Free online calculator
Ovulation calculator
Predict fertile window and period.
Ovulation and cycle predictor
Enter your last period date and cycle length to predict ovulation, fertile window, and upcoming cycles.
How to use this calculator
- 1
Enter the first day of your last period
Use the date picker to select the date your most recent period started.
- 2
Set cycle length
Enter your average cycle length in days — the default is 28. Normal range is 21–35 days.
- 3
Set period duration
Enter how many days your period typically lasts. The default is 4 days.
- 4
Read your predictions
The calculator shows your ovulation date, fertile window, next period date, and three upcoming cycles.
Formula
Ovulation date: period start + (cycle length − 14) days
Fertile window: ovulation date − 5 days through ovulation date + 1 day
Next period: period start + cycle length days
All date arithmetic uses UTC to avoid daylight-saving time shifts.
Worked example
Last period April 1, cycle 28 days, duration 4 days:
Ovulation = April 1 + 14 = April 15. Fertile window: April 10–16. Next period: April 29.
FAQ
How accurate is the ovulation estimate? +
This calculator uses the standard assumption that ovulation occurs 14 days before the next period. For regular cycles this is a reasonable estimate, but actual ovulation can vary by several days. These figures are for planning only and not a substitute for medical guidance.
Does cycle length include the period itself? +
Yes. Cycle length is measured from the first day of one period to the first day of the next. A 28-day cycle with a 4-day period means 24 days of non-period days after bleeding stops.
What if my cycle is irregular? +
This calculator works best with consistent cycles. If your cycle varies significantly from month to month, use your average over the last 3–6 cycles as the input. Consider speaking with a healthcare provider if cycles are very irregular.
What is the fertile window? +
The fertile window includes the five days before ovulation and the day after, as sperm can survive for several days. The most fertile days are the two days before and the day of ovulation itself.
How many cycles does this predict? +
The calculator shows the current cycle (from your last period) plus three upcoming cycles, giving you a four-cycle outlook.
About this calculator
This ovulation calculator estimates your next period date, ovulation day, and fertile window based on the first day of your last period and your average cycle length. It also projects three upcoming cycles to help you plan ahead. These are statistical estimates for informational purposes and are not a substitute for medical advice.
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