Ovulation Calculator
Find your ovulation date and fertile window from your last period
Enter the first day of your last period plus your average cycle and period length, and this tool works out your estimated ovulation date and fertile window automatically. It's useful if you're trying to conceive or simply want to keep track of your cycle. Everything is calculated inside your browser, so none of your personal information is sent to a server.
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🔒 Everything runs 100% in your browser. Your files and input are never uploaded to any server.
How to use
- Select the first day of your last period.
- Enter your average cycle length (21–45 days) and period length (3–10 days).
- Review your next estimated ovulation date, fertile window, and the six-cycle forecast table.
- Copy the results if you want to keep a record.
FAQ
- How accurate is the ovulation estimate?
- It's an estimate based on average cycle math, and your actual ovulation date can vary from person to person and month to month. Don't rely on it for medical diagnosis or as a form of birth control.
- How is the fertile window calculated?
- The fertile window is shown as the five days before your estimated ovulation date through the day after it.
- Is my data saved?
- No. The dates and cycle details you enter are calculated only in your browser — nothing is sent to or stored on a server.