Bra Size Chart: The Complete Fitting Guide (With Calculator)

Why the +4 rule has been giving women the wrong size for 80 years

Nearly every mainstream bra fitting guide, from department stores to lingerie brands, still teaches the "+4 rule": measure your underbust in inches, then add 4 (or 5) to get your band size. A 30-inch underbust becomes a 34 band. A 32-inch underbust becomes a 36 band.

This rule was created in the 1940s. At that time, bras had no elastic in the band. The rigid cotton and satin construction needed extra room to close around the body. Adding 4–5 inches was the only way the bra could physically fit.

Modern bras use stretch fabric and elastic. They are designed to fit snugly at the actual underbust measurement. When you add 4 inches using the old formula, you end up in a band that is 2–4 sizes too large. The band rises up your back, the cups gap and pull away from your body, and the straps have to do all the support work. Shoulder strap grooves are the most visible result.

A correctly fitting bra provides 80% of its support from the band. The straps carry the remaining 20%. If your straps are digging in, your band is almost certainly too big.

Research from the University of Portsmouth's breast biomechanics unit, across 13 years of 3D scanning and clinical measurement, found that the majority of women are wearing bras that are too large in the band and too small in the cup. The band being too large is almost always the root cause of the other fit problems.

How to measure yourself correctly

You need a soft measuring tape. Measure in inches for US/UK sizing or centimeters for EU sizing. Measure over a thin, unpadded bra or no bra at all. A padded bra adds 1–2 inches to your bust measurement and throws off the cup calculation. If you also need your dress, jean, or top sizes, our women's clothing size chart covers every category across US, UK, EU, and AU with the same measurement-first approach.

Step 1: Measure your band size (underbust)

Wrap the tape firmly around your ribcage, directly under your bust. Keep it level all the way around. Breathe out, then measure. The tape should be snug but not cutting in.

• If your measurement is an even number, that is your band size.
• If your measurement is an odd number, round up to the next even number. A 31-inch underbust becomes a 32 band.
• Do not add 4 inches.

Step 2: Measure your bust (overbust)

Lean forward at a 45-degree angle and let your breasts hang naturally. Wrap the tape loosely around the fullest part of your bust. The tape should skim the surface without pressing in. Stand back up and note the measurement.

Measuring while leaning forward captures the full projected volume of your breast tissue. Measuring upright misses tissue that falls toward the sides, which leads to a cup size that is too small.

Step 3: Calculate your cup size

Subtract your band size from your bust measurement. Each inch difference equals one cup size:

Difference (inches)	US Cup Size	UK Cup Size	EU Cup Size
Less than 1"	AA	AA	AA
1"	A	A	A
2"	B	B	B
3"	C	C	C
4"	D	D	D
5"	DD	DD	E
6"	DDD / F	E	F
7"	G	F	G
8"	H	FF	H
9"	I	G	I
10"	J	GG	J
11"	K	H	K

Bra size calculator

US bra size chart

The chart below shows US bra sizes from AA to K cups, across band sizes 28 through 46. Sizes highlighted in the most common range (32–38 band, A–DDD cup) are stocked by most major retailers. Sizes outside that range are considered extended sizing and require specialty brands.

Band	AA	A	B	C	D	DD	DDD/F	G	H
28	28AA	28A	28B	28C	28D	28DD	28DDD	28G	28H
30	30AA	30A	30B	30C	30D	30DD	30DDD	30G	30H
32	32AA	32A	32B	32C	32D	32DD	32DDD	32G	32H
34	34AA	34A	34B	34C	34D	34DD	34DDD	34G	34H
36	36AA	36A	36B	36C	36D	36DD	36DDD	36G	36H
38	38AA	38A	38B	38C	38D	38DD	38DDD	38G	38H
40	40AA	40A	40B	40C	40D	40DD	40DDD	40G	40H
42	42AA	42A	42B	42C	42D	42DD	42DDD	42G	42H
44	44AA	44A	44B	44C	44D	44DD	44DDD	44G	44H
46	46AA	46A	46B	46C	46D	46DD	46DDD	46G	46H

International bra size conversions

The US and UK band numbers are identical. The cup letters match through D, then diverge. EU sizing uses centimeter-based band numbers and different cup letters above D. French sizing follows EU cups but adds 15 to the band number.

