🌸 Ovulation Calculator
Enter the first day of your last period and your usual cycle length to estimate when you ovulate and your most fertile days.
How the Ovulation Calculator works
Ovulation usually happens about 14 days before the next period starts. The fertile window is the 5 days before ovulation plus the day of ovulation, because sperm can live up to about 5 days.
Step-by-step calculation
- 1Enter last period date and cycle length.
- 2Ovulation ≈ period + cycle − 14 days.
- 3Fertile window = 5 days before through ovulation day.
Worked example
If your last period started on March 1 and your cycle is 28 days, ovulation is around March 15, the fertile window is about March 10–15 and the next period is due around March 29.
Ovulation Calculator examples
| Average cycle length (days) | Estimated ovulation |
|---|---|
| 21 | Thursday, January 8, 2026 |
| 28 | Thursday, January 15, 2026 |
| 35 | Thursday, January 22, 2026 |
| 42 | Thursday, January 29, 2026 |
Fixed values: First day of last period: Jan 1, 2026. Other inputs kept at the calculator's default values. Results calculated by this calculator.
Why use an Ovulation Calculator?
Plan for pregnancy by knowing your most fertile days.
Important factors that can affect your results
- Cycles vary month to month.
- Stress and illness shift ovulation.
- Not reliable as birth control.
Frequently asked questions
How accurate is this?
It is an estimate based on an average cycle. Cycles vary, so ovulation tests or tracking body temperature give more precise timing.
Can I use it to avoid pregnancy?
No. Calendar estimates are not reliable birth control.
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Last reviewed November 2026 · How we build and check our tools · Report a problem