Period Tools

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

The calculator estimates your next period by projecting forward from your last period start date. It then estimates ovulation as 14 days before that predicted next period, using the luteal-phase method. The fertile window is highlighted 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 rather than being sent to a server or stored in a database. Results are for general informational planning only and aren’t a guarantee of timing. If you have irregular cycles, recent changes (like stopping hormonal birth control), or any health concerns, consider discussing your situation with a healthcare provider.

What inputs do I need to get predictions for the next 6 cycles?

Enter the first day of your last period, your average cycle length (days from one period start to the next), and your average period/bleeding length.

How does the calculator estimate ovulation?

It uses a luteal-phase method: ovulation is estimated as 14 days before the predicted start of your next period.

What counts as my fertile window in this calculator?

The fertile window is a six-day span made up of the five days before ovulation plus ovulation day itself, because conception is possible across the full window.

Can I use the calculator if my cycles are irregular?

You can, but treat results as approximate. If your cycle length varies widely, predictions can drift—using a longer recent cycle for later planning and a shorter one for early estimates can help.

Does the calculator store or send my period dates anywhere?

No—inputs run in your browser, aren’t sent to a server, and aren’t stored in a database. Refreshing the page clears your inputs.

Are period and ovulation predictions accurate?

For regular cycles, predictions are often within a day or two for many people, but accuracy decreases when cycles vary by more than about a week between months or after factors that change cycle timing.