Period Tools

Period Tools’ Period Calculator helps you plan ahead by predicting your next 6 periods, estimated ovulation days, and your most fertile window. Enter the first day of your last period, your typical cycle length, and your average period (bleeding) length to generate a calendar view.

The calculator estimates your next period by projecting forward from your last period start date, then estimates ovulation as 14 days before that predicted next period. It highlights the fertile window as the five days leading up to ovulation plus ovulation day, since sperm can survive inside the body for up to five days.

Your inputs run in your browser with no data sent to a server or stored in a database. Use results for general planning only, not as medical advice or a guarantee of timing. If you’re dealing with irregular cycles or any health concerns, consider speaking with a healthcare provider.

How do I get predictions for my next 6 cycles?

Provide the first day of your last period, your average cycle length (from one period start to the next), and your average bleeding length. The calculator then produces period dates, an estimated ovulation day, and the fertile window for the next six cycles.

How is ovulation estimated?

Ovulation is estimated using a luteal-phase method: it’s calculated as 14 days before the predicted next period start date. The fertile window is then the five days leading up to ovulation plus ovulation day.

What is the fertile window in this calculator?

The fertile window is the six-day span made up of the five days before ovulation and ovulation day itself. Conception is possible across the full window, not only on the single ovulation day.

Can I use the calculator if my cycles are irregular?

You can, but treat results as approximate. If your cycle length varies widely, consider using your longest recent cycle length for later planning and your shortest for early estimates, and talk with a healthcare provider if irregularity may have a medical cause.

Does the Period Calculator save my data?

Your inputs stay in your browser and are not sent to a server or saved in a database. Refreshing the page clears your inputs.

How accurate are period and ovulation predictions?

For regular cycles, predictions typically land within one to two days most of the time. Accuracy drops when cycles vary by more than about a week between months, or after changes like coming off hormonal birth control.