Period Calculator
Enter the first day of your last period, your usual cycle length in days, how many days your period lasts and how many cycles to look ahead. Press Predict Periods. The calculator adds your cycle length to that first day once for each cycle and lists the start and end date of every period. It uses the same cycle length every time, so the dates are only as close as your average is.
How to use the Period Calculator
- Enter your numbers in the Period Calculator above.
- Press the Predict Periods button to see the result.
- To try other numbers, change any value and press the button again.
The period prediction
Start of period k = first day of last period + k × cycle length · · · End of period k = its start + period length − 1 day
k runs from 1 to the number of cycles you ask for. Cycle length is the number of days from the first day of one period to the first day of the next. A period length of 5 counts the start day and the 4 days after it. The calculator counts calendar days from the date you enter, so it gives the same dates in every time zone.
Worked example
Your last period started on April 3, 2026, your cycle is 30 days and your period lasts 5 days. The next five periods start on May 3, June 2, July 2, August 1 and August 31, and the first one ends on May 7. With a 28-day cycle they start on May 1, May 29, June 26, July 24 and August 21. The first start moves 2 days earlier, the second 4, and the fifth comes 10 days sooner.
Frequently asked questions
How long is a normal menstrual cycle?
The Office on Women's Health says the typical cycle is 28 days long, and that periods still count as regular if they usually come every 24 to 38 days. Your cycle can change from month to month. This calculator does not model that. It uses the one cycle length you enter for every cycle.
How do I find my cycle length?
Count the days from the first day of one period to the first day of the next. Mark the day each period starts on a calendar for a few months, then take the average of the gaps. Enter that average as your cycle length.
When should I talk to a doctor about my cycle?
The Office on Women's Health says to talk to a doctor or nurse if your cycles are longer than 38 days or shorter than 24 days, or if you are worried about your cycle. It also says to talk to one if you have no period for 90 days and are not pregnant or breastfeeding.