Bra Sister Sizes: Complete Chart and Fit Guide

  Bra sister sizes are sizes with approximately the same cup capacity but different band lengths. To sister-size down, choose one smaller band and one larger cup—for example, 34D to 32DD/E. To sister-size up, choose one larger band and one smaller cup—for example, 34D to 36C.

Sister sizing is most useful when the cups fit but the band does not. It is usually best to move only one band step and compare the same bra style, because wire width, cup placement, straps and support change as the band changes.

What Are Bra Sister Sizes?

Sister sizes belong to the same approximate cup-volume family. A 32DD/E, 34D and 36C can hold a similar amount of breast tissue, but they are not identical bras.

• 32DD/E has a shorter band and cups proportioned for a smaller frame.

• 34D has the middle band and frame proportions.

• 36C has a longer band, with cup and strap placement scaled for a broader frame.

Cup letters are relative to the band. A 32D cup has less capacity than a 38D cup even though both use the letter D. That is why a cup letter should never be interpreted without its band number.

Sister sizes preserve approximate capacity—not exact shape. Brands use different patterns, materials, wires and grading systems, so the same labeled size can fit differently across styles.

Bra Sister Sizing at a Glance

What you notice	What to try	Example starting from 34D
Band rides up or shifts; cups fit	Down one band and up one cup	32DD/E
Band is genuinely too tight; cups fit	Up one band and down one cup	36C
Band fits; cups spill	Keep the band and increase the cup	34DD/E
Band fits; cups gape	Test a smaller cup or different cup shape	34C or another style
Band and cups are both wrong	Reassess both measurements	Do not use a simple sister-size swap
Wires are too wide, narrow, tall or low	Change cup shape or brand	Compare plunge, demi, balconette or full-cup bras

The rule: smaller band = larger cup letter; larger band = smaller cup letter.

Bra Sister-Size Chart

This chart follows the U.S.-style labels commonly displayed in Lavinia Lingerie size selectors: A, B, C, D, DD/E, DDD/F, G and H. Some imported brands use U.K. or European notation, so always follow the size system shown on the specific product page.

Your usual size	Sister size down: firmer band	Sister size up: looser band
32B	30C	34A
32C	30D	34B
32D	30DD/E	34C
32DD/E	30DDD/F	34D
34B	32C	36A
34C	32D	36B
34D	32DD/E	36C
34DD/E	32DDD/F	36D
34DDD/F	32G	36DD/E
34G	32H	36DDD/F
36B	34C	38A
36C	34D	38B
36D	34DD/E	38C
36DD/E	34DDD/F	38D
36DDD/F	34G	38DD/E
36G	34H	38DDD/F
38C	36D	40B
38D	36DD/E	40C
38DD/E	36DDD/F	40D
38DDD/F	36G	40DD/E
40C	38D	42B
40D	38DD/E	42C
40DD/E	38DDD/F	42D
42C	40D	44B
42D	40DD/E	44C

The chart is a starting point, not a fit guarantee. A neighboring size can preserve volume while changing cup width, depth, wire position and strap spacing.

How to Calculate Your Sister Bra Size

To sister-size down for a firmer band

1. Reduce the band by one standard step—typically two inches in U.S. sizing.

2. Increase the cup by one step in that brand’s cup sequence.

Examples:

• 36C becomes 34D.

• 38D becomes 36DD/E.

• 40DD/E becomes 38DDD/F.

Use this direction when the current cups fit but the band rides up, shifts or requires the tightest hook while still new.

To sister-size up for a looser band

1. Increase the band by one standard step.

2. Decrease the cup by one step in that brand’s cup sequence.

Examples:

• 32DD/E becomes 34D.

• 34D becomes 36C.

• 38DDD/F becomes 40DD/E.

Use this direction only when the cups fit and the band remains uncomfortably tight on the loosest hook.

U.S. vs. U.K. and European Sister Sizes

The sister-size principle is the same across sizing systems, but cup letters do not advance in the same sequence. U.S. brands may use DD and DDD, U.K. brands may include double letters such as FF and GG, and continental European brands may use single letters with band sizes such as 70, 75 and 80.

