Period Calculator
Wondering when your next period is due? Enter the first day of your last period and your average cycle length to estimate your next period date, ovulation day, and five-day fertile window. This free period calculator runs entirely in your browser, so your personal dates never leave your device.
Medical disclaimer: Estimates only, not medical advice. Cycles vary; consult a healthcare professional.
Frequently Asked Questions
How accurate is this period calculator?
It provides estimates only, not medical advice. It assumes ovulation happens 14 days before your next period and that your cycle length stays constant — but real cycles vary with stress, illness, travel, and hormones. Track several cycles to improve the estimate, and consult a healthcare professional for anything important.
How is the fertile window calculated?
Ovulation is estimated at 14 days before your predicted next period. Because sperm can survive for up to 5 days, the fertile window shown here is the 5-day span ending on the estimated ovulation day.
What if my cycles are irregular?
The calculator uses your average cycle length, so irregular cycles make its predictions less reliable. If your cycle regularly falls outside 21–45 days or varies a lot from month to month, talk to a healthcare professional rather than relying on an average.
Can I use this as birth control or to plan a pregnancy?
No — do not rely on these estimates for contraception. The fertile window is an approximation and ovulation timing can shift from cycle to cycle. If you are trying to conceive, use it as a rough guide alongside ovulation predictor tests and your doctor's advice.