Donaldson R F Ovulation Calculator
Estimate your most fertile days based on the first day of your last period and your average cycle length.
How this works
Ovulation typically happens about 14 days before your next period starts. The fertile window is generally the 5 days before ovulation plus ovulation day itself, since sperm can survive several days while the egg survives about 24 hours. This calculator works best for cycles between 21 and 35 days.
This is an estimate for general reference only, not medical advice, and should not be relied on alone for contraception. Cycles can vary from month to month even when generally regular. Talk to a healthcare provider for guidance on fertility or contraception.