Band size conversion chart

US / UK (inches)	EU (cm)	FR / ES (cm)	AU / NZ	JP (cm)	IT (cm)
28	60	75	8	60	0
30	65	80	10	65	1
32	70	85	12	70	2
34	75	90	14	75	3
36	80	95	16	80	4
38	85	100	18	85	5
40	90	105	20	90	6
42	95	110	22	95	7
44	100	115	24	100	8

Cup size conversion chart

This is where most international guides get it wrong. EU cup letters match US up to D, then the EU system inserts an extra E where the US goes straight to DD. Always check this chart when buying from European or French brands. Assuming the letter is the same will put you in the wrong cup.

US	UK	EU / IT	FR / ES	AU / NZ	JP
AA	AA	AA	AA	AA	AA
A	A	A	A	A	A
B	B	B	B	B	B
C	C	C	C	C	C
D	D	D	D	D	D
DD	DD	E	E	DD	E
DDD / F	E	F	F	E	F
G	F	G	G	F	G
H	FF	H	H	FF	H
I	G	I	I	G	I
J	GG	J	J	GG	J
K	H	K	K	H	K

Practical note on brand sizing

European lingerie brands (Simone Perele, Chantelle, Prima Donna) follow EU sizing. When they say "DD," they mean what Americans call DD. But when a German catalog lists an E cup, that is a US DD. French brands like Aubade use FR sizing, so their 90C is a US 34C. Always confirm by checking the brand's own size chart before purchasing.

Sister sizing: the concept most bra guides skip

Cup letters are not absolute measurements. A D cup on a 30 band holds less volume than a D cup on a 38 band. The letter only describes the difference between your underbust and your bust, not the total size of your breast.

Sister sizes are bra sizes that share the same cup volume, just on a different band. Moving one band size up means the cup has to go down one letter to keep the same volume. Moving one band size down means the cup goes up one letter.

Example: These three bras hold identical cup volume:

• 32D (smaller band, larger relative cup)
• 34C (standard fit)
• 36B (larger band, smaller relative cup)

Sister sizing is useful when a bra fits perfectly in the cup but the band is slightly off. If a 34C fits in the cup but the band is too tight, try a 36B. If the band is fine but the cups overflow, try a 34D instead of going up a full size in both dimensions.

Sister size chart

If you wear...	Sister size down (smaller band)	Sister size up (larger band)
32A	30B	34AA
32B	30C	34A
32C	30D	34B
32D	30DD	34C
32DD	30DDD	34D
34A	32B	36AA
34B	32C	36A
34C	32D	36B
34D	32DD	36C
34DD	32DDD	36D
34DDD	32G	36DD
36A	34B	38AA
36B	34C	38A
36C	34D	38B
36D	34DD	38C
36DD	34DDD	38D
38B	36C	40A
38C	36D	40B
38D	36DD	40C
38DD	36DDD	40D

Sister sizing only works within a range of about two band sizes. A 28G and a 38B technically have the same volume difference number, but the actual breast shape and projection they fit well are completely different. Sister sizing is a small adjustment tool, not a whole-size substitution.

The 7-point fit test

Put on your bra and check each point below. A bra that passes all seven fits correctly. If it fails more than two, you need a different size.

1. The center gore

The center gore is the panel between the cups. It should lie flat against your sternum with no gap. If it lifts away from your body, your cups are too small. Go up a cup size, or try a sister size with a smaller band.

2. The band position

The band should sit level all the way around, parallel to the floor. If it rides up at the back, the band is too large. A correctly fitting band stays within one inch of level even when you raise your arms.

