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Ovulation Calculator
About the Ovulation Calculator
Ovulation is when an egg is released, and the few days around it are your most fertile. This calculator estimates your ovulation day and fertile window from your last period and cycle length.
How to use the Ovulation Calculator
- Enter the first day of your Last Period.
- Enter your average Cycle Length in days.
- Press Calculate to see your estimated ovulation date and fertile days.
Formula and method
Ovulation day ≈ first day of last period + (cycle length − 14)
Sperm can survive for up to five days, so the fertile window runs from about five days before ovulation to one day after.
Worked example
With a 28-day cycle starting on 1 March, ovulation is around 15 March, and the fertile window is roughly 10 to 16 March.
Tips
- Cycles vary from month to month, so treat the result as an estimate.
- Ovulation predictor kits and basal body temperature charts can confirm the timing.
- If you have irregular cycles or have been trying to conceive for a year (six months if over 35), speak to a doctor.
Frequently Asked Questions
No. Calendar-based estimates are not a reliable form of contraception.
Use your average cycle length, but expect the window to shift. Tracking symptoms or using test strips helps.