DelaDela Tools · runs on your device
Ovulation calculator
A calendar estimate of a fertile window, using your period date and usual cycle length. It estimates timing rather than confirming ovulation.
Your dates
Enter your details to see your estimate.
Your estimate
Enter your dates to see an estimate.
Actual timing can differ from these dates. Temperature, tests and cervical fluid can each add helpful context to the picture.
How this is calculated
Ovulation day = max(period length + 1, min(cycle length − 3, cycle length − 14)). This starts from cycle length minus 14 and bounds the estimate using the lengths you entered. The displayed fertile window runs from five days before the estimated ovulation day through that day. This calculator cannot confirm when ovulation occurs. Read more about the app's estimates and their limits in our methodology.
The app may display an additional day around its estimate. These are display conventions, not confirmed fertile or safe-day boundaries. The fertile window guide explains the biological context.
Never use this as contraception. Calendar estimates cannot reliably prevent pregnancy, and this tool is not medical advice.