3. The band tension

Slide two fingers under the back band. You should feel resistance, but your fingers should be able to move. If three or more fingers slide under easily, the band is too loose. If you cannot fit one finger, it is too tight. A new bra should fasten on the loosest hook so you can tighten it as the fabric stretches.

4. The underwire placement

The underwire should follow your natural breast crease exactly, sitting on your ribcage and chest wall, not on breast tissue. If the wire sits on the side of your breast, the cup is too small. If it digs into the front of your armpit, the cup is too narrow (you need a wider wire shape, not a bigger size).

5. The cup coverage

Your breast tissue should fill the cup completely with no gaps, bubbles, or spillage. Gaping cups mean the cup is too large, or the style is wrong for your breast shape. Spillage over the top or sides means the cup is too small. Spillage specifically at the outer edge of the underwire often means you need a wider wire, not a bigger cup.

6. The straps

Straps should stay in place when you roll your shoulders forward and back. They should not dig in or leave marks after a full day of wear. If you must tighten straps all the way to get support, your band is too big. The straps are compensating for a band that is not doing its job. Slide two fingers under the strap: slight resistance is correct.

7. The swoop and scoop test

This one is almost never mentioned in mainstream guides. Lean forward, reach into the side of your bra, and scoop all breast tissue forward and into the cup. Stand upright. If the cup now overflows, you had been leaving breast tissue in your armpit area uncaptured, which means your cup was too small to begin with. Many women discover they are 1–2 cup sizes larger once they capture all their tissue.

Common fit mistakes (and what's actually causing them)

Symptom	Common (Wrong) Fix	Actual Cause	Real Fix
Straps digging in	Loosen the straps	Band is too large, straps compensating	Go down a band size
Straps falling off	Tighten the straps	Straps too far apart for narrow shoulders, or band too large	Try a racerback style or go down a band
Center gore floating	Buy a different brand	Cup too small	Go up a cup size
Spillage at armpit	Go up a cup	Cup width too narrow for breast root	Try a wider-wire style in the same size
Wrinkling cups	Go down a cup	Cup too large, or wrong shape for breast type	Go down a cup or try a demi/balconette shape
Band riding up	Tighten the straps	Band is too large	Go down 2 band sizes and up 1 cup (sister size)
Underwire poking chest	Bend the wire	Cup too small, pushing wire forward	Go up a cup size
Quadra-boob (double bubble)	Buy a minimizer	Cup too small	Go up 1–2 cup sizes

Bra sizing by breast shape

Two women with identical measurements can wear the same bra size but need completely different bra styles. Breast shape determines which bra style fits, not just the size number. The same principle holds across your whole wardrobe. Our guide to dressing to flatter your figure covers how cut, structure, and fabric placement work together for every body type.

Full on bottom

More tissue below the nipple line than above. Balconette bras and plunge bras work poorly because the cup shape is cut for balanced projection. Full-coverage and T-shirt bras with a rounded cup base give the best fit. Avoid bras with rigid horizontal seams across the lower cup, which dig in.

Full on top

More tissue above the nipple line. Plunge and balconette styles leave the upper cup empty and create a gap at the top. Full-coverage bras with a higher cup front and a gentle gather work better. Molded cups in T-shirt bras often fit this shape poorly due to their preset projection.

Wide-Set Breasts

Breast tissue starts further out on the chest, with natural space between the breasts. Center gores will often float even in the correct size because the underwire spacing does not match the breast placement. Low-center-gore plunge bras and soft-cup bralettes often fit better than structured underwire styles.

Close-Set Breasts

Breast tissue starts near the center of the chest. Plunge bras and styles with a narrow center gore fit best. Wide-gore bras sit on breast tissue rather than bone, which causes discomfort and the floating gore problem.

