Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your ovulation date, fertile window (the 6 days most likely to result in conception), and next expected period.
Reviewed by Richard Ross · Last updated April 2026
How Ovulation Calculator works
The menstrual cycle and ovulation
A typical menstrual cycle lasts 21–35 days, counted from the first day of one period to the first day of the next. Ovulation — the release of an egg from the ovary — typically occurs 14 days before the next period, regardless of overall cycle length. For a 28-day cycle this is around day 14; for a 35-day cycle, around day 21.
The fertile window
The fertile window is the period during which conception is possible. Sperm can survive in the female reproductive tract for up to 5 days, while an egg survives only 12–24 hours after ovulation. This creates a fertile window of approximately 6 days: the 5 days before ovulation and the day of ovulation itself. The chance of conception is highest in the two days before and on the day of ovulation.
Limitations of calendar-based ovulation tracking
This method assumes regular cycle lengths and consistent timing of ovulation. In reality, ovulation can shift by several days each cycle due to stress, illness, travel, or hormonal fluctuations. Women with irregular cycles will find LMP-based calculations less reliable. Ovulation predictor kits (OPKs), which detect the LH surge that precedes ovulation, are more accurate for irregular cycles.
Not a contraceptive method
This calculator is designed to help those trying to conceive, not to prevent pregnancy. The "rhythm method" or calendar-based contraception has a typical-use failure rate of 24% per year — far higher than barrier methods or hormonal contraception. If you are trying to avoid pregnancy, speak to a healthcare provider about appropriate contraception.
Frequently asked questions
How do I know when I'm ovulating?
Signs of ovulation include a rise in basal body temperature (0.2–0.5°C above baseline), changes in cervical mucus (becoming clear, slippery, and stretchy — similar to raw egg white), mild pelvic pain (mittelschmerz), and an LH surge detectable by ovulation predictor kits. OPKs are the most practical way to confirm ovulation timing.
Can I get pregnant outside my fertile window?
The probability is very low but not zero. Sperm can survive for up to 5 days, and ovulation timing can vary. For reliable contraception, do not rely on avoiding fertile days — use appropriate contraceptive methods.
My cycle is irregular — how accurate is this calculator?
For irregular cycles, calendar-based predictions are unreliable. Ovulation predictor kits, basal body temperature charting, or monitoring cervical mucus are more accurate tracking methods. If your cycle varies by more than 7 days, speak to a GP or gynaecologist as irregular cycles can indicate hormonal imbalances.
What is the best time to have sex when trying to conceive?
The highest conception probability is in the two days before ovulation and on the day of ovulation itself. Having sex every other day from about 4–5 days before your estimated ovulation date through the day after gives the best coverage. Daily sex is no more effective and may reduce sperm quality in some men.
What is the fertile window?
The fertile window is the period during which pregnancy is possible from unprotected sex. Because sperm can survive in the reproductive tract for up to 5 days and an egg survives only 12-24 hours after ovulation, the fertile window spans approximately 6 days: the 5 days before ovulation and the day of ovulation itself. The highest probability of conception comes from intercourse in the 1-2 days before ovulation and on the day itself.
How can I tell when I am ovulating?
Several methods indicate ovulation. Basal body temperature (BBT) rises by approximately 0.2°C after ovulation — tracking this daily with a basal thermometer identifies the pattern over several months. Cervical mucus changes: around ovulation it becomes clear, stretchy, and egg-white-like (compared to dry or creamy at other times). LH (luteinising hormone) surge tests detect the hormone spike that triggers ovulation 24-36 hours before it occurs. Ovulation pain (Mittelschmerz) is a one-sided cramping felt by about 20% of women at ovulation.
Does stress affect ovulation?
Yes. Chronic stress elevates cortisol, which can suppress GnRH (gonadotropin-releasing hormone) and disrupt the hormonal cascade that triggers ovulation. This can delay or prevent ovulation entirely, lengthening the cycle or causing anovulatory cycles (cycles without ovulation). Acute short-term stress around the time of ovulation can also delay it. Women with irregular cycles from stress often find that cycle regularity improves when stress is reduced, though this varies significantly between individuals.
Is it possible to get pregnant outside the fertile window?
The probability is very low but not zero. Cycle length varies month to month even in regular cycles, meaning ovulation timing shifts. If a woman ovulates earlier or later than usual in a given cycle, intercourse outside the predicted fertile window could coincide with an unexpected fertile time. No method of natural family planning based on cycle tracking is 100% effective — the fertility awareness method (FAM) has a typical-use failure rate of around 24% per year.
How does age affect ovulation and fertility?
Ovarian reserve (the number and quality of eggs) declines with age. Women are born with all the eggs they will ever have — approximately 1-2 million at birth, declining to around 400,000 at puberty. By the mid-30s, egg quality and quantity decline more rapidly. At 30, the monthly chance of conception is approximately 20%. By 40, it falls to around 5%. Ovulation still occurs regularly in many women in their late 30s and early 40s, but the eggs are more likely to have chromosomal abnormalities, increasing miscarriage risk.
What is PCOS and how does it affect ovulation?
Polycystic ovary syndrome (PCOS) is one of the most common hormonal disorders in women of reproductive age, affecting approximately 1 in 10. It is characterised by irregular or absent ovulation, elevated androgens (male hormones), and polycystic ovaries on ultrasound. Women with PCOS may not ovulate every month — or at all — making cycle-based ovulation prediction unreliable. Treatment options for PCOS-related fertility issues include clomifene citrate (to trigger ovulation), metformin (for insulin resistance), and in some cases IVF.
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This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.