Free Bra Size Calculator

Getting Started with the Bra Size Finder

This free online bra size calculator acts as a comprehensive bra size finder that supports the three most common international sizing systems: US, UK, and EU. Simply enter your bust and underbust measurements—either in inches or centimeters—and the tool instantly calculates your band size, cup letter, and a full list of sister sizes that share the same cup volume. Whether you are shopping at a local boutique or ordering from an overseas brand, the tool converts your size across all three charts, eliminating guesswork.

Taking Accurate Measurements

Reliable measurements are the foundation of a well‑fitting bra. Follow these guidelines to get the most consistent numbers:

• Bust measurement: While braless or wearing a non‑padded bra, stand straight and wrap a soft measuring tape around the fullest part of your bust. The tape should be snug but not tight—you should be able to breathe in and out without strain. Record the number; if you round, always round up.
• Frame (underbust) measurement: Place the tape directly under your bust, circling the rib cage. Keep the tape level and pull it tight, especially when you exhale. Round this value to the nearest whole unit.

For extra accuracy, measure twice and average the results. Avoid measuring during the days leading up to your period, as temporary swelling can affect the numbers.

Converting Measurements to Cup Size

Once you have both numbers, subtract your frame measurement from your bust measurement. The resulting difference (in inches or centimeters) tells you which cup letter corresponds to your size in each system. Use the international chart below:

Difference (in [cm])	US Cup	Australian Cup	UK Cup
< 1 [< 2.5]	AA	AA	AA
1 [2.5]	A	A	A
2 [5.1]	B	B	B
3 [7.6]	C	C	C
4 [10.2]	D	D	D
5 [12.7]	DD / E	DD	DD
6 [15.2]	DDD / F	E	E
7 [17.8]	DDDD / G	F	F
8 [20.3]	H	G	FF
9 [22.9]	I	H	G
10 [25.4]	J	I	GG
11 [27.9]	K	J	H
12 [30.5]	L	K	HH
13 [33.0]	M	L	J
14 [35.6]	N	M	JJ
15 [38.1]	O	N	K
16 [40.6]	P	O	KK
17 [43.2]	Q	P	L
18 [45.7]	R	LL	M

Note that US and UK cups follow different progressions after D. US brands often use DD, DDD, G, etc., while UK systems advance to E, F, FF, and G. The chart above gives you the exact mapping.

Understanding Sister Sizes

Sister sizes are different bra size labels that actually hold the same breast volume. Manufacturers keep cup volume constant by adjusting the letter as the band changes: when the band gets larger, the cup letter gets smaller, and vice versa. For example, 34C, 36B, and 32D all share the same volume.

Why does this matter? Brands vary in how they cut a band; if a 34C feels too tight in the band, you can try a 36B (sister size up) while keeping the same cup volume. The calculator automatically generates a complete sister‑size table for your measured size, making it easy to experiment with alternatives.

Ensuring a Proper Fit

A correct measurement is only the starting point. When trying on bras, follow these steps:

1. Bend forward and arrange your breasts into the cups.
2. Hook the band on the loosest set of hooks. A new bra should be snug on this setting—stretched elastic will eventually need the inner hooks.
3. Check the band level. It must be horizontal all around; if it rides up at the back, the band is too loose.
4. Slide one finger under the band. If you can fit two fingers, choose a smaller band or use a sister size with a larger cup.
5. Adjust the straps so they sit comfortably without slipping or digging.
6. Look at the cups – no overflow, no gaping. The center gore (the fabric between cups) should lie flat against your sternum.

Signs of an Incorrect Fit

If you notice any of the following, your bra probably does not fit correctly:

• Red marks from straps or underwire – straps are too tight or the band is not providing enough lift; an underwire that pokes means the cup is too small.
• Spillage at the top or sides – the cup is too small; try one letter up or a sister size.
• Gaping cup fabric – the cup is too large or the shape is wrong for your breast.
• Band rides up – the band is too long; go down a band size and adjust the cup letter accordingly (calculator can help).
• Neck or back pain – the bra is not supporting properly; re‑measure and consider a professional fitting.

Bra Types for Different Needs

Knowing your correct size is helpful for any bra style you choose:

• Everyday comfort – Non‑padded, wireless bras offer breathability.
• Sports & activity – Sports bras minimize movement; choose encapsulation or compression styles.
• Maternity & nursing – These bras have stretchy fabrics and clips for nursing access.
• Special occasions – Push‑up, cage, and stick‑on bras alter your shape for different outfits.

With the measurement method, the cup chart, and an understanding of sister sizes, you are ready to use this bra size measurement tool with confidence. It converts between US, UK, and EU sizing and helps you find your perfect fit regardless of where you shop.