Period Calculator: Estimate Your Next Period

Enter the first day of your last period and your usual cycle length to see estimated upcoming dates on your menstrual calendar.

Calendar estimates for general information only. Dates may vary. Do not use for diagnosis, medical decisions, or contraception.

How the period calculator works

The calculator adds your average cycle length to the period start date you enter. It then uses your usual bleeding duration to estimate the end date. It does not measure hormones or detect changes in your body.

A worked example

If your last period started on October 1 and your cycle is 28 days long, the next estimated start is October 29. With five days of bleeding, October 29 through November 2 are highlighted, including both dates. Changing the bleeding duration changes the estimated end, not the next start.

Recording and calculating cycle length

Record the actual first day of each period and count the days until the next one starts. You need two start dates to measure one complete cycle. Record bleeding duration separately, and keep predicted dates distinct from periods that have actually happened.

Four start dates give you three complete cycles. Cycles of 27, 29, and 28 days average 28 days. Cycles of 24, 28, and 32 days have the same average but vary more. Keep the individual lengths alongside the average; do not invent a date you cannot remember.

Reading your menstrual calendar

Dark rose marks the period you entered; light pink shows later estimates. Each prediction repeats the same cycle length. Compare predicted dates with what you observe, but remember that a difference alone does not explain its cause.

Why period dates can change

Cycles can change from month to month. Stress, weight changes, hormonal contraception, and stages such as puberty or perimenopause can affect the pattern. The calculator cannot distinguish these causes or confirm or rule out pregnancy.

If your cycles are irregular

An average can hide substantial differences between cycles. This tool accepts cycles of 21–35 days and bleeding durations of 2–7 days. These are tool limits, not a definition of a healthy cycle. Do not change your data to make it fit. Discuss changes or concerns with a healthcare professional.

Period and ovulation predictions are different

Predicting your next period does not confirm when you ovulate. The ovulation calculator estimates a date; the fertile window calculator estimates a range of days. Neither should be used to avoid pregnancy.

Frequently asked questions

How is my next period estimated?

We add your average cycle length to the first day of your last period. Your usual period length determines how many days are highlighted. For example, a 28-day cycle places the next estimated start 28 days after the date you enter.

Will my period start on the date shown?

The date is an estimate, not a guarantee. The tool repeats the cycle length you enter without detecting changes in your body. Results refer to the cycle starting on your entered date, so some estimates may already be in the past.

Can I use this with irregular periods?

This tool accepts cycles of 21–35 days and periods lasting 2–7 days; these are input limits, not diagnostic cutoffs. If your pattern falls outside them, do not substitute a different value. Irregular cycles, hormonal contraception, and changes after pregnancy can make forecasts unreliable. Discuss persistent changes with a healthcare professional.

What do the highlighted calendar dates mean?

The darker rose highlight marks the period start you entered, extended by your usual period length. Lighter pink dates show estimated following periods across the three displayed months. We do not reconstruct earlier cycles.

How do I calculate my average cycle length?

Count from the first day of one period through the day before the next period starts. Add the lengths of several completed cycles, then divide by the number of cycles. For example, 27, 29, and 28 days average 28 days. An average does not remove cycle-to-cycle variation.

What is the difference between cycle length and period length?

Cycle length is the interval between the start of one period and the start of the next. Period length is how many days the bleeding lasts. For example, you might have a 28-day cycle with five days of bleeding. Enter these separately so the calendar can estimate both start and end dates.

Reviewed by a physician