How to use the ovulation calculator
- Enter the first day of your last period.
- Enter your average cycle length: the number of days from the first day of one period to the first day of the next. If it varies, use the average of your last few cycles.
- Leave the luteal phase at 14 days unless you have tracked it. This is the time between ovulation and your next period, and it is fairly constant for each person.
- See your fertile window, likely ovulation day and next period for the coming cycles.
How the fertile window is estimated
Fertile window = 5 days before ovulation + ovulation day + 1 day after
Sperm can survive in the body for up to about 5 days, while the egg lives for about 12–24 hours after ovulation. That is why the most fertile days are the two or three days before ovulation and the day itself.
Example
Your period started on 14 October and your cycle is 28 days. Your next period is expected on 11 November, so ovulation is likely around 28 October and your fertile window runs from about 23 to 29 October.
Signs of ovulation
- Clear, stretchy, slippery cervical mucus, similar to raw egg white.
- A small rise in resting body temperature (about 0.2–0.5 °C) after ovulation.
- A positive result on an ovulation (LH) test, usually 24–36 hours before ovulation.
- Mild one-sided pelvic pain in some women.
When calendar estimates are less reliable
Calendar methods work best with regular cycles. They are less accurate after stopping hormonal contraception, after giving birth, while breastfeeding, near menopause and with conditions such as polycystic ovary syndrome (PCOS). If you have been trying to conceive for 12 months (or 6 months if you are 35 or over) without success, see a doctor.
Frequently asked questions
Can I use this to avoid pregnancy?
No. Ovulation can come earlier or later than expected, so a calendar alone is not a reliable method of contraception. Ask a health professional about effective methods.
When can I take a pregnancy test?
Most home tests are reliable from the first day of your missed period. The calculator shows that date.
What if my cycles are irregular?
Use your average and treat the dates as rough. Ovulation tests and tracking signs give a better picture.
Is my data stored?
No. Everything stays in your browser.
Sources: Wilcox AJ et al., "Timing of sexual intercourse in relation to ovulation", New England Journal of Medicine, 1995; NHS, "Trying to get pregnant"; American College of Obstetricians and Gynecologists, "Fertility awareness-based methods of family planning".
More health calculators
Last reviewed: 2026-11-09. These tools give general information, not medical advice. Talk to a doctor about your health. See our medical disclaimer.