Period Calculator
Your next period is expected one full cycle after the first day of your last one. Enter your dates below to see the next four period start dates — plus the fertile window and estimated ovulation day for each of those cycles, so you can plan around all of it at once.
This cycle on a calendar
Your next cycles at a glance
Each later cycle assumes the same average length. Real cycles drift by a few days, so treat dates further out as rougher estimates.
DueDateCalc works out your due date, week-by-week timeline and milestones.
How period prediction works
"Cycle length" means the first day of one period to the first day of the next. If your last period started on the 1st and your average cycle is 28 days, the next is expected on the 29th. The table above repeats that step so you can see several months ahead — useful for holidays, events, and knowing when a period is late enough to be worth a pregnancy test.
How accurate is it?
Only as steady as your cycles are. Variation of up to about a week between your shortest and longest cycles is common and usually nothing to worry about; stress, illness, travel and changes in weight or exercise can all shift a single cycle by several days. Predictions further ahead stack that uncertainty, which is why the dates above get rougher the further out they go. If your cycles are regularly shorter than 21 days, longer than 35 days, or have become persistently irregular, that is worth raising with a clinician rather than averaging away here.
Late period?
If your expected date has passed, the most common explanations are a later-than-usual ovulation (which pushes everything back), pregnancy, or an anovulatory cycle. A home pregnancy test taken on or after the day of the missed period is the reliable next step — our pregnancy test calculator shows the earliest sensible testing day for your cycle.