Track your fertile window, ovulation day, and upcoming periods
Ovulation typically occurs 12–16 days before your next period. Sperm can survive for up to 5 days in the reproductive tract, making the 5 days before ovulation plus ovulation day itself the peak fertility window — a total of 6 days when pregnancy is possible.
The luteal phase (time between ovulation and next period) is fairly constant at 12–14 days for most women. If you have irregular cycles, tracking your basal body temperature or using ovulation test strips alongside this calculator will give more accurate results.
Enter the first day of your last menstrual period, your average cycle length, and your luteal phase length. The calculator instantly shows your estimated ovulation date, fertile window, next period date, and a 3-month color-coded calendar.
This calculator is for informational and family planning purposes only. It is not a reliable method of contraception. Consult your doctor for medical advice.
You are most fertile during the 5 days before ovulation and on the day of ovulation itself. This is called the fertile window. The day before and the day of ovulation are the two most fertile days. Having unprotected sex during this window gives you the highest chance of conceiving.
Signs of ovulation include a rise in basal body temperature (BBT), clear egg-white cervical mucus, mild one-sided pelvic pain (Mittelschmerz), and a positive ovulation predictor kit (OPK). Most women ovulate 14 days before their next period starts.
For irregular cycles, use your average cycle length over the last 3–6 months as your input. Irregular cycles make predicting ovulation harder — OPK strips or a fertility monitor will give more accurate day-by-day results.
It's unlikely but possible for women with shorter cycles (21–24 days). If your period lasts 5–7 days and ovulation happens early, sperm from sex near the end of your period could survive until ovulation.
The luteal phase is the time between ovulation and the start of your next period. It typically lasts 12–14 days and is relatively consistent. A luteal phase shorter than 10 days can sometimes make it difficult to sustain a pregnancy.
This calculator provides an estimate based on average cycle patterns. It is accurate for women with regular cycles. Individual results vary — factors like stress, illness, and hormonal changes can shift your ovulation date by several days.
Ovulation typically occurs 12–16 days before the start of the next period, not 14 days after the last period (a common misconception that only holds for textbook 28-day cycles). This calculator works backwards from your expected next period using the luteal phase — the time between ovulation and next period — which is relatively consistent within an individual at around 12–14 days. This gives a more accurate ovulation estimate than counting forward from your last period.
Sperm can survive in the reproductive tract for up to 5 days in optimal cervical mucus conditions, while an egg is viable for only 12–24 hours after release. This means conception can result from intercourse up to 5 days before ovulation, but not from intercourse more than a day after. Peak fertility is the 2–3 days immediately before ovulation.
Cervical mucus becomes clearer, slippery, and stretchy (like raw egg white) approaching ovulation — the most reliable physical sign. Basal body temperature (BBT) rises by 0.2–0.5°C after ovulation due to progesterone; daily BBT tracking with a sensitive thermometer can confirm the post-ovulatory rise (though it confirms ovulation has occurred rather than predicting it). Mittelschmerz — mild one-sided pelvic pain — is felt by some women at ovulation.
If your cycles vary by more than a week from month to month, or if you've recently stopped hormonal contraception (which can take several months to re-establish regular ovulation), predictions will be less reliable. Tracking multiple cycles improves accuracy considerably.
Fertility window methodology follows NHS guidance on the menstrual cycle.
Figures are estimates for guidance only. See about this site — how we source data and what these tools can and cannot do.
Ovulation is triggered by a surge in luteinising hormone (LH) — typically occurring 24–36 hours before the egg is released. The egg itself survives for only 12–24 hours after release. Sperm, however, can survive in the female reproductive tract for up to 5 days under favourable conditions. This means the fertile window extends from roughly 5 days before ovulation through to the day of ovulation itself — six days in total — with pregnancy most likely to result from intercourse in the 2–3 days immediately preceding ovulation.
For a woman with a regular 28-day cycle, ovulation typically occurs around day 14. However, this is an average — significant variation exists between women and within the same woman across cycles. Ovulation can be affected by stress, illness, significant weight change, travel across time zones, and hormonal factors. Apps and calculators based purely on cycle length provide an estimate, not a certainty.
Basal Body Temperature (BBT): Core body temperature rises by approximately 0.2°C (0.4°F) after ovulation due to progesterone release. By measuring temperature daily with a basal thermometer (before getting up), you can identify the post-ovulatory rise — but this confirms that ovulation has already occurred, making it useful for understanding your patterns rather than predicting future ovulation.
LH Ovulation Test Strips: These detect the LH surge in urine, typically 24–36 hours before ovulation. Testing daily from around day 10–12 of a 28-day cycle (or 3–4 days before expected ovulation) gives advance warning. These strips are widely available and relatively inexpensive, making them one of the most practical methods for timing intercourse.
Cervical Mucus Monitoring: Cervical mucus changes throughout the cycle in response to oestrogen and progesterone. At ovulation, it becomes clear, slippery, and stretchy (sometimes described as resembling raw egg white) — this consistency optimises sperm survival and transport. Monitoring these changes, combined with calendar tracking, improves prediction accuracy.
Cycle length is considered irregular when it varies by more than 7–9 days from month to month. Common causes include polycystic ovary syndrome (PCOS), thyroid disorders, hyperprolactinaemia, perimenopause, and significant changes in weight or exercise. PCOS, which affects approximately 1 in 10 women of reproductive age, typically causes irregular or absent ovulation — cycles may be 35–90+ days and ovulation may be unpredictable or absent in some months.
For women with irregular cycles, calendar-based ovulation prediction is less reliable. LH testing (using more strips, starting earlier in the cycle) or ultrasound follicle monitoring are more accurate approaches. If cycles are consistently very long, very short, or absent, investigation of the underlying cause is recommended before relying on timing-based approaches to conception.
Fertile window timing follows published research on sperm survival (Wilcox et al., NEJM 1995) and egg viability. LH surge timing references standard reproductive endocrinology guidelines. PCOS prevalence figures are from the European Society for Human Reproduction and Embryology (ESHRE). This calculator is for informational and family planning purposes only and is not a reliable method of contraception. Always consult a healthcare provider for fertility concerns.
Researched and maintained by Iulian, founder of Flux Media Systems. General information, not professional advice — about this site & our sources →