Ovulation and Fertile Window Calculator
Estimate ovulation and the fertile window for this cycle and the next two.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
next period = first day of the last period + cycle length ovulation = next period − 14 days fertile window = the 5 days before ovulation, and the day itself
Fourteen days before the next period is an average, and the real day can be earlier or later. The window starts before ovulation because sperm can survive for several days.
What to watch out for
Use the average of a few recent cycles if the length changes a little from month to month. Irregular cycles make these dates unreliable.
Frequently Asked Questions
When does ovulation happen?
About 14 days before the next period, which is an average. From 1 January 2027 with a 28-day cycle, the estimate is 15 January 2027.
What is the fertile window?
The 5 days before ovulation and the day itself, because sperm can survive for several days. In the same example, that runs from 10 January 2027 to 15 January 2027.
Can I use this as contraception?
This is only an estimate and is not reliable as contraception, because cycles vary and sperm can survive for several days.
What if cycles are irregular?
These dates become unreliable, because they rely on a predictable cycle length. Periods stopping unexpectedly, severe pain or unusual bleeding are worth raising with a doctor or midwife.