Ovulation Calculator
Estimate ovulation and fertile windows, compare projected cycles and download a private calendar. Not contraception advice.
Private by design
Your inputs stay on your device and are not sent to our servers. Optional usage analytics does not receive your files or tool inputs.
Understand your cycle
Estimates only. Do not use these dates to identify safe days or prevent pregnancy.
Assumptions and clinical sources
Expected next period = period start + cycle length. Estimated ovulation = next period minus the selected luteal phase. The highlighted window covers five days before through one day after that estimate. Ovulation varies; cycles, illness and other factors can shift it. Calendar results cannot confirm ovulation, diagnose fertility or identify safe days for contraception.
NHS: Periods and fertility ↗Health New Zealand: Timing for conception ↗Calculated locally. Inputs are not uploaded or saved by this tool.
A clearer calendar, privately
Enter your period date and usual cycle length.
How to use Ovulation Calculator
- Enter the first day of your last period and usual cycle length.
- Adjust period duration, regularity and luteal-phase assumptions.
- Review projected cycles and download a summary or estimated calendar.
What are the limits of Ovulation Calculator?
Calendar estimates cannot confirm ovulation or diagnose fertility. Irregular cycles reduce reliability. Do not use this tool to identify safe days or prevent pregnancy. Inputs stay in browser memory.