When to Sister Size Up or Down

The decision in one table

Only the first two rows are sister sizing. The rest change the cup volume, which a sister size never does.

Sister size down: band down, cup up

Use it when:

• the band rides up at the back,
• a nearly new bra needs the tightest hook,
• the straps dig in because the band is not holding, or
• your measured underbust is closer to the band below.

This is by far the more common direction. Many people wear a band one or two sizes too big, often because their size was worked out by adding inches to the underbust.

Sister size up: band up, cup down

Use it when:

• the band is painful or restricts breathing on the loosest hook, and
• the cups contain you with no spilling, and
• the band is still comfortable when you fasten the bra back to front around your ribs.

The third check matters. If the band feels fine back to front, the band is not the problem: the cups are too small and are pulling it tight. Go up a cup on the same band instead. The test is described in is my bra band too tight?

When your size is out of stock

Either neighbour will hold the same volume. To choose:

• Prefer the firmer band in most cases. It will loosen with wear, and you can use an extender for the first few weeks if needed.
• Prefer the looser band if your ribcage is bony, if you are pregnant or expect to gain weight, or if this brand is known for firm bands.
• Be warier with moulded or padded cups. They have a fixed shape, and the small changes in wire width between sister sizes show more than they do in a soft cup.

When a brand runs tight or loose

Band tension differs from brand to brand. If you are a secure 32E in most brands and a new one feels like a vice, its 34DD may be your size there. Being a different size in different brands is normal.

When not to sister size

• To fix cups. Spilling and gaping need a different cup volume.
• More than one step. The further you go, the more the wires, straps and cup shape drift from what suits your frame. See do sister sizes really fit the same?
• As a habit. If you sister size up every time for comfort and your bands always ride up, the cups are probably too small.
• Without knowing your starting point. If your current size was a guess, measure first.

Frequently asked questions

Is it better to sister size up or down?

Go by the band. If it is loose, go down a band and up a cup. If it is tight, go up a band and down a cup. When you only need a stand-in because your size is sold out, the firmer band is usually the better choice because bands stretch with wear.

Should I buy my sister size or wait for my true size?

If the sister size passes the fit checks, it is a perfectly good bra. If you notice the band is slightly loose or the wires sit a little wide, keep it as a spare and get your true size when it is back in stock.

Can a sister size be my real size?

Yes. If you have been wearing a band that is too big, your sister size on the smaller band is your real size. The terms only describe the relationship between two sizes.