You measure yourself, follow every step in the size guide, and end up with a number that lands exactly between two sizes. Underbust of 33". Waist of 28.5". A bust difference that puts you halfway between a D and a DD.
This is the most common moment in online shopping for underwear. Almost everyone has it. And almost everyone guesses.
There's a better answer — and it's simpler than you might think.
The general rule: size up
For most underwear most of the time, size up. Wireless bras feel softer when the band is slightly looser. Shapewear stays comfortable longer when it isn't fighting you for breath. Bodysuits drape more naturally when the fabric has a little ease in it.
This rule comes from one truth about everyday underwear: the discomfort that ends a day always comes from something being too tight, not too loose. By 5 PM, the bra that fit perfectly in the morning has decided to remind you it exists. The shapewear that smoothed everything at 9 AM is now telling you to find a bathroom. Sizing up is your hedge against that hour.
Most women are wearing one band size too big and one cup size too small — and most haven't re-measured in years. The "size up" rule corrects for a different kind of mistake than that one. It assumes you've already measured correctly and just need to decide between two valid options.
The three times you don't size up
The rule has exceptions. Three of them.
The first is firm shapewear for an event. If you're buying a sculpting bodysuit specifically for a wedding, a photo session, or another fitted-dress moment, keep your usual size. The "sculpted" feel comes from a snug fit. Size up and you'll lose the effect you bought it for. Most women don't wear firm shapewear for eight hours anyway — it's a tool for a moment, not a workday.
The second is when your measurement lands exactly on an even number. If your underbust measures exactly 32" (not 32.5", not 31.5"), go with the 32 band. Don't round up to 34 because you "feel like" you're between. The chart is the truth in that moment.
The third is when you've been wearing the wrong bra for years. If your current bras feel "fine" and you haven't re-measured since college, your sense of "right size" is probably miscalibrated. Re-measure first, follow the chart, and only then think about up or down.
Sister sizing — the move most online shoppers don't know
When the cup of a bra fits well but the band doesn't, there's a smarter move than swapping to a completely different size. It's called sister sizing.
Cup size isn't an absolute volume — it's a relationship to the band. A 34D and a 32DD hold almost exactly the same amount of bust tissue. They're "sister sizes" because they share cup volume but have different bands.
The rule:
- Going down one band size → go up one cup letter (volume stays the same)
- Going up one band size → go down one cup letter
A practical example: you measured at 34D, ordered it, and the band rides up your back all day. The cup fits beautifully. Don't go to a 34C — you'll lose cup space too. Try a 32DD instead. Same cup, tighter band that stays put.
What to do at checkout when you still aren't sure
Three honest steps:
- Pick the larger size first. It's a softer landing for the days you'll actually wear it.
- Read the fit note on the product page. Most ILARIA products say whether they run small, true to size, or generous. That note matters more than the size chart for the final decision.
- Don't agonize. The first exchange is free.
That last one matters most. You don't have to be right on the first order. You have to be willing to swap once if you're wrong.
What to do next
If you want a personalized answer instead of a rule, take the 60-second Comfort Match quiz — it asks the questions a size chart can't (how you want it to feel, what your current pieces get wrong) and recommends from there.
If you want to re-measure first, the size guide walks you through it in four steps.
And if you've already bought from us and the size isn't quite right, email us — we'll guide you to the right sister size for your next try.
ILARIA is about comfort that lasts past the first try-on — bras, bodysuits, and shapewear that still feel good hours later. First exchange is always free. Read more in our fit and help center.
