Ovulation Calculator

Find your fertile window, likely ovulation day and next period for the next three cycles.

Result
Fertile window—
Most fertile days—
Estimated ovulation—
Next period—
Pregnancy test from—
Due date if you conceive—

Next 3 cycles

Period startsFertile windowOvulation

How to use the ovulation calculator

  1. Enter the first day of your last period.
  2. Enter your average cycle length: the number of days from the start of one period to the start of the next (usually 21–35).
  3. If you know it, adjust the luteal phase (the days from ovulation to your next period). Otherwise leave it at 14.

You'll see your fertile window, most fertile days, estimated ovulation date, next period and the next three cycles.

How it's calculated

Ovulation day = First day of period + (Cycle length − Luteal phase) Fertile window = 5 days before ovulation → ovulation day

Example: with a 28-day cycle starting March 1, ovulation is around March 15, the fertile window is March 10–15, and the next period is due March 29.

Signs of ovulation

  • Cervical mucus becomes clear, slippery and stretchy, like raw egg white.
  • Basal body temperature rises slightly (about 0.3–0.5 °C / 0.5–1 °F) after ovulation.
  • Ovulation test strips detect the LH surge 24–36 hours before ovulation.
  • Some people feel mild one-sided pelvic pain (mittelschmerz).

For general information only. Not a contraceptive method or medical advice. If you've been trying to conceive for 12 months (6 months if you're over 35), talk to a doctor.

Frequently asked questions

When am I most fertile?
You're most fertile in the 2–3 days leading up to and including ovulation. Sperm can survive up to five days, so the fertile window spans about six days: the five days before ovulation plus ovulation day.
How is ovulation calculated?
Ovulation usually happens about 14 days before your next period (the luteal phase). So in a 28-day cycle it's around day 14, and in a 32-day cycle around day 18. If you know your luteal phase length from tracking, enter it for a more personal estimate.
How accurate is an ovulation calculator?
It's an estimate based on averages. Cycles vary, stress and illness can shift ovulation, and many people don't ovulate exactly mid-cycle. Ovulation tests (LH strips), basal body temperature and cervical mucus tracking confirm ovulation more reliably.
When should I take a pregnancy test?
For the most accurate result, test on or after the day your period is due. Some sensitive tests can detect pregnancy a few days earlier, but early negatives are common.
Can I use this as birth control?
No. Calendar-based estimates are not reliable enough to prevent pregnancy. Talk to a healthcare provider about contraception.