bra size in the US system, or those experiencing discomfort or poor fit with their current bras. This includes individuals who have recently experienced body changes, are looking for new bra styles, or are shopping for bras online where trying them on isn’t immediately possible.

Common misconceptions: A frequent misunderstanding is that a larger cup size letter (like DD compared to B) automatically means a larger bust overall. This isn’t true, as cup size is dependent on the band size. A 32DD bra has less overall breast volume than a 38B bra. Another misconception is that all bras of the same stated size will fit identically. Manufacturing variations, style differences (balconette, push-up, full coverage), and fabric types significantly impact how a bra feels and fits, even within the same size.

US Cup Size Formula and Mathematical Explanation

The calculation of your US bra cup size involves determining your band size first and then calculating the difference between your bust and underbust measurements. This difference dictates the cup volume.

Step-by-step derivation:

1. Measure Underbust: Wrap a soft measuring tape snugly around your ribcage, directly under your bust. Ensure the tape is level. Note this measurement in inches. This is your Underbust Measurement (U).
2. Measure Bust: Wrap the tape loosely around the fullest part of your bust, usually over the nipples. Keep the tape level and ensure it’s not constricting. Note this measurement in inches. This is your Bust Measurement (B).
3. Calculate Bust-Underbust Difference: Subtract your Underbust Measurement from your Bust Measurement. This gives you the Difference (D).
   
   D = B - U
4. Determine Cup Size: Compare the Difference (D) to the standard US cup size increments. Each approximately 1-inch difference corresponds to a cup size increase, starting from A.
   • If D ≈ 1 inch: A Cup
   • If D ≈ 2 inches: B Cup
   • If D ≈ 3 inches: C Cup
   • If D ≈ 4 inches: D Cup
   • If D ≈ 5 inches: DD Cup (or E Cup)
   • If D ≈ 6 inches: DDD Cup (or F Cup)
   • And so on…
5. Combine Band and Cup: Your bra size is a combination of your band size (derived from the underbust measurement) and your calculated cup size. For example, a 34C bra means a band size corresponding to a 34-inch underbust and a C cup.

Formula for Cup Volume (Approximation): While the letter denotes the difference, the actual volume can be approximated using the formula for the volume of a prolate spheroid (an ellipse rotated around its major axis), assuming the breast is roughly this shape.

Cup Volume (V) ≈ (4/3) * π * (Bust Circumference at Fullest Part / 2) * (Underbust Circumference at Fullest Part / 2) * (Difference in Inches / 2)

A simplified approximation used in many calculators, focusing on the *difference* (D) in inches to determine cup letter, is sufficient for most users. The calculator above uses this difference method.

Variables Table

Variable	Meaning	Unit	Typical Range
U	Underbust Measurement	Inches	28 – 50+
B	Bust Measurement	Inches	30 – 60+
D	Bust-Underbust Difference	Inches	0.5 – 15+
Cup Size Letter	Breast Volume relative to Band Size	Letter (A, B, C…)	A to K+
Cup Volume	Approximate volume of breast tissue in the cup	Cubic Inches	~10 – 100+

Practical Examples (Real-World Use Cases)

Example 1: Standard Fit Scenario

Scenario: Sarah is shopping for a new everyday bra and wants to ensure she’s wearing the correct size. She measures herself at home.

Inputs:

• Underbust Measurement (U): 34 inches
• Bust Measurement (B): 37 inches

Calculation:

• Difference (D) = B – U = 37 – 34 = 3 inches
• Based on the difference of 3 inches, the cup size is C.
• Assuming a standard band fit for a 34 underbust, Sarah’s estimated US bra size is 34C.

Calculator Output:

• Primary Result: 34C
• Intermediate Values:
• Underbust: 34 inches
• Bust: 37 inches
• Difference: 3 inches
• Band Size: 34
• Cup Size: C

Interpretation: A 34C bra is likely to provide a good starting point for Sarah. She should look for bras labeled ’34C’ and ensure the band is snug and the cups cover her breasts without gaping or spilling.

Example 2: Potential Size Upgrade Scenario

Scenario: Maria has been wearing a 36B for years but feels her bras are digging in at the top of the cup and spilling over slightly.

Inputs:

• Underbust Measurement (U): 36 inches
• Bust Measurement (B): 40.5 inches

Calculation:

• Difference (D) = B – U = 40.5 – 36 = 4.5 inches
• A difference of 4.5 inches typically falls between a D and DD cup. Many systems round this up or offer options like D/DD. For simplicity, let’s consider it leaning towards DD.
• Her band size is 36. Her estimated US bra size is 36DD.

Calculator Output:

• Primary Result: 36DD
• Intermediate Values:
• Underbust: 36 inches
• Bust: 40.5 inches
• Difference: 4.5 inches
• Band Size: 36
• Cup Size: DD

Interpretation: Maria’s measurements suggest she might need a larger cup size than her current 36B. The calculated 36DD indicates a larger cup volume is needed to accommodate her bust while maintaining the same band snugness. This change could resolve the discomfort she’s experiencing.

