Period Calendar and Next Period Predictor

What it is

The period calendar takes the first day of your last period, your average cycle length and how many days you bleed, then lists the next 3 to 6 expected start and end dates and draws a month-by-month calendar with the period days shaded. It also counts the days from a date you choose, today by default, to the next expected start. The page opens with an example, a period that began 10 days ago, so you see results at once. If you save the real start dates of your periods they stay in this browser only, and once you have three or more the tool works out your average cycle and offers it to you. Predicted starts can be exported as an .ics file. It is a rough estimate for reference, it does not diagnose or treat anything, and it must not be used as birth control.

How to use

1. Enter the first day of your last period. The date shown at the start is an example.
2. Enter your average cycle (21 to 45 days), your period length (1 to 10 days) and how many predictions to show (3 to 6).
3. Read the next expected start, the countdown, the table and the calendar.
4. To keep a real start date, press the save button. Records are kept in date order, up to 24.
5. With three or more records the average cycle appears. Press the use-this-value button to copy it into the cycle field.
6. To use your calendar app, press the download button for the .ics file.

How it works

• Expected start = last start + cycle length x prediction number. Expected last day = expected start + period length - 1 day.
• Countdown is the number of days from the date checked to the first expected start on or after it. On that day it shows as today.
• Average cycle puts saved start dates in order and averages the gaps between neighbours. The shortest and longest gaps are shown too, and a spread above 7 days triggers an irregularity warning. The rounded average can only be copied into the cycle field when it falls between 21 and 45 days.
• Storage is the browser’s local storage. If it is blocked, the page says so and works for the current screen only.
• The .ics file has one all-day event per predicted start, titled Predicted period start.
• Terms follow the KDCA National Health Information Portal guide to understanding the normal menstrual cycle (checked via a summary). The prediction itself is a plain estimate, the start date plus the cycle length. (Accessed 2026-10-04)
• Dates use calendar days with no time zone, and 29 February is handled in leap years.

Examples

A last period starting on 1 January 2026, a 28-day cycle and a 5-day period.

No.	Expected start	Expected last day
1	2026-01-29	2026-02-02
2	2026-02-26	2026-03-02
3	2026-03-26	2026-03-30

If your saved starts are 1 January, 29 January, 28 February and 29 March, the gaps are 28, 30 and 29 days and the average is 29. These dates are only an example.

Good to know

Cycles can shift with stress, illness, weight changes, childbirth and breastfeeding. This prediction assumes an average, can be wrong, and does not diagnose or treat anything. If your periods are very irregular or suddenly change, ask a clinician. The dates you enter and the records you save stay in this browser.