Enter your last period and average cycle length to see your next six projected period dates.
How to Use the Period Calculator
Enter the first day of your last period and your average cycle length, and the calculator lists your next six projected period start dates, each one calculated by adding your cycle length to the last.
This one's built for a specific, unglamorous purpose: knowing roughly when to pack extra supplies, plan around a trip, or block out a date on a calendar. It isn't trying to diagnose anything — it's a projection tool, and a fairly blunt one.
Next period = Last period start + cycle length
(repeat, adding cycle length each time, for six periods)The projection is only as good as the cycle length you feed it, and that number moves around more than most people assume. Stress, illness, travel, changes in sleep or exercise, and plain biological variation can all shift a cycle by several days in either direction — even for someone whose cycles are usually pretty regular. That's compounding: the first projected date carries whatever uncertainty your average already has, but the sixth one has that same uncertainty multiplied six times over, since each date is built on the assumption that every cycle before it landed exactly on schedule.
Practically, that means treat the next date or two as reasonably reliable and the further-out dates as rough placeholders worth double-checking closer to the time. If your actual cycles vary widely around your stated average, this gap will be larger than it looks on the page. Most cycles land within a few days of the stated average, but a single stressful month is enough to shift the actual start date noticeably — which is exactly why lining up plans mid-cycle rather than at the last minute leaves the most margin for error. Unlike the Ovulation Calculator, which counts backward from the next period to estimate fertile days, this tool works forward from the last one — the more direct, familiar way most people already think about tracking a cycle.
This is a projection, not a diagnosis. It applies one average cycle length forward repeatedly — it doesn't evaluate whether your cycles are actually regular. If your periods vary widely from cycle to cycle, expect these dates to drift, and mention persistent irregularity to a doctor rather than relying on this tool to track it.