Ovulation Calculator
Enter the first day of your last period and your average cycle length, a whole number from 21 to 35 days. The calculator shows your estimated ovulation date, a 7-day fertile window around it and the day your next period should start. It works from the calendar alone, so treat the dates as estimates.
How to use the Ovulation Calculator
- Enter your numbers in the Ovulation Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The ovulation prediction
Ovulation = first day of last period + (cycle length − 14) days · · · Fertile window = 5 days before ovulation to 1 day after · · · Next period = first day of last period + cycle length
The calculator assumes ovulation comes 14 days before your next period, not 14 days after your last one. So it counts cycle length minus 14 days from the first day of your last period. The fertile window it shows runs seven days, from 5 days before that date to the day after it.
Worked example
Last period started January 5, 2026, with a 28-day cycle: ovulation January 19, fertile window January 14 to January 20, next period February 2. With a 30-day cycle every date moves 2 days later, to ovulation on January 21 and the next period on February 4.
Frequently asked questions
How accurate is calendar tracking?
It is an estimate that assumes ovulation 14 days before your next period. If your cycle length changes from month to month, the real date moves too. Ovulation predictor kits (LH strips) and basal body temperature tracking measure your own cycle instead.
When in the fertile window is conception most likely?
In a 1995 study of 221 women published in the New England Journal of Medicine, Wilcox and colleagues found that pregnancies came only from sex in the 6 days that ended on the day of ovulation. The window this calculator shows runs one day longer, to the day after ovulation.
What cycle lengths does it accept?
Whole numbers from 21 to 35 days. For anything else, such as 36 or 28.5, the calculator shows a message and no dates. It does not round your number or fall back to 28 days.
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.