Ovulation Calculator
Predict your fertile window and ovulation date based on your cycle length and last period start date — useful for conception planning or cycle awareness.
How to use the Ovulation Calculator
- Enter your inputs into the Ovulation Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The ovulation prediction
Ovulation ≈ Next period date − 14 days · · · Fertile window: 5 days before ovulation through ovulation day
Ovulation typically occurs 14 days before the next period (not 14 days after the last one). The fertile window includes the days sperm can survive — about 5 days before ovulation plus the day itself.
Worked example
Last period started January 5, 28-day cycle: next period February 2; ovulation ≈ January 19. Fertile window: January 14–19. Pregnancy is most likely from sex during these 6 days.
Frequently asked questions
How accurate is calendar tracking?
Within 2–3 days for women with regular cycles. Less accurate for irregular cycles. Ovulation predictor kits (LH strips) or basal body temperature tracking are more precise.
When in the fertile window is conception most likely?
The day of ovulation and the 1–2 days before. Conception rates per cycle peak around day −1 to day 0, with declining odds further from ovulation.
Can I ovulate during my period?
Very rare in regular cycles (~28 days) but possible with short cycles (~21 days). Without confirmation by other methods, don't assume "safe" days based on calendar alone.