Period Calculator
Add your cycle length to the first day of your last period to predict the next one. For a 28-day cycle that began on 1 January 2024, the next three periods are expected on Jan 29, Feb 26 and Mar 25, with ovulation around Jan 15. This is an estimate, not medical advice.
How to use
- Enter the first day of your last period, your usual cycle length in days and how many days your period usually lasts. If you are unsure, leave 28 and 5.
- Choose how many upcoming periods to show, from 3 to 6.
- Read the dates in the table and find them on the calendar below. Red days are the period, the outlined days are the fertile window, and the filled day is the estimated ovulation.
How it works
Cycle length is counted from the first day of one period to the day before the next one. The next period is predicted as the first day of your last period plus the cycle length, and each following period adds the cycle length again. The period itself covers the number of days you entered.
Ovulation and the fertile window use the same method as the Ovulation Calculator on this site: ovulation is estimated 14 days before the next period starts, and the fertile window is the 5 days before ovulation plus the day of ovulation.
Real cycles vary by a few days from month to month, so predictions get less reliable the further ahead they go. Stress, illness, travel, breastfeeding, hormonal birth control and conditions such as PCOS can all shift a cycle.
This is NOT birth control and not medical advice. It cannot tell you whether you are pregnant. See a doctor if you miss several periods in a row, if cycles are often shorter than 21 or longer than 35 days, or if bleeding is very heavy or painful.
Values used
| Item | Value |
|---|---|
| Cycle length accepted | 21 to 45 days |
| Period length accepted | 2 to 10 days |
| Upcoming periods shown | 3 to 6 |
| Ovulation | 14 days before the next period starts |
| Fertile window | The 5 days before ovulation, plus ovulation day |
Sources and disclaimer
Cycle and period lengths: NHS and Office on Women's Health (US HHS) guidance on the menstrual cycle. Fertile window: Wilcox, Weinberg and Baird, New England Journal of Medicine 1995.
This tool is not medical advice and is not a contraceptive method.
Sources: NHS · Office on Women's HealthLast reviewed 2026-10-15
FAQ
How is the next period date calculated?
The tool adds your cycle length to the first day of your last period. With a 28-day cycle starting on 1 January, the next period is expected on 29 January. Later periods add the same number of days again.
What is a normal cycle length?
Adult cycles commonly run 21 to 35 days and often vary by a few days each month. The tool accepts 21 to 45 days. If yours is often outside that range, ask a doctor.
My period is late. Am I pregnant?
This calculator cannot say. A late period has many possible causes. A home pregnancy test is reliable from the first day of a missed period, and a doctor can help if the test is negative but your period still does not come.
Is my data sent anywhere?
No. The dates are calculated in your browser and nothing you enter leaves your device.