Fertility Window Calculator
6-Day Fertile Period Prediction
How It Works
The Science
Your fertile window is the 6-day period ending on ovulation day: 5 days before plus ovulation day itself. This is based on Wilcox et al. (1995) research showing sperm can survive up to 5 days in the reproductive tract, while the egg remains viable for only 12–24 hours after release. We estimate ovulation by subtracting 14 days from your predicted next period (cycle length minus luteal phase).
How to Use
Enter the first day of your last period and your average cycle length. The calculator predicts your next ovulation and highlights the 6-day fertile window. The chart shows where you are in your current cycle. Save results to track patterns across months.
Pro Tips
- Peak fertility is the day before ovulation and ovulation day itself — aim for these if timing intercourse.
- Cycle length can vary month to month; tracking for 3+ months improves prediction accuracy.
- BBT and ovulation test strips can confirm ovulation after it happens, but this tool helps you plan ahead.
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