How much blood can period underwear hold?

It depends on the absorbency level. As a rough guide, Light holds about one pad's worth, Regular about two, Heavy about three to four, and Overnight even more. Brands design each level for different flow days.

On your heaviest days you can either choose an Overnight style or use period underwear as backup with a tampon or cup. Many women mix levels across their cycle — lighter pairs at the start and end, heavier pairs in the middle.