The menstrual cycle calculator predicts your next period, fertile window, and ovulation date based on your last period and average cycle length. Results are estimates — individual cycle variation is normal and expected.
Calculate Your Cycle
How to Use the Menstrual Cycle Calculator
This menstrual cycle calculator predicts your next 3 periods, each cycle's fertile window, and estimated ovulation date based on your entered cycle information.
Step 1: Enter Your Last Period Start Date
Enter the first day of your most recent menstrual period — the day bleeding began, not when spotting started. This is used as the anchor for calculating all future cycle dates.
Step 2: Enter Your Cycle Length
Enter your average cycle length in days. If you're not sure, count the days from the first day of one period to the first day of your next period. Track several cycles to find your typical average — many women range between 26 and 32 days.
Understanding Your Fertile Window
The fertile window is the 6-day period when pregnancy is possible. It includes the 5 days before ovulation and ovulation day itself. For a 28-day cycle, this typically falls around days 10–15 of the cycle. The most fertile days are the 1–2 days surrounding ovulation.
FAQ
Is this period calculator free?
Yes, completely free with no signup required. All calculations run in your browser.
Is my data private?
Yes, all calculations run locally in your browser. No data is sent to any server.
How is the fertile window calculated?
Ovulation typically occurs around 14 days before the next period in a standard 28-day cycle. The fertile window is the 6-day window ending on ovulation day (5 days before ovulation plus ovulation day itself). Sperm can survive 3-5 days in the reproductive tract, making the days before ovulation also fertile.
What is a normal menstrual cycle length?
A normal cycle ranges from 21 to 35 days, with 28 days being average. Period duration is typically 2-7 days. Cycles shorter than 21 days or longer than 35 days may warrant evaluation by a healthcare provider, especially if this represents a change from your typical pattern.
Can I use this to prevent pregnancy?
No. This calculator provides estimates only and should not be used as contraception. Cycle length and ovulation timing vary significantly even in regular cycles, and this tool does not account for that variation. Consult a healthcare provider about evidence-based contraceptive methods.
What factors affect cycle regularity?
Stress, significant weight changes, intense exercise, thyroid disorders, polycystic ovary syndrome (PCOS), perimenopause, certain medications, and illness can all affect cycle length and regularity. Tracking your cycle for several months helps identify your personal baseline.