When is the best time to conceive?
1-2 days before ovulation offers peak fertility when sperm are waiting for the egg.
Health Fitness
Fertility Window Calculator
Calculate fertility window with your details.
Personalized estimate
Use realistic inputs for the most useful results.
What this calculator does
A fertility window calculator identifies the days in your menstrual cycle when conception is most likely. The fertile window spans 5 days before ovulation through 1 day after—about 6 days total. Ovulation typically occurs 14 days before your next period, so tracking cycle length helps predict fertile days.
How it works
Enter your last menstrual period date and cycle length. The calculator estimates ovulation (cycle length - 14 days) and shows the 6-day fertile window.
Formula
Ovulation = LMP + (Cycle Length - 14). Fertile Window = Ovulation - 5 days to Ovulation + 1 day.
Tips for using this calculator
- Track cycles for 3+ months for accuracy
- Combine with ovulation predictor kits
- Peak fertility is 1-2 days before ovulation
- Sperm survive up to 5 days; eggs survive 12-24 hours
- Irregular cycles reduce prediction accuracy
Frequently asked questions
How accurate is the calculator?
Good for regular cycles. Irregular cycles benefit from ovulation kits or tracking.
Can I use this to avoid pregnancy?
Not recommended as sole method—sperm longevity and cycle variation make timing uncertain.