Projected (Narrow and Deep) Breasts

Less surface area, more forward projection. Molded foam cups often leave gaps at the sides because they are shaped for a wider, shallower breast. Seamed cups with vertical stitching (which follow the shape rather than imposing one) and balconette styles tend to fit this shape better.

Shallow (Wide and Flat) Breasts

More surface area, less forward projection. Balconette bras and demi cups with a lower cut front work well. Deep-projection molded cups sit away from the breast, creating the appearance of empty cups even in the right size.

Pendulous Breasts

Tissue that hangs lower with elongated shape. Full-coverage styles with a tall cup height and strong bottom-band support are the priority. Underwire bras with a firm lower cup base work better than soft-cup styles, which allow further descent under the weight of the tissue.

Sports Bra Sizing

Sports bras do not follow standard bra sizing rules. Most compression-style sports bras use S/M/L/XL and are built around band sizing, not cup sizing — which is why they fit poorly for anyone outside a B–C cup range.

Encapsulation vs. Compression

Compression sports bras press both breasts against the chest as a single unit. They work for A–C cups. Above a C cup, compression creates "uniboob" and provides inadequate support during high-impact movement.

Encapsulation sports bras have individual cups that hold each breast separately, similar to a regular underwire bra. These are necessary for D cups and above in any high-impact activity. Studies on breast movement during running show that unsupported breasts can move up to 8 cm vertically — encapsulation bras reduce this to under 2 cm.

Sports Bra Size Conversion

Regular Bra Size	Sports Bra (XS–XL Scale)	Notes
28–30 A/B	XS	Check brand chart — may run large
32 A/B/C	S	Standard fit
34 A/B/C	S–M	Try both
34 D/DD	M	Use encapsulation style
36 A/B/C	M	Standard fit
36 D/DD	M–L	Use encapsulation style
38 A/B/C	L	Standard fit
38 D/DD/DDD	L–XL	Use high-impact encapsulation
40+ any cup	XL–XXL	Use underwire sports bra with full cup

The jump test: put on a sports bra and jump in place for 30 seconds. If you feel significant bounce or the bra shifts, it is not providing adequate support regardless of how it fits standing still. For building a full warm-weather wardrobe around breathable fabrics and active lifestyles, our summer outfits guide has 27 looks that balance coverage, comfort, and style.

When Your Bra Size Changes

Bra size is not fixed. Most women's size changes multiple times across their lifetime, and the changes are often different in band and cup.

Pregnancy and Nursing

Breast size typically increases 1–2 cup sizes during the first trimester and may continue increasing. Band size often increases as the ribcage expands. During nursing, size fluctuates daily with milk production — many women find soft-cup nursing bras with stretch fabric more practical than fitted underwire during this period. Post-weaning, breast tissue composition changes: the same measurement may produce a different shape, which means the previously-fitting bra style may no longer fit correctly even if the numbers are the same.

Weight Changes

A 10-pound weight change typically shifts band size by one increment (e.g., 34 to 36). Cup size can go either direction depending on where fat distributes on your specific body. There is no formula; remeasure when you notice fit changes. After a size shift, it helps to rebuild your wardrobe with pieces that work for your current shape. Our plus size spring outfit ideas are a good starting point for curve-flattering styles across a range of sizes.

Hormonal Changes

Breast size fluctuates through the menstrual cycle — many women are 1 cup size larger during the week before their period. If your bra fits well most of the month but feels tight one week, this is the cause. Perimenopause and menopause also change breast tissue density, which can shift both size and shape even without significant weight change.

Bra Lifespan

A bra's elastic degrades. After 6–9 months of regular wear (3–4 times per week with washing), the band stretches and loses tension. A bra that once fit on the loosest hook now fits on the tightest — and then no longer fits at all. This is not a size change. It is the end of the bra's useful life. Replace rather than compensate.

Rotate between at least 3 bras to extend each one's lifespan. Washing on a gentle cycle in a lingerie bag, and air-drying (never the dryer), preserves elastic significantly longer than machine-drying.