How Do Bra Size Work?

Bra sizes are based on your band size (the number) and your cup size (the letter). These measurements are usually determined by using a tape measure around your rib cage and bust. The number/letter is your bra size. Finding the right bra size is crucial for the right support and for a smooth comfortable fit. To all the girls and women who have worn an ill-fitting bra or wrong bra size know that this can ruin a great outfit and even worse the entire day with backaches and pain.

How 'the Number' and 'the Letter' work together on a bra size?

Understanding this is where the magic will happen for you when out bra shopping solo. Have you heard of sister sizing? That's what all of this is about... The band size will dictate how much cup volume your 'cup size' has.

How to measure bra size with measuring tape?

Lots of us are wearing the wrong size, which means we don't look or feel as good as we should! Follow below to make sure you've got both your band and cup sizes just right.

How to measure bra size without measuring tape?

An accurate way to measure your bra size is with a measuring tape, that doesn’t mean you can’t figure out your correct bra size without one! It’s actually pretty easy to measure your correct bra size without a measuring tape, as long as you have an existing bra for reference. You can use our Sizing Guide tool to help you learn your bra size without a tape measure so you can find the perfect bra fit straight away, or you can follow this guide to work your bra size out yourself. STEP 1: Enter the size of the bra you’re currently wearing Once you’ve opened any products, click ‘Sizing Guide’ and on the page choose your current band and cup size. STEP 2: How does the bra band feel? The band is the section that wraps around your torso under your boobs and around your back. Is your bra band too loose, too tight or just right ? STEP 3: How does the bra cup feel? If the cup gapes it is too loose, and if it creates over spill, it is too tight. The ideal cup will match the curve of the breast perfectly. STEP 4: Discover your perfect bra size without a measuring tape! If your band fits perfectly but the cup is too big or too small, you simply need to move up or down a size. For example, a 32B would move to a 32A if the cup is too big or a 32C if the cup is too small. If your band is too big or small but the cup is just right, you need to move up or down a band size and move your cup size to your sister size. A sister size is the equivalent cup size on a different band length. When you move up a band size, you move down a cup size, and if you move down a band size you move up a cup size (provided the same cup size is needed). For example, a 32B is equivalent in terms of cup size to a 34A or a 30C. Congrats, you’ve found the perfect bra size! Now you know your correct bra size, it’s time to go bra shopping! If you can't find your size or if you'd like a custom-fitted size, please explore our custom bra service.

How can I tell if my bra fits properly? Let’s check each part of the bra. When trying on your next bra, focus on the straps, the cups, the band, the underwire and the gore for the ultimate in comfort and support. Straps: The straps are tightened so that they fit snug and comfortably on your shoulders. They shouldn’t slip off or dig into your shoulders. The straps should also be positioned in the center of each of your shoulders — not too far on the edge where it can slip off and not too close to the neck where it can feel uncomfortable. Cups: The cups should lay flush against your breast tissue without causing them to flow out of or gape within the cups. Band: The band should feel snug and comfortable on the loosest set of hooks. This allows you to move to the inner hooks to tighten the bra as it stretches with regular wear. Gore: Also known as the bridge of the bra which connects the two cups, the gore should lay flush against your sternum. It shouldn’t hover over the chest or dig in. Underwire: The underwire should lay comfortably under the breast tissue, providing support and without digging into the sides of the breast tissue or into the underarm area. The underwire shouldn't lay on your breast tissue or below your rib cage.

2026 Bra Size Calculator & Chart | Measure Your Size Easily

Q: What does the number on a bra size mean?

The number or the 'band size' is a representation of your rib cage girth. We get this size by taking a measurement around your rib cage, about where the bra band sits. This measurement is taken in inches. As mentioned, this number gives us a guide to what size bra we need to accommodate the circumference of your rib cage, overlying skin and subcutaneous tissue.

Q: What does the letter on a bra size mean?

The letter component is an indication of cup volume. The amount of actual breast tissue/fat in volume.

PART 1 BRA SIZE CALCULATOR

Enter your measured Under Bust and Over Bust in inches. The calculator checks the approved HSIA matrix and returns a size only when that exact measurement combination appears in the chart.

Step 1: Measure your Under Bust. Take a snug measurement around your rib cage directly under your bust.

