Period Calculator

Estimate your next period

This period calculator estimates when your next period will start and end, your likely ovulation day and your fertile window, using your own cycle length and period length. It also lists the expected dates of your next four periods.

How to use it

1. Enter the first day of your last period. It starts filled with the date 14 days ago, so change it to your real date.
2. Enter your cycle length in days (15 to 60) — the number of days from the start of one period to the start of the next.
3. Enter your period length in days (1 to 15).
4. Read the next period dates, estimated ovulation, fertile window and the upcoming cycle view.

How the dates are calculated

• Next period start = last period start + cycle length.
• Expected end = next period start + period length − 1 day.
• Estimated ovulation = next period start − 14 days.
• Fertile window = 5 days before ovulation to 1 day after it.

Example: last period started 1 October 2026, with a 28-day cycle and a 5-day period. Next period: 29 Oct to 2 Nov 2026. Estimated ovulation: 15 Oct. Fertile window: 10 Oct to 16 Oct.

Upcoming cycles in the example

Things to keep in mind

• The calculator assumes every cycle has the same length. Real cycles often vary by a few days.
• The 14-day rule for ovulation is an average; your own ovulation day may differ.
• Use it for planning, not diagnosis, and not as a method of contraception.

Frequently asked questions

How do I find my cycle length?

Count the days from the first day of one period to the first day of the next. If it changes, use an average of your last few cycles.

Is my data saved?

No. The dates are calculated in your browser and nothing is stored; reloading the page resets the form.

What if my cycles are very irregular?

The estimates will be less reliable. If your cycles are very irregular or symptoms feel unusual, speak to a healthcare professional.