Thirdlove Bra Calculator

An easy-to-use tool to find your accurate ThirdLove bra size, including our signature half-cup sizes. Get the comfort and support you deserve.

Your Recommended Bra Size

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Band Size: —

Cup Size: —

Measurement Difference: —

This is a starting point. Your perfect fit may also depend on breast shape and bra style. Consider trying one of our sister sizes if needed.

What is a ThirdLove Bra Calculator?

A ThirdLove Bra Calculator is a specialized tool designed to help you find your most accurate bra size based on the unique sizing system used by ThirdLove. Unlike many standard calculators, our method accounts for not just band and bust measurements but also incorporates our signature half-cup sizes, ensuring a more precise and comfortable fit. If you’ve ever struggled with cups that gap or overflow, our bra size calculator can provide the clarity you need. This tool is for anyone seeking to end the guesswork and find a bra that feels like it was made for them.

Many people are surprised to learn their true size. A common misunderstanding is that bra sizing is universal across all brands, but variations are common. The ThirdLove calculator helps establish your baseline size specifically for our products, giving you the best starting point for a perfect fit.

ThirdLove Bra Calculator Formula and Explanation

The calculation for your bra size involves two key components: your band size and your cup size. Here’s a simple breakdown of how we determine it:

1. Band Size Calculation: We take your underbust measurement. If the number is odd, we add 1 to make it even. If it’s even, that is your band size. This ensures your band is snug and supportive.
2. Cup Size Calculation: We subtract your final band size from your bust measurement. The difference in inches determines your cup letter.

The core formula is: Cup Difference = Bust Measurement – Band Size

Variables Explained

The variables used to determine your bra size. All measurements should be in inches for this table.

Variable	Meaning	Unit	Typical Range
Underbust Measurement	The measurement around your rib cage directly beneath your breasts.	inches / cm	28 – 48 in
Bust Measurement	The measurement around the fullest part of your bust.	inches / cm	30 – 55 in
Band Size	The numbered part of your bra size, rounded to an even number.	(Unitless Number)	30 – 48
Cup Difference	The difference between your bust and band size.	inches	0 – 9 in

Cup Size Chart

This chart maps the difference between your bust and band size to the corresponding ThirdLove cup size.

Difference (inches)	Cup Size	Difference (inches)	Cup Size
< 0.5″	AA	4.5″	D½ (E)
1″	A	5″	E (DD)
1.5″	A½	5.5″	E½ (F)
2″	B	6″	F
2.5″	B½	6.5″	F½ (G)
3″	C	7″	G
3.5″	C½	7.5″	G½ (H)
4″	D	8″	H

Practical Examples

Example 1: Finding a Common Size

• Inputs: Underbust = 31 inches, Bust = 36 inches
• Band Calculation: 31 is odd, so add 1. Band Size = 32.
• Cup Calculation: 36 (Bust) – 32 (Band) = 4 inches difference.
• Result: A 4-inch difference corresponds to a D cup. The final size is 32D.

Example 2: Using a Half-Cup Size

• Inputs: Underbust = 34.5 inches (entered as cm and converted), Bust = 39.5 inches
• Band Calculation: Round 34.5 to 35, which is odd. Add 1. Band Size = 36.
• Cup Calculation: 39.5 (Bust) – 36 (Band) = 3.5 inches difference.
• Result: A 3.5-inch difference corresponds to a C½ cup. The final size is 36C½. This demonstrates the importance of the correct bra fit.

How to Use This ThirdLove Bra Calculator

1. Select Your Unit: Start by choosing whether you will enter your measurements in inches or centimeters.
2. Measure Your Underbust: Wrap a soft measuring tape snugly around your ribcage, just below your bust. Make sure the tape is level. Enter this number into the “Underbust Measurement” field.
3. Measure Your Bust: Wrap the tape around the fullest part of your bust, making sure it’s not too tight or too loose. Enter this value into the “Bust Measurement” field.
4. Read Your Results: The calculator will instantly display your recommended band size, cup size, and full bra size. The chart will also update to show a visual of your proportions.
5. Interpret the Results: Use the calculated size as your primary starting point when shopping. Remember to also check our guidance on how to measure bra size for a perfect fit.

Key Factors That Affect Bra Fit

• Breast Shape: Are your breasts round, teardrop, slender, or side-set? Shape affects how you fill a cup. A tool like the find my bra size quiz can help identify styles for your shape.
• Band Snugness: The band provides about 80% of the support. It should be snug and level, with only enough room to slide two fingers underneath.
• Strap Tension: Straps should not dig in or carry the weight of your bust. They are there for shaping and security, not heavy lifting.
• Gore Tacking: The center panel of the bra (the gore) should lie flat against your sternum without gaps.
• Cup Spillage: If you have “quad-boob” or spillage at the sides, you likely need a larger cup size or a different style.
• Sister Sizing: If a bra almost fits, you might need a “sister size.” For example, if a 34C is too loose in the band, try its sister size 32D, which has a smaller band but the same cup volume.

Frequently Asked Questions (FAQ)

1. What if my measurements are in centimeters?

Our calculator has a unit switcher. Simply select “cm” and enter your measurements. The calculator will automatically convert them to inches for the standard sizing formula.

2. I got a half-cup size. Is that real?

Yes! ThirdLove pioneered half-cup sizes to help you find a more precise fit. A B½ cup, for example, is halfway between a B and a C cup.

3. What is a sister size?

Sister sizes are bra sizes with the same cup volume but different band lengths. For instance, a 32D, 34C, and 36B are sister sizes. Knowing this can help you find a better fit if your band is too tight or loose. Check our bra sister sizes guide for more info.

4. My calculated size feels wrong. What should I do?

A calculator is an excellent starting point, but fit can be affected by breast shape and bra style. Use your result as a guide and be prepared to try a sister size up or down. Our Fit Experts are also available to help!

5. How often should I measure myself?

We recommend measuring yourself every 6-12 months, or after significant body changes like weight fluctuation or pregnancy.

6. Why does the band size formula add a number?

The traditional method of adding to the underbust measurement helps translate a raw body measurement into a standardized, even-numbered band size that corresponds with manufactured bra bands.

7. Can I use this calculator for other brands?

While it will give you a good estimate, this calculator is optimized for ThirdLove’s specific sizing, including half-cups. Other brands may fit differently.

8. What’s more important, the band or the cup?

Both are crucial, but the band provides the vast majority of support. An ill-fitting band is the most common reason for bra discomfort. A correct bra fit starts with the band.