How the calculator works
The second half of the cycle (the luteal phase, from ovulation to the next period) is fairly constant at around 12–16 days, while the first half varies more. That's why ovulation is counted back from the next expected period rather than forward from the last one.
Why the fertile window is six days
An egg survives for about 12–24 hours after it's released, but sperm can live for up to five days in the body. So sex in the five days before ovulation can still lead to pregnancy, and the two to three days just before ovulation are the most fertile of all.
Signs of ovulation
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white.
- Basal body temperature rises by about 0.2–0.5 °C after ovulation and stays up until your period.
- Ovulation predictor kits detect the LH surge 24–36 hours before ovulation.
- Some people notice mild one-sided pain (mittelschmerz) or breast tenderness.
Irregular cycles
If your cycle length varies by more than a week, calendar estimates become unreliable. Use the average of your last few cycles, and combine it with ovulation tests or temperature tracking. Very irregular cycles can have medical causes, such as PCOS or thyroid problems, so it's worth mentioning to a doctor.
Important
Calendar methods alone aren't reliable contraception. If you've been trying to conceive for a year (or six months if you're over 35) without success, talk to your doctor. Your dates are calculated in your browser and never stored or sent anywhere.
Frequently asked questions
When do I ovulate?
Usually about 14 days before your next period. In a 28-day cycle that’s around day 14.
How many days am I fertile each month?
About six: the five days before ovulation and the day of ovulation.
Can I get pregnant right after my period?
It’s possible with short cycles, because sperm can survive up to five days and you may ovulate early.
Is an ovulation calculator accurate?
It gives a good estimate for regular cycles, but ovulation can vary. Ovulation tests are more precise.
Can I use this as birth control?
No. Calendar estimates alone are not reliable enough to prevent pregnancy.
Last updated October 2026 · More health & fitness calculators