Know your cycle, day by day.
Enter the first day of your last period, your average cycle length and how long your period usually lasts. You will instantly see when your next period is due, your estimated ovulation day, your fertile window and the dates of your next six cycles.
Your cycle details
The first day of your most recent menstrual period.
Average number of days between periods.
How many days your period typically lasts.
Your next period is estimated by adding your cycle length to the first day of your last period. Ovulation is estimated 14 days before the next period, and the fertile window covers the 5 days before ovulation plus ovulation day. Results update as you type.
What this means for you right now
Where you are in your cycle
Key dates in your current cycle
Your next six cycles
Every row is calculated from your last period date and cycle length. The highlighted row is your upcoming cycle.
| Cycle | Period starts | Period ends | Ovulation | Fertile window |
|---|
Cycle tracking, explained
How Period Predictions Work
Period predictions are based on the average length of your menstrual cycle. By adding your cycle length to the first day of your last period, the calculator estimates when your next period will start. Predictions for subsequent cycles repeat this pattern. Ovulation is estimated to occur 14 days before the start of the next period.
Understanding Your Fertile Window
The fertile window spans approximately 6 days each cycle, the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the reproductive tract, so intercourse during this window has the highest chance of conception. Peak fertility is 1-2 days before ovulation.
Common questions
How accurate are period predictions?
Predictions are most accurate for women with regular cycles. If your cycle varies by more than a few days from month to month, predictions become less reliable. Stress, illness, weight changes, and hormonal factors can all affect cycle timing.
What is a normal cycle length?
Normal menstrual cycles range from 21 to 35 days, with 28 days being the average. Cycles are counted from the first day of one period to the first day of the next. Consistent cycles within this range are considered normal, even if they are not exactly 28 days.
Can I use this as birth control?
Calendar-based fertility awareness methods are not highly reliable for birth control (failure rates of 12-24% with typical use). Cycles can vary, and ovulation timing is not perfectly predictable. If avoiding pregnancy, use this tool only alongside more reliable contraception methods.
This calculator gives an estimate, not a medical diagnosis. Predictions assume a regular cycle: if your cycles are irregular, vary by more than a few days, or you experience unusually heavy bleeding, severe pain, or a missed period, talk to your gynecologist. Calendar predictions alone are not a reliable form of contraception. Always follow the guidance of your healthcare provider.