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About This Calculator
Menstrual cycles typically range from 21 to 35 days, with the average being 28 days, though individual variation is completely normal. This calculator predicts upcoming period start dates, fertile windows, and ovulation days based on your cycle history. Tracking patterns over time helps identify irregularities that may warrant medical attention.
Quick Tips
- 1 Track 3+ cycles to find your true average length — 28 days is just a guideline.
- 2 Ovulation typically occurs 14 days before your next period starts.
- 3 Cycle lengths between 21-35 days are considered medically normal.
Example Calculation
Last 3 periods: Jan 2, Jan 30, Feb 27 with 5-day duration.
Average cycle: 28 days | Next period: March 27, 2026 | Ovulation: March 13
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.
Frequently Asked Questions
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.
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.
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.