🌸 Ovulation Calculator
Ovulation usually happens about 14 days before your next period starts, and the egg survives roughly a day while sperm can wait up to five — that is why the fertile window stretches several days before ovulation day itself.
Give the first day of your last period and your average cycle length, and the calculator marks your ovulation day, your full fertile window and your next expected period.
How to use this calculator
Enter the first day of your last period and your average cycle length to find your ovulation day and fertile window.
- Pick the First day of last period on the date field.
- Enter your Average cycle length in days.
- Read the headline ovulation day.
- Note the fertile window and the next expected period below it.
Frequently asked questions
How is ovulation day estimated?
Ovulation usually happens about 14 days before the next period, so the tool adds your cycle length minus 14 days to your last period start. With a last period of 1 September 2026 and a 28-day cycle, ovulation falls on 15 September 2026.
Why is the fertile window six days long?
The egg survives roughly a day but sperm can wait up to five, so the window runs from five days before ovulation through the day after. That is 10 to 16 September in the example above.
Can I rely on this to avoid pregnancy?
Not alone. Cycles vary with stress, illness and travel, and this is a calendar estimate. Combine it with a proper fertility tracking method if pregnancy prevention matters.
What if my cycles are irregular?
The estimate gets less reliable the more your cycle length swings. Track several months to find your true average, or talk to your doctor if cycles are very unpredictable.