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Fertility Window Calculator

Find your expanded 10-day fertility window with LH surge, peak days, and conception probability.

Ovulation calculator·Period calculator·Due date calculator

How it works

The calculator estimates ovulation as LMP + cycle length − 14. Because sperm survive up to 5 days and the egg stays viable 12–24 hours, the expanded fertility window covers 10 days (7 days before to 2 days after ovulation). The LH surge starts 24–36 hours before ovulation. Daily conception probabilities are based on Wilcox et al. 1995.

Frequently Asked Questions

What is the expanded fertility window?+
The classic window covers 6 days (5 before ovulation + the ovulation day). This calculator extends it to 10 days. Sperm occasionally survive 6–7 days, and ovulation timing varies. The expanded window gives more confidence when trying to conceive.
What does the LH surge mean?+
Luteinizing hormone (LH) spikes 24–36 hours before ovulation. Over-the-counter LH urine tests detect this surge and are the most reliable predictor of imminent ovulation.
Which day has the highest conception probability?+
Ovulation day (~33 %) and the day before (~31 %). Intercourse every 1–2 days during the peak window (day −2 to ovulation day) maximizes your chance.
How accurate is the prediction?+
For regular cycles (variation ≤ 2 days) the estimate hits in roughly 80 % of cases. For irregular cycles, PCOS, or breastfeeding, accuracy drops — combine with LH tests and basal body temperature.
What if my cycle is longer or shorter than 28 days?+
The luteal phase (post-ovulation) is almost always 14 days. In a 35-day cycle ovulation is on day 21; in a 24-day cycle it is on day 10. The calculator adjusts automatically.
Can I use this for contraception?+
No. Sympto-thermal methods (NFP) require basal body temperature and cervical mucus on top of calendar tracking. Calendar-only predictions have a Pearl Index of 9–40 and are not safe contraception.

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Background

Fertility Window Calculator: 10 Days, LH Surge & Peak Days

8 min