Ovulation & Fertile Window Calculator
Calculate your estimated ovulation date and fertile window. Based on the standard luteal phase model and adjusted for your cycle length.
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When Ovulation Occurs?
Ovulation typically occurs 14 days before your next period (the luteal phase), NOT 14 days after your last period. This only coincides for a perfect 28-day cycle. For a 35-day cycle, ovulation is around day 21. For a 24-day cycle, around day 10.
What's the key thing to understand about The Fertile Window?
Sperm can survive 3–5 days in the reproductive tract. An egg is viable for 12–24 hours after ovulation. The fertile window is therefore approximately 5 days before ovulation through the day after. The most fertile days are the 2–3 days leading up to and including ovulation.
What do I need to know about Natural Fertility Signs?
Cervical mucus becomes clear, slippery, and egg-white-like around ovulation — the most reliable natural sign. Basal body temperature (BBT) rises slightly (0.2–0.5°C) after ovulation. Ovulation predictor kits (OPKs) detect the LH surge 24–36 hours before ovulation occurs.
When to Seek Medical Advice?
Calculator results are estimates based on population averages and statistical formulas — they are not medical diagnoses. Consult your GP if calculator results suggest a health concern, if you have symptoms requiring attention, or if you are managing a chronic condition where precise monitoring is important. Regular health checks (annual GP review, NHS Health Check for those aged 40-74) provide professional assessment using validated clinical tools. Use health calculators to inform conversations with your GP, not to replace them. These tools give a useful starting estimate, but only a healthcare professional can assess your individual circumstances — always seek medical advice for any persistent, worsening, or concerning symptoms rather than relying on a calculator result alone.