Period & Ovulation Calculator
Predict your next period, ovulation date, and fertile window
Enter the first day of your last period along with your average cycle length and period length, and this tool shows your next three expected periods, your estimated ovulation date, and your fertile window at a glance. It's handy whether you're trying to conceive or just want to know when your next period is due. All calculations happen instantly in your browser, so the dates you enter are never sent to any server.
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🔒 Everything runs 100% in your browser. Your files and input are never uploaded to any server.
How to use
- Use the date picker to enter the first day of your last period.
- Enter your average cycle length (21–45 days) and average period length (3–10 days).
- Check the top of the screen for your next period start date, estimated ovulation date, and fertile window.
- Review the three-cycle forecast table to see the expected period and ovulation dates for each upcoming cycle.
FAQ
- How are the ovulation date and fertile window calculated?
- Ovulation is estimated as 14 days before your next expected period, and the fertile window is shown as the five days leading up to ovulation plus the day after.
- Can the results differ from what actually happens?
- Yes. This calculator is a prediction based on average cycle math, and real ovulation and period timing can shift with your health and other factors, so treat the results as a general guide only.
- Are the dates I enter saved anywhere?
- No. Everything is calculated inside your browser and nothing is sent to or stored on a server.