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Period Calculator

Enter your last period, its length and your cycle length to predict the next periods, ovulation and fertile windows.

Formulas last reviewed: September 2026

Disclaimer: Results are general estimates for information and education only. They are not medical advice, a diagnosis or a treatment plan. Please talk to a doctor or qualified health professional before making health, diet, exercise or pregnancy decisions.

How are periods predicted?

A menstrual cycle is counted from the first day of one period to the first day of the next. Most adults have cycles of 21 to 35 days, with bleeding that lasts 2 to 7 days.

If your cycles are fairly regular, your next period can be predicted by adding your average cycle length to the first day of your last period. The calculator also marks ovulation and the fertile window.

How it is calculated

Next period = first day of last period + cycle length
Ovulation = next period − 14 days

Worked example

Last period 1 January, 28-day cycle, 5-day period

Next periods
Thu, 29 Jan 2026, Thu, 26 Feb 2026
Bleeding days
5 days from each start
Ovulation before the 2nd period
Thu, 15 Jan 2026

When to see a doctor

  • Cycles shorter than 21 or longer than 35 days.
  • Periods lasting more than 7 days or very heavy bleeding.
  • Missing three or more periods in a row when not pregnant.
  • Severe pain, bleeding between periods or after intercourse.

Predictions are less accurate if your cycles vary. Stress, travel, illness, weight changes and some medicines can shift a period.

Frequently asked questions

Is a late period always pregnancy?

No. Stress, illness, weight change, thyroid problems, PCOS and travel can also delay periods. A pregnancy test gives a clear answer.

How do I count my cycle length?

From the first day of bleeding to the day before the next bleeding starts. Average three to six cycles for a steady number.

Can my cycle change over time?

Yes, it is common in teenagers, after childbirth and approaching menopause for cycles to change.

Are irregular periods a problem?

Occasional variation is normal. Frequent irregularity may point to conditions such as PCOS or thyroid disorders and is worth discussing with a doctor.