Period Calculator
Predict your upcoming periods, ovulation day, and fertile window from the start of your last period and your cycle length.
Independently verified for accuracy
Calculator by Toolsloft ↗- Next period
- 2026-06-29
- Ovulation
- 2026-06-15
- Fertile window start
- 2026-06-10
- Fertile window end
- 2026-06-16
Upcoming periods
- 2026-06-29
- 2026-07-27
- 2026-08-24
This calculator forecasts your upcoming periods, your estimated ovulation day, and the fertile window around it from the date your last period started and your typical cycle length. Enter those two figures, along with how long your period lasts and how many cycles ahead you want to see, to get a simple calendar of likely dates. The results are estimates and real cycles often shift by a few days.
How this is calculated
The next period is the last period date plus the cycle length, and that step repeats for each upcoming cycle you ask for. Ovulation is estimated about 14 days before the next period, and the fertile window spans from five days before ovulation to one day after it. Cycle timing varies from month to month and between people, so treat every date here as an estimate rather than a guarantee.
How to use
- Enter the date your last period started.
- Enter your average cycle length in days and how many days your period lasts.
- Choose how many upcoming cycles you want to forecast.
- Read your next period date, ovulation estimate, fertile window, and the list of upcoming periods.
Examples
- 28-day cycle from June 1:
next period 2026-06-29, ovulation 2026-06-15 - 31-day cycle from March 15:
next period 2026-04-15, ovulation 2026-04-01
FAQ
- How are future periods predicted?
- Each upcoming period is the start date of your last period plus your cycle length, then plus another cycle length for the period after that, and so on for the number of cycles you request.
- How is the fertile window estimated?
- Ovulation is estimated about 14 days before the next period starts. The fertile window covers the five days before ovulation through the day after it, since that is when conception is most likely.
- How accurate are these dates?
- They are estimates based on an average cycle. Real cycles vary from month to month, so use the dates as a guide rather than a precise schedule.