Period Calculator
Predict your next 6 periods, ovulation dates, and fertile windows based on your cycle.
Predict your next six menstrual periods, along with estimated ovulation dates and fertile windows for each cycle, based on your typical cycle length and period duration.
How Period Calculator Works
Starting from the first day of your last period, the calculator projects your next six period start dates by simply adding your entered cycle length repeatedly (next period = last period + cycle length, then + cycle length again, and so on) — assuming your cycle length stays consistent across each projected cycle.
Each period's end date is calculated from your entered typical period duration, and ovulation for each projected cycle is estimated using a standard 14-day luteal phase counted backward from that cycle's next period — the same logic used across the site's other fertility calculators, applied here across a rolling six-cycle forecast.
The fertile window for each cycle spans five days before through one day after that cycle's estimated ovulation date, giving a full multi-month view of expected periods and fertile windows at once rather than calculating just a single upcoming cycle.
See It In Action
Who Uses Period Calculator and Why
- Getting a multi-month forecast of upcoming periods and fertile windows at once, rather than one cycle at a time.
- Planning travel, events, or appointments around several projected future periods.
- Checking an estimated ovulation date for a cycle several months out.
Mistakes to Avoid
- Trusting the later projected cycles (5th or 6th out) as precisely as the very next one — the forecast assumes a perfectly consistent cycle length, and real cycles often vary by a few days month to month, so accuracy naturally decreases further out.
- Not noticing this calculator uses a fixed 14-day luteal phase assumption rather than a personalized one, unlike some of the site's other fertility tools that let you enter your own.
- Ignoring a pattern of highly irregular real periods compared to the prediction — occasional variation is normal, but a persistent, large mismatch is worth mentioning to a healthcare provider.
Tips for Best Results
- Treat the next one or two projected cycles as reasonably reliable, and the later ones as a rough general outlook rather than a firm plan.
- If you want a more personalized ovulation estimate for a single upcoming cycle, the dedicated ovulation calculator lets you enter your own luteal phase length.
Fixing Common Problems
My actual period arrived several days off from the prediction. — A few days of natural variation is common even with a fairly regular cycle — if this happens occasionally, it's not a cause for concern; track your actual cycles over a few months to refine future predictions.
Terms Explained
Cycle length: The number of days from the first day of one period to the first day of the next — the core input this calculator repeats forward to project future periods.