Bra Sister Sizes Explained: Chart and Fit Guide

 

Bra sister sizes have approximately the same cup volume but different band sizes. 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.

This adjustment can solve a band-fit problem without substantially changing cup capacity. It is most reliable one step away from your usual size, within the same bra style and brand. Sister sizes are a fitting tool, not exact duplicates: cup width, strap position, wire shape and support change with the band.

 

What Are Bra Sister Sizes?

Bra sister sizes are neighboring sizes built around roughly equivalent cup capacity. The band number changes, so the cup letter must move in the opposite direction to preserve volume.

For example, 32DD/E, 34D and 36C belong to the same sister-size family. Their cups hold a similar volume, but the bras do not fit identically:

• 32DD/E has a shorter, firmer band and a cup scaled for a 32-inch size range.

• 34D has the middle band and frame proportions.

• 36C has a longer band with cups and straps positioned for a broader frame.

Cup letters are relative to band size. A 32D cup is smaller 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.

 

Bra Sister Sizing at a Glance

If this happens	Try this change	Example from 34D
Band rides up or feels too loose, while cups fit	Go down one band and up one cup	32DD/E
Band feels too tight, while cups fit	Go up one band and down one cup	36C
Band and cups are both too small	Go up in cup volume; do not use a simple sister size	Try 34DD/E or reassess both measurements
Band and cups are both too large	Go down in cup volume; do not use a simple sister size	Try 34C or reassess both measurements
Wires are too wide, narrow, tall or low	Try another cup shape or style	Compare plunge, balconette, demi or full-cup designs

The sister-size rule: smaller band = larger cup letter; larger band = smaller cup letter.

Bra Sister-Size Chart

The chart below follows the U.S.-style progression commonly displayed on Lavinia Lingerie product selectors: A, B, C, D, DD/E, DDD/F, G and H. Some brands use U.K. or European notation, so always follow the exact size options and conversion information shown on the product page.

Your current 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
36B	34C	38A
36C	34D	38B
36D	34DD/E	38C
36DD/E	34DDD/F	38D
36DDD/F	34G	38DD/E
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. A 34D in one bra can feel different from a 34D in another because cup depth, fabric, wire shape and manufacturing standards vary.

How Do You Calculate a Sister Size?

Use one of these two calculations:

To sister-size down

1. Reduce the band by one standard step, usually two inches.

2. Increase the cup by one step.

Examples:

• 36C becomes 34D.

• 38D becomes 36DD/E.

• 40DD/E becomes 38DDD/F.

Choose this direction when the cups fit but the band rides up, shifts or feels too loose.

To sister-size up

1. Increase the band by one standard step, usually two inches.

2. Decrease the cup by one step.

Examples:

• 32DD/E becomes 34D.

• 34D becomes 36C.

• 38DDD/F becomes 40DD/E.

Choose this direction when the cups fit correctly but the band remains uncomfortably tight on the loosest hook.

How Do U.S., U.K. and European Cup Sizes Affect Sister Sizing?

The sister-size method remains the same, but cup letters do not progress identically across every sizing system. U.S. labels may use DD and DDD, U.K. labels may include double letters such as FF or GG, and many continental European brands use single letters while bands may be shown as 70, 75, 80 and so on.

Do not calculate across systems using cup letters alone. First confirm the sizing system used for ordering, then move one step along that system’s cup sequence. Lavinia product selectors often combine equivalent U.S. labels—for example, DD/E and DDD/F—to make ordering clearer. The sewn-in manufacturer tag may still display its original regional size.

How to Know Whether You Need a Sister Size

Begin with a bra that is not stretched out. Fasten a new bra on its loosest hook so the tighter hooks remain available as the elastic relaxes over time. Then check the band and cups separately.

Signs the band may be too large

• It rises noticeably between the shoulder blades.

• It shifts when you raise your arms.

• The front slips downward or the wires move away from the breast crease.

• You begin a new bra on 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

• It feels painful or restricts comfortable breathing.

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

• The band distorts or pulls the side seams backward.

• Red marks remain painful or pronounced rather than fading normally.

Before increasing the band, confirm that the cups are not too small. Too-small cups can borrow space from the band and make the entire bra feel tight.

Use the independent band test

To isolate band fit, 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 likely came from cups that were too small or too shallow. If the band is still painfully tight, a larger band or a different band construction may be appropriate.

Check the Cups Before Changing Band Size

A sister size preserves approximate volume; it does not correct every cup problem.

Signs the cups may be too small

• Breast tissue spills over the top, sides or beneath the cup.

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

• The underwire rests on breast tissue.

• The cup compresses or creates a flattened shape.

• The band feels tighter only when the cups are worn normally.

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.

• The wire extends far beyond the breast root.

• The cup collapses or loses support during movement.

