Period Calculator
Predict your next period, ovulation date, and fertile window from your cycle.
Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01
How to use this period calculator
- Enter the first day of your last period using the date picker.
- Set your cycle length in days (21–45; the default is 28).
- Set your period duration in days (1–10; the default is 5).
- Press Calculate to see your next 6 periods, ovulation date, and fertile window.
- Use Copy to save the result to your clipboard, or Reset to start over.
About this period calculator
The period calculator predicts your next six menstrual periods along with your estimated ovulation date and fertile window, based on the first day of your last period, your cycle length, and your period duration.
The period calculator works by adding multiples of your cycle length to the start date of your last period. Each predicted period start date is calculated as: last period date + (n × cycle length), and the end date is start + duration − 1. Ovulation is estimated as last period date + cycle length − 14, which places it 14 days before the next expected period. The fertile window spans five days before to five days after the ovulation date. This is an estimate for general information and is not a substitute for advice from a qualified clinician. Predictions are based on a regular cycle; natural variation in cycle length is normal and results may differ if your cycle is irregular.
For example, if your last period started on 2026-05-01 with a 28-day cycle and 5-day duration, the tool predicts your next period from May 29 to Jun 2, ovulation on May 15 (14 days before the next period), and a fertile window from May 10 to May 20. This is useful for anyone tracking their cycle, planning ahead, or identifying their likely fertile days.
FAQ
- What does the period calculator predict?
- The period calculator forecasts your next six menstrual period start and end dates, along with an estimated ovulation date and fertile window, based on your last period date, cycle length, and period duration.
- What formula does the period calculator use?
- Each future period start is: last period date + (n × cycle length). Ovulation is estimated as last period date + cycle length − 14, placing it 14 days before the next expected period. The fertile window runs from 5 days before to 5 days after the ovulation date.
- How accurate are the predictions?
- Results are estimates based on a perfectly regular cycle. Natural variation in cycle length is common, so actual dates may differ. These predictions are for general planning only and are not a substitute for medical advice from a qualified clinician.
- What cycle length should I enter?
- Enter the average number of days between the first day of one period and the first day of the next. A typical range is 21–45 days; the tool defaults to 28. If your cycle varies, use your usual average.
- What happens if I enter a cycle length outside 21–45 days?
- The tool shows an error message and does not calculate. Cycle length must be between 21 and 45 days, and period duration must be between 1 and 10 days, to produce a valid result.
- Does this tool store my data?
- No. The period calculator runs entirely in your browser. Your dates are never sent to a server, logged, or retained after you close the page.