Do not calculate across systems by matching letters alone. First identify the ordering system used on the product page, then move one cup step within that system. Lavinia selectors often combine equivalent U.S. display labels such as DD/E and DDD/F, while a manufacturer tag may show its original regional size.

How to Know Whether You Need a Sister Size

Start with a bra that has not stretched out. A new bra should normally feel secure on the loosest hook, leaving the tighter hooks available as the elastic relaxes with wear.

Signs the band may be too large

• The band rises between the shoulder blades.

• It shifts when you raise your arms.

• The front slips downward or the wires leave the breast crease.

• A new bra already requires the tightest hook.

• The straps must be overtightened to create lift.

If the cups fit, try one band smaller and one cup larger.

Signs the band may be too small

• The band causes pain or restricts comfortable breathing.

• The closure, elastic or wire digs despite correct cup placement.

• Side seams are pulled sharply backward.

• The bra feels tight even when the cups are removed from the equation.

Cup overflow can make a correct band feel too tight. Check cup size before moving to a larger band.

Use the independent band test

Put the bra on backward and upside down so the cups hang away from the breasts. Fasten it on the loosest hook. If the band now feels comfortable, the original tightness probably came from cups that were too small or too shallow. If it remains painfully tight, try a larger band or a different band construction.

The common “two-finger rule” can be a quick comfort check, but it is not a precise sizing method because fabrics stretch differently. Band position, stability and comfort are more useful clues.

Check the Cups Before Changing the Band

A sister size cannot fix every cup problem.

Signs the cups may be too small

• Tissue spills over the top, sides or beneath the cups.

• The center gore floats away from the sternum in a standard underwire bra.

• Underwires rest on breast tissue.

• The cups compress or flatten the bust.

• The band feels tight only when the bra is worn normally.

Keep the band and increase the cup if the band already fits.

Signs the cups may be too large—or the wrong shape

• Fabric wrinkles or leaves empty space.

• A molded cup gaps at the top or bottom.

• Wires extend far beyond the breast root.

• The cups collapse or move during normal activity.

Gaping does not always mean the cup volume is too large. The cup may be too tall, shallow or open for your shape. A different neckline or construction can fit better than a smaller cup.

Position the breast tissue correctly

Lean slightly forward and guide all breast tissue from the side into each cup. Set the wire into the natural crease beneath the breast, then adjust the straps. This “scoop and swoop” step reveals the true cup fit and prevents unnecessary size changes.

When Sister Sizing Works Best

Sister sizing works best for a one-step band adjustment when cup volume is already correct. It can help when:

• Your usual size is temporarily unavailable.

• You fall between two band sizes.

• A particular brand runs firmer or stretchier.

• Your measurements have changed slightly.

• One bra construction fits differently from your everyday style.

Unlined and stretch-lace bras

Stretch lace and seamed cups can adapt more easily to subtle shape differences than rigid molded cups. A sister size may work well if the band needs a minor adjustment and the cup edge remains smooth.

Balconette and demi bras

Balconette and demi cups make upper-cup fit especially visible. Sister sizing can improve the band, but it will not fix a neckline that cuts into upper fullness or gaps on a shallower bust. Compare cup shapes within Lavinia’s balconette bra collection.

Molded and T-shirt bras

Molded cups hold a predetermined shape. A sister size may have similar capacity yet gap because its height, width or depth no longer matches your body. Compare the adjacent size with a different construction from the molded cup bra collection.

Strapless and convertible bras

The band provides nearly all the anchoring in a strapless bra. A smaller sister size can help only when the current band is genuinely loose; it should never force wires onto breast tissue. Shop convertible and strapless bras by cup construction and dress requirements as well as size.

Longline and fuller-bust bras

Longline bras distribute pressure across more of the torso, while fuller-bust styles often use stable fabric, wider bands and reinforced cups. Sister sizing may refine the band, but frame proportions become especially important. Compare longline bras and full-figure bras in the same style whenever possible.

When You Should Not Sister-Size

Do not use a simple sister-size swap when both the band and cups are wrong.

• Band fits; cups spill: keep the band and increase the cup.

• Band fits; cups gape: keep the band and test another cup size or shape.

• Band rides up; cups spill: reduce the band and increase cup volume enough to correct both problems.

