Ovulation Calculator
Predict your ovulation day and most fertile window based on your cycle length with this free ovulation calculator in your browser.
Enter details
Result
Enter a date to see your estimate.
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the first day of your last period and your average cycle length.
- Read your estimated ovulation day and fertile window.
How it works
Ovulation is about one luteal phase (default 14 days) before your next period. The fertile window spans roughly the 5 days before to 1 day after ovulation.
Worked example
28-day cycle. Ovulation ≈ day 14; fertile window ≈ days 9–15.
Tips
- Estimate only — not medical advice. Dates and fertile windows vary between people and cycles; confirm with a healthcare professional.
Frequently asked questions
How reliable is this?
Cycles vary, so it's an estimate. Ovulation tests or tracking give a more personal result.
Privacy?
Runs entirely in your browser. Your inputs are not saved or transmitted.
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Cite this tool
Zerdly. (2026). Ovulation Calculator. Retrieved from https://www.zerdly.com/tools/ovulation-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.