Period Calculator: Predict Your Next Period Date
Enter your last period date and average cycle length to predict your next period, ovulation window, and fertile days. Free, private, no signup.
How the Period Calculator Works
This calculator uses your last period start date and average cycle length to estimate your next period, ovulation day, and fertile window. It also shows the current phase of your cycle.
Cycle Phases
Period Calculator FAQ
Period calculators are estimates based on average cycle length. They are 80–90% accurate for women with regular cycles (28 ± 3 days). Stress, illness, travel, and hormonal changes can shift your cycle, so use the calculator as a guide, not a guarantee.
A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the average. Cycles shorter than 21 days or longer than 35 days may warrant a conversation with your healthcare provider.
Your fertile window is typically 5 days before ovulation plus the day of ovulation. For a 28-day cycle, this is roughly days 10–15. Sperm can survive up to 5 days in the reproductive tract, so intercourse before ovulation can still result in pregnancy.
No. This calculator provides estimates only and should not be used as a reliable form of birth control. Natural family planning methods require daily temperature tracking, cervical mucus monitoring, and professional guidance. Consult a healthcare provider for contraception advice.
Common reasons for a late period include stress, significant weight change, intense exercise, travel across time zones, illness, polycystic ovary syndrome (PCOS), thyroid issues, perimenopause, and pregnancy. If your period is more than a week late consistently, consult a healthcare provider.