Select your Under Bust measurement

Step 2: Measure your Over Bust. Bend 45 degrees to measure the fullest part of your bust.

Select your Over Bust measurement

Recommended HSIA Size

—

Select both measurements to see your recommended HSIA size.

Half-inch handling: Under Bust is matched to the lower whole inch; Over Bust is matched to the next whole inch. Need help? Contact us.

PART 2 HOW TO MEASURE BRA SIZE?

1. How Do Bra Sizes Work?

Your recommended HSIA bra size combines a band number and a cup letter. Measure your Under Bust and Over Bust in inches, then use the calculator or the approved HSIA chart. Because the chart contains specific measurement ranges, do not estimate a size by subtracting one measurement from the other.

(1) What does the number on a bra size mean?

The number is the recommended HSIA band size returned by the chart. It is based on a snug Under Bust measurement taken directly beneath the bust.

(2) What does the letter on a bra size mean?

The letter is the cup result at the intersection of your Under Bust column and Over Bust row. The approved sequence is A, B, C, D, DD/E, DDD/F, G, H, I, J, K and L.

2. Step-by-Step Guide to Measure Your Bra Size

Step 1 — Under Bust: Take a snug measurement around your rib cage directly under your bust.

Step 2 — Over Bust: Bend forward about 45° and measure around the fullest part of your bust.

Use inches. The calculator accepts half-inch measurements: Under Bust values are matched to the lower whole-inch chart value, while Over Bust values are matched to the next whole-inch chart row.

3. How Should a Bra Fit?

Once you have your recommended HSIA size, check the fit on your body. Product construction and availability can vary, so the chart result is a starting point rather than a substitute for trying on the bra.

Straps: They should stay comfortably on the shoulders without slipping or digging in.
Cups: Breast tissue should sit fully inside the cups without spilling or gaping.
Band: It should feel snug and level on the loosest hooks.
Gore: The center bridge should sit flat against the sternum.
Underwire: It should surround the breast tissue without resting on it or digging into the underarm.

PART 3 HSIA BRA SIZE CHART

Use the approved standard HSIA chart below for your band-and-cup result. Wireless styles use the separate S–5XL conversion table, while Sister Sizes retain the existing equivalent-cup-volume relationships.

1. Standard Bra Size Chart

HSIA bra size chart for Under Bust 24 to 51 inches and Over Bust 28 to 62 inches

Measurement steps and HSIA bra size chart page 1, covering Under Bust 24 to 33 inches

HSIA bra size chart page 2, covering Under Bust 34 to 51 inches

Supported results cover even band sizes 28–48 and cup sizes A, B, C, D, DD/E, DDD/F, G, H, I, J, K and L. Availability may vary by product.

View the accessible text version of the size chart

Bra size chart for under bust measurements 24 to 37 inches.
	Under Bust
Over Bust	24-25"	26-27"	28-29"	30-31"	32-33"	34-37"
28"	28A
29"	28B
30"	28C	30A
31"	28D	30B
32"	28DD/E	30C	32A
33"	28DDD/F	30D	32B
34"	28G	30DD/E	32C	34A
35"	28H	30DDD/F	32D	34B
36"	28I	30G	32DD/E	34C
37"	28J	30H	32DDD/F	34D	36A
38"	28K	30I	32G	34DD/E	36B
39"	28L	30J	32H	34DDD/F	36C	38A
40"		30K	32I	34G	36D	38B
41"		30L	32J	34H	36DD/E	38C
42"			32K	34I	36DDD/F	38D
43"			32L	34J	36G	38DD/E
44"				34K	36H	38DDD/F
45"				34L	36I	38G
46"					36J	38H
47"					36K	38I
48"					36L	38J
49"						38K
50"						38L

Bra size chart for under bust measurements 38 to 51 inches.
	Under Bust
Over Bust	38-40"	41-43"	44-45"	46-48"	49-51"
41"	40A
42"	40B
43"	40C	42A
44"	40D	42B
45"	40DD/E	42C
46"	40DDD/F	42D
47"	40G	42DD/E	44A
48"	40H	42DDD/F	44B
49"	40I	42G	44C	46A
50"	40J	42H	44D	46B
51"	40K	42I	44DD/E	46C	48A
52"	40L	42J	44DDD/F	46D	48B
53"		42K	44G	46DD/E	48C
54"		42L	44H	46DDD/F	48D
55"			44I	46G	48DD/E
56"			44J	46H	48DDD/F
57"			44K	46I	48G
58"			44L	46J	48H
59"				46K	48I
60"				46L	48J
61"					48K
62"					48L

Measurements outside a colored chart cell do not have an approved result. The calculator will show “Not available” instead of estimating a nearby size.

