Ovulation Reverse Calculator
Reverse-calculate ovulation and fertile days from your next period
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🔒 Everything runs 100% in your browser. Your files and input are never uploaded to any server.
This tool works backward from your expected next period start date, estimating your ovulation date as 14 days before it and showing your most fertile window (5 days before ovulation through 1 day after). It's handy for anyone tracking their cycle or planning for pregnancy, and every calculation happens right in your browser with nothing sent to a server.
How to use
- Select your expected next period start date in the date field.
- The estimated ovulation date appears automatically above the input.
- Check the fertile window shown below it (5 days before to 1 day after ovulation).
- Change the date anytime to instantly recalculate for a different cycle.
FAQ
- How is the ovulation date calculated?
- It subtracts 14 days from your expected next period start date, a common reverse-calculation method based on typical luteal phase length.
- Can I use this for contraception?
- No. Ovulation timing can shift with hormonal changes, stress, and overall health, so this should not be relied on as a birth control method.
- Is my date data stored anywhere?
- No, all calculations happen locally in your browser and the date you enter is never sent to or stored on a server.