Ovulation Calculator
Ovulation happens about 14 days before your next period starts. Your fertile window is the five days before ovulation plus the day itself â six days in total.
Example: For a 28-day cycle with a last period starting 1 January 2026, ovulation falls around 15 January 2026, the fertile window runs 10 January 2026 to 15 January 2026, and the next period is due 29 January 2026.
Enter the first day of your last period to see your estimate.
These are estimates from calendar dating only. Irregular cycles make them unreliable.
Find an OB-GYN near youHow this is calculated
This calculator counts back 14 days from your next expected period to estimate ovulation, then opens the fertile window five days earlier to account for sperm survival of up to five days. The egg itself stays viable for around 24 hours. Cycle lengths outside 21â35 days are clamped to that range, and irregular cycles make these dates unreliable.
This tool is for information only. It does not diagnose, treat, or replace advice from a licensed clinician. Reviewed against ACOG guidance.
Frequently Asked Questions
How do I know when I am ovulating?
Ovulation typically occurs about 14 days before your next period, not 14 days after your last one. For a 28-day cycle that lands around day 14, but for a 35-day cycle it is closer to day 21. Physical signs include clearer, stretchier cervical mucus and a small rise in basal body temperature after ovulation has already happened.
How long is the fertile window?
About six days: the five days before ovulation plus the day of ovulation. Sperm can survive in the reproductive tract for up to five days, which is why intercourse before ovulation can still lead to pregnancy.
Is this calculator accurate if my periods are irregular?
No. Calendar-based estimates assume a reasonably consistent cycle length. If your cycles vary by more than a few days, the predicted ovulation date can be off by a week or more. An OB-GYN can offer follicular monitoring or ovulation predictor kits instead.
Can I get pregnant outside my fertile window?
It is much less likely but not impossible, because ovulation can happen earlier or later than predicted. Treat the fertile window as a guide for timing, not as a guarantee in either direction.
How soon after ovulation can I take a pregnancy test?
Most home pregnancy tests are reliable from the first day of a missed period, roughly 14 days after ovulation. Testing earlier risks a false negative because hCG levels are still low.
Should I see a doctor if I am struggling to conceive?
Consider consulting an OB-GYN after 12 months of trying if you are under 35, or after 6 months if you are 35 or older. Seek advice sooner if your cycles are very irregular or absent.