How to Use This US Cup Size Calculator

1. Gather Supplies: You’ll need a soft, flexible measuring tape.
2. Measure Underbust: Stand straight and exhale. Wrap the tape measure snugly around your rib cage, directly beneath your breasts. Ensure the tape is level all the way around and comfortable, not digging in. Record this number in inches (e.g., 32, 34, 36).
3. Measure Bust: Wrap the tape measure loosely around the fullest part of your bust. Usually, this is over your nipples. Ensure the tape is parallel to the floor and not compressing your breasts. Record this number in inches (e.g., 35, 38, 40).
4. Input Measurements: Enter your underbust measurement into the “Underbust Measurement (Inches)” field and your bust measurement into the “Bust Measurement (Inches)” field.
5. Calculate: Click the “Calculate Cup Size” button.

How to read results:

• The primary result (large, colored box) will display your estimated US bra size (e.g., 34C).
• The “Intermediate Results” section will show your raw measurements, the calculated difference, your determined band size, and the corresponding cup letter.
• The explanation below the results clarifies the simple formula used.
• The table provides context by showing common size combinations.
• The chart visually represents how bust measurement increases cup volume for a given band size.

Decision-making guidance: Use the calculated size as a starting point. Bra fit is highly personal. If the calculated size feels slightly off, consider trying a size up or down in the band or cup. For example, if you calculated 34C but the band feels too tight, try a 36B. If the cups gap, try a 34B. If they spill over, try a 34D or even a 36C.

Key Factors That Affect US Cup Size Results

While the calculator provides a good estimate based on standard measurements, several factors can influence your actual bra size and fit:

1. Band Fit: The band provides about 80-90% of a bra’s support. If the band is too loose, the cups won’t sit correctly, leading to inaccurate cup size assessment. Conversely, a band that’s too tight can make the cup size appear smaller. Always ensure the band fits snugly but comfortably, parallel to the floor.
2. Body Shape and Breast Shape: Breasts come in various shapes (e.g., round, conical, teardrop, East-West). Some bra styles are better suited for certain shapes. A calculator provides a generic cup volume; your specific breast shape might require a different style or even a sister size for optimal fit. Understanding different bra styles can be very helpful here.
3. Measurement Technique: Inaccurate measurements are the most common reason for ill-fitting bras. Measuring too loosely, too tightly, or not keeping the tape level can lead to incorrect results. Ensure consistency and comfort during measurement.
4. Changes in Body Weight or Hormones: Fluctuations in weight, pregnancy, breastfeeding, or hormonal changes (like during your menstrual cycle) can affect breast size and shape. Your bra size might change temporarily or permanently due to these factors. Regularly re-measuring is advised.
5. Bra Construction and Brand Variations: Different manufacturers use slightly different sizing standards and construction methods. A 34C in one brand might fit like a 34B or 34D in another. Even within the same brand, push-up bras, balconettes, and full-coverage bras can fit differently. Comparing bra brands can reveal these nuances.
6. Posture: Standing or sitting with poor posture can affect how your measurements are taken and how a bra ultimately fits. Good posture ensures your bust is presented in its natural position for accurate measurement.
7. Underwire Placement: The underwire should ideally encompass the entire breast tissue without digging into it or resting on the chest wall. If underwires are uncomfortable or don’t sit correctly, it might indicate a band or cup size issue, or simply the wrong style for your breast shape.
8. Inflation/Deflation: While not directly related to the calculator’s inputs, factors like heat or cold can slightly affect tissue volume, though this is usually negligible for standard bra sizing.

Frequently Asked Questions (FAQ)

What’s the difference between US and UK cup sizes?

UK bra sizing often uses different increments for cup sizes beyond D (e.g., DD, E, F, FF, G) compared to the US system (DD, DDD, G, H, etc.). Additionally, band sizing can sometimes differ slightly. It’s essential to know which system you are using when purchasing bras internationally.

Does the calculator account for sister sizing?

This calculator provides a primary estimated size based on your direct measurements. Sister sizing refers to maintaining the same cup volume but adjusting the band size (e.g., 34D and 36C are sister sizes). If your calculated band size feels incorrect, you might need to explore sister sizes. For example, if 34C feels too tight, try 36B; if it feels too loose, try 32D.

Why do I spill out of the cups?

Spilling out of the cups usually means the cup size is too small or the bra style is unsuitable for your breast shape. Try going up one cup size (e.g., from C to D) or trying a different style like a full-coverage or balconette bra. Ensure your underwire is sitting in your inframammary fold (the natural crease under your breast).

Why does my bra ride up in the back?

If the back band rides up towards your shoulders, it typically means the band is too large. The bra needs to fit snugly around your ribcage to provide proper lift and support. Try a smaller band size (e.g., from 36 to 34) and adjust the cup size accordingly (often up one size, like 36B to 34C).

What does ‘full coverage’ mean for a bra?

Full coverage bras are designed to cover the entire breast, minimizing spillage from the top and sides. They typically offer more support and are suitable for larger busts or for wear under non-stretchy or revealing clothing.

How often should I measure my bra size?

It’s recommended to measure your bra size at least every 6-12 months, or whenever you notice significant changes in your weight, breast shape, or if your current bras no longer fit comfortably. Hormonal changes, pregnancy, and breastfeeding can also necessitate frequent re-measuring.

Can I use clothing measurements for this calculator?

No, this calculator requires specific underbust and bust measurements taken with a soft measuring tape. Standard clothing measurements are not accurate for determining bra size.

Is this calculator accurate for all body types?

This calculator uses the standard US bra sizing method, which is a widely accepted starting point. However, individual body shapes and breast shapes vary significantly. While accurate for many, it’s always best to use the result as a guide and try bras on for the best fit. Consider consulting a professional fitter if you have persistent fit issues.