Ovulation Calculator
Estimate your ovulation day, fertile window and next expected period from your last period and cycle length.
Estimate your fertile window
Enter the first day of your last period and your average cycle length.
Result
Formula used
Ovulation day = LMP + (cycle length − 14) days
The fertile window spans 5 days before ovulation through 1 day after, covering sperm survival time and the egg's short viability window.
How to use this calculator
- Enter the first day of your last period.
- Optionally enter your average cycle length — otherwise 28 days is assumed.
- Press Calculate to see your estimated ovulation day and fertile window.
This tool provides general informational estimates only and is not a substitute for medical advice.
FAQ
Frequently asked questions
How is ovulation day estimated?
Ovulation typically occurs 14 days before your next period, so it's estimated as the first day of your last period plus (cycle length minus 14) days.
What is the fertile window?
The fertile window is the estimated 6-day span with the highest chance of conception — about 5 days before ovulation through the day after.
Does cycle length affect the result?
Yes. Longer cycles push ovulation later and shorter cycles bring it earlier, since ovulation is counted backward from the next period.
Is this a reliable form of contraception or conception planning?
No. This is a statistical estimate for general informational purposes only, not a medical or contraceptive method.