Ovulation Calculator – Fertile Window Estimate
Estimate your fertile window from cycle length and last period date.
- Free
- Instant result
- Private · on-device
- No sign-up
How to use: Enter the first day of your last period and average cycle length. See estimated ovulation and next period dates.
How to use the Ovulation Calculator
Enter the first day of your last menstrual period and your average cycle length (often 28 days, but many people range from about 21 to 35). The calculator estimates ovulation day and a fertile window using a standard calendar method.
Ovulation is estimated about 14 days before the next period starts. The fertile window usually covers a few days before ovulation plus ovulation day, because sperm can survive several days and the egg about a day.
This is an educational estimate only—not medical advice and not a contraceptive method. Cycle length varies with stress, illness, travel, and hormonal conditions. Apps and clinic guidance that use LH tests or ultrasound are more precise.
If you are trying to conceive, combine calendar estimates with other signs your clinician recommends. If cycles are highly irregular, talk to a healthcare professional rather than relying on a single average length.
Related tools on this site include the pregnancy due-date calculator once conception is confirmed, and the date-difference tool for counting days between appointments.
Frequently asked questions
How is ovulation estimated?
Often cycle length minus luteal phase (commonly ~14 days) after LMP day 1.
Is this a contraception method?
No. Do not rely on calendar estimates alone to prevent pregnancy.
What if my cycles are irregular?
Estimates are less reliable — speak with a clinician and consider ovulation tests.
Can I estimate my due date from ovulation?
Yes once pregnancy is confirmed; clinicians may use LMP or early ultrasound.