Gaping does not automatically mean the cup volume is too large. A cup can be too tall, too shallow or too open for your breast shape. In that case, changing the style may work better than changing the size.

Position the breast tissue correctly

Lean slightly forward and guide the breast tissue from the side and beneath the arm into each cup. Set the wire in the natural crease under the breast, then adjust the straps. This “scoop and swoop” step reveals the true fit and can prevent an unnecessary size change.

When Does Sister Sizing Work Best?

Sister sizing works best for one-step band adjustments when the cup volume already feels correct. It can be useful when:

• Your usual size is temporarily unavailable.

• You fall between two band sizes.

• A particular brand runs firmer or stretchier in the band.

• Your measurements have changed slightly.

• A special bra construction fits differently from your everyday style.

Convertible and strapless bras

The band supplies most of the stability in a strapless bra. If the cups fit but the bra slides down, one sister size smaller may provide better anchoring. It should remain comfortable and must not force the wires onto breast tissue. Compare Lavinia’s convertible and strapless bras by cup construction as well as size.

Longline bras and bustiers

An extended band distributes pressure across more of the torso, but boning and firm fabric may make it feel more structured. A neighboring sister size can help, although waist and torso proportions also matter. Explore longline bras, bustiers and corsets with the intended outfit before removing tags.

Balconette and demi bras

The open neckline and wider-set straps of a balconette make cup shape especially visible. Sister sizing can refine the band, but it will not fix a cup edge that cuts into upper fullness or gaps on a shallower bust. Compare balconette bras with stretch lace, molded cups and seamed unlined cups.

Everyday and molded bras

Molded cups keep a predetermined shape, so a size with equivalent volume may still gap if the cup geometry does not match your body. When shopping the complete bra collection, compare both an adjacent sister size and a different cup shape.

When Should You Not Sister-Size?

Do not rely on sister sizing when both the band and cups are wrong. Change the component that needs correction instead:

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

• Band fits; cups gape: keep the band and test a smaller cup or a different shape.

• Band rides up; cups also spill: reduce the band and increase the cup enough to correct both problems—not merely one sister step.

• Wires repeatedly sit too wide or narrow: try a different cup shape, wire profile or brand.

• Straps fall despite adjustment: check strap placement, cup size and frame width rather than tightening endlessly.

Avoid moving more than one sister size unless a fitter or detailed try-on confirms the result. The farther you travel along a sister-size family, the more the frame changes. For example, 32DD/E and 36C 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: Which Should You Change?

Fit problem	Band change	Cup change
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	No change	Up one or more cups
Band correct; cups too large	No change	Down a cup or change cup shape
Band too loose; cups too small	Down a band	Increase enough to correct volume
Bra fits in size but wires or neckline do not	Usually no change	Change bra shape or construction

The band should provide the foundation. Straps fine-tune the fit; they should not carry most of the bust’s weight.

How to Compare Two Sister Sizes at Home

Try the same bra style in both sizes whenever possible. Comparing different designs introduces too many variables.

1. Fasten each bra on the loosest hook.

2. Position all breast tissue inside the cups.

3. Confirm that the band remains 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, raise your arms and walk for several minutes.

8. Put on a close-fitting top to compare the silhouette.

Keep the size that remains secure and comfortable through movement—not simply the one that looks best while standing still.

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 equivalent 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 if the 36 band is too loose, or 38B if the cups fit but the 36 band is genuinely too tight.

Is 36C the same as 34D?

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

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

Only if the cups fit correctly and the band is still too tight during an independent band test. If the cups are too small, increase the cup size first because undersized cups can make the band feel tight.

Should I sister-size down for more support?

You may sister-size down when the current band is too loose. A firmer band can improve stability, but a band that is already correct should not be downsized merely to create more compression.

Can I move two sister sizes away?

It is usually better to stay within one sister-size step. Moving two or more bands changes wire width, cup placement, strap position and 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 conventions. Even two styles from one brand may fit differently because a plunge, balconette and molded cup 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 which system the product uses, then move exactly one cup step within that system. Do not convert by matching letters alone.

Can sister sizing fix cup gaping?

Only when the gaping is caused by an incorrect band-and-cup combination. 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 temporarily adds length to the existing band without changing cup volume, wire width or strap placement. It can help with a new, slightly firm band or temporary body changes, but it should not compensate for clearly incorrect sizing.

Find the Size That Supports You Today

Your best bra size is the one that fits the specific bra and your body now—not simply the size you have always purchased. Use sister sizing for a precise one-step band adjustment, verify the cup shape and test the bra while moving. When the band stays level, the cups contain smoothly and the wires follow your natural shape, the bra can support beautifully without demanding constant attention.

By Lavinia Lingerie Editorial Team · Updated 2026-08-06.