• Wires are consistently too wide or narrow: try another cup shape, wire profile or brand.

• Straps repeatedly fall: evaluate strap placement, cup size and frame width.

• The center gore is painful or too wide: compare plunge or narrow-gore constructions.

Avoid moving more than one sister-size step without a thorough fitting. The farther you travel from the original band, the more the frame changes. A 32DD/E and 36C may have similar capacity, but the 36 band is longer, the cups may be wider and the straps may sit farther apart.

Sister Size vs. Cup Size: What Should Change?

Fit problem	Band action	Cup action
Band too loose; cups correct	Down one band	Up one cup
Band too tight; cups correct	Up one band	Down one cup
Band correct; cups too small	Keep band	Increase cup
Band correct; cups too large	Keep band	Decrease cup or change shape
Band too loose; cups too small	Reduce band	Increase enough to correct volume
Size seems right but wires or neckline do not	Usually keep band	Change cup shape or style

The band provides the foundation. Straps should fine-tune the fit—not carry most of the bust weight.

How to Compare Two Sister Sizes at Home

Compare the same bra model in both sizes whenever possible.

1. Fasten each new bra on the loosest hook.

2. Position all breast tissue inside the cups.

3. Confirm that the band stays level.

4. Check that the wires surround the breast tissue.

5. Confirm that the cups neither spill nor leave empty space.

6. Adjust the straps without overtightening.

7. Sit, bend, walk and raise your arms.

8. Wear a close-fitting top to compare the silhouette.

9. Keep the size that remains comfortable and secure through movement.

Browse Lavinia’s complete bra collection by size and construction, then verify the exact product’s size selector before ordering.

Frequently Asked Questions About Bra Sister Sizes

What is the sister size of 34D?

The closest sister sizes of 34D are 32DD/E for a firmer band and 36C for a looser band. They have approximately similar cup capacity but different frame proportions.

What is the sister size of 36C?

The closest sister sizes of 36C are 34D and 38B. Choose 34D when the 36 band is too loose, or 38B when the cups fit but the 36 band is genuinely too tight.

Is 36C the same as 34D?

No. They have similar cup capacity, but 36C has a longer band and differently scaled cups, wires and strap placement. They are neighboring alternatives, not identical sizes.

What is the sister size of 38D?

The nearest sister sizes of 38D are 36DD/E for a firmer band and 40C for a looser band.

Should I sister-size up if my bra feels tight?

Only if the cups fit correctly and the band still feels too tight during the independent band test. If the cups are too small, increase cup volume first.

Should I sister-size down for more support?

Sister-size down when the current band is too loose. A firmer band can improve stability, but a correctly fitting band should not be downsized simply to create more compression.

Can I move two sister sizes away?

It is generally better to stay within one band step. Moving farther changes wire width, cup placement, strap position and overall support enough that the bra may no longer suit your frame.

Why does the same bra size fit differently between brands?

Brands use different patterns, materials, wire shapes and sizing systems. Even two bras from one brand can fit differently because plunge, balconette and molded cups are engineered for different shapes.

Are sister sizes the same in U.S. and U.K. sizing?

The calculation is the same, but the cup-letter sequences differ. Confirm the system used for ordering and move one cup step within that system rather than matching letters alone.

Can sister sizing fix cup gaping?

Only when an incorrect band-and-cup combination causes the gap. Gaping often indicates a cup-shape mismatch, so a lower cup, stretch edge or different construction may work better.

Is a bra extender the same as sister-sizing up?

No. An extender adds temporary length to the existing band without changing cup volume, wire width or strap placement. It can help with a slightly firm new band or temporary body changes.

Can I use a sister size when my usual size is sold out?

Yes, if the adjacent band fits and the cup shape still suits your body. Try to move only one step, use the same bra style and confirm the fit through normal movement.

Find the Size That Fits This Bra and Your Body

Your best bra size is the one that fits the specific design and your body today—not simply the number and letter you have always purchased. Use sister sizing for a precise one-step band adjustment, then verify the cups, wires, straps and movement. When the band stays level and the cups contain smoothly, the bra can support beautifully without demanding constant attention.

By Lavinia Lingerie Editorial Team · Updated 2026-06-05.