2. Wireless Bra Size Chart

Use your Under Bust range and regular HSIA band-and-cup result to find the corresponding Wireless size.

Wireless bra sizes by Under Bust range, band size and cup size.
Under Bust	Band Size	A	B	C	D	DD/E
28-29"	32	S	S	S	M	M
30-31"	34	S	S	M	M	L
32-33"	36	M	M	L	L	XL
34-37"	38	L	XL	XL	2XL	2XL
38-40"	40	XL	2XL	2XL	3XL	3XL
41-43"	42	2XL	3XL	3XL	4XL	4XL
44-45"	44	3XL	4XL	4XL	5XL	5XL
46-48"	46	4XL	5XL	5XL
49-51"	48	5XL

Wireless conversions cover Under Bust 28–51 inches, band sizes 32–48 and cups A–DD/E. A blank cell means the approved source contains no Wireless size for that combination.

3. Bra Sister Sizes

(1) What is Bra Sister Size?

Sister sizes are groups of bra-size equivalents that are related by cup volume. For example, a 36F (DDD) cup belongs to the same family as a 38E (DD), a 40D, and a 42C. Stick with us — we’ll explain. But just know that your perfect fit may still be out there waiting for you. And we’re going to help you discover yours.

(2) Do sister sizes really work?

Do you come across the situation that the bra you would like to buy runs out of your size?
Do you feel that your current bra does not fit (band and/or cups) and need a better fit?
If so, you need to know bra sister sizes, which can be a tremendous help for your bra fitting.

(3) How to use bra sister sizes?

Find your current bra size, and your sister sizes are the ones listed in the same row (going across). So, for example, if you’re a 34C, your sister sizes are 30E, 32D, 36B, and 38A. Each row is a family of sister sizes.

(4) Bra sister sizes chart

Below is a sister size chart from which you can find bra sizes that are related to your actual size. Start with the size for which you have been measured. Moving along the same row, you will find the sister sizes that have the same cup volume.

Bra sizes with equivalent cup volume, arranged by band size.
30	32	34	36	38	40	42	44	46
30B	32A	34AA
30C	32B	34A
30D	32C	34B	36A
30E	32D	34C	36B	38A
30F	32E	34D	36C	38B	40A
30G	32F	34E	36D	38C	40B	42A
	32G	34F	36E	38D	40C	42B	44A
		34G	36F	38E	40D	42C	44B	46A
			36G	38F	40E	42D	44C	46B
				38G	40F	42E	44D	46C
					40G	42F	44E	46D
						42G	44F	46E
							44G	46F
								46G

Now you know the sister sizes, and hopefully this information helps alleviate some frustration when bra shopping. Sister sizing may not work for every body or every style, so the best option is still to get fitted and try on the bras that interest you.

PART 4 FAQs ABOUT BRA SIZE MEASUREMENT

1. What Measurement Range Does the Calculator Support?

The approved chart accepts Under Bust measurements from 24 to 51 inches and valid Over Bust measurements from 28 to 62 inches. Depending on the measurement combination, it can return even band sizes 28–48 and cup sizes A–L. Not every Over Bust value is valid for every Under Bust range.

2. What If My Result Says “Not Available”?

That measurement combination falls outside the approved colored cells in the HSIA chart. Check that the tape is positioned correctly and measure again. If the result is still unavailable, please contact our fit experts rather than choosing a nearby size automatically.

3. How Often Should I Remeasure My Bra Size?

Remeasure every 6–12 months, or whenever your weight, hormones, or fit change. Breasts and ribcage fluctuate with age, pregnancy, and fitness, so a size that fit last year may not fit today.

SAVE UP TO $40

HOW TO MEASURE BRA SIZE