Fertile Window Calculator
Find the days in your cycle when pregnancy is most likely — your fertile window.
The Fertile Window Calculator is being built
This free calculator will appear here shortly. Meanwhile, the formula and ranges below show exactly how it works.
- First day of last period
- Average cycle length (days)
- Fertile window dates
- Peak fertility days
About the Fertile Window Calculator
Sperm can survive up to about 5 days and an egg about 12–24 hours, so the fertile window is the 5 days before ovulation plus ovulation day.
Formula explained
Fertile window = ovulation day − 5 days → ovulation day
Ovulation day ≈ first day of last period + (cycle length − 14). The 2 days just before ovulation are usually the most fertile.
Frequently asked questions
Can the fertile window be used for contraception?
Calendar methods are not very reliable for avoiding pregnancy, especially with irregular cycles. Talk to a doctor about effective contraception.
How long should we try before seeing a doctor?
After 12 months of trying if under 35, or after 6 months if 35 or older. Sooner if periods are irregular.
Medical disclaimer: These tools give general information and estimates only. They are not a diagnosis and do not replace advice from a qualified doctor. Always talk to your doctor about your results, and seek urgent medical help in an emergency.