Period CalculatorFree online fitness & health calculator — instant results, no sign-up
Predict your next periods.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Period Calculator
The Period Calculator forecasts the calendar dates for your next four menstrual cycles based on a standard recurring interval. It helps individuals track future menstruation dates for travel planning, event scheduling, or general health monitoring. The tool calculates start and end dates using your last menstrual period date, cycle duration, and bleeding duration.
- First day of last period
- The calendar start date of your most recent menstrual cycle bleeding.
- Cycle length
- The average number of days from the start of one period to the start of the next, typically between 21 and 35 days.
- Period length
- The typical duration of menstrual bleeding in days, typically between 3 and 7 days.
Formula and method
Start Date_i = LMP + (i × Cycle Length) End Date_i = Start Date_i + Period Length − 1
- iCycle index for upcoming periods (1, 2, 3, and 4)
- LMPFirst day of the last menstrual period (date)
- Cycle LengthNumber of days between cycle starts
- Period LengthNumber of days of menstrual flow
For each of the next four cycles (i = 1 to 4), the start date is determined by adding i times the cycle length to the start date of your last period. The ending date of each bleeding phase is computed by adding the period length minus one day to that cycle's start date, ensuring the first day itself is counted as day one of bleeding.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- First day of last period1969-11-02
- Cycle length28 days
- Period length5 days
Results
- Period 1Sun, November 30, 1969 – Thu, December 4, 1969
- Period 2Sun, December 28, 1969 – Thu, January 1, 1970
- Period 3Sun, January 25, 1970 – Thu, January 29, 1970
- Period 4Sun, February 22, 1970 – Thu, February 26, 1970
How to use the Period Calculator
- 1Identify the start date of your most recent period (LMP).
- 2Multiply your cycle length in days by the cycle number (1 through 4) and add that number of days to the LMP to find each cycle's start date.
- 3Add your period length in days to the cycle start date and subtract one day to determine each cycle's end date.
- 4Display the resulting date ranges formatted by day of the week, month, day, and year.
Reading your result
Each result line displays the projected date range for that cycle's menstrual flow, from the first bleeding day to the final bleeding day. Period 1 is the next upcoming period, followed consecutively by Periods 2, 3, and 4. These ranges assume your cycle length and bleeding duration remain constant over the next four cycles.
Limitations
The calculator assumes an unvarying cycle length, but natural menstrual cycles frequently fluctuate by several days due to stress, hormonal shifts, illness, or lifestyle changes. It does not track ovulation, luteal phase variations, or irregular cycles caused by conditions like PCOS, nor should it be relied upon as a method of contraception.
Common questions
How is cycle length measured?+
Cycle length is counted from the first calendar day of bleeding in one period up to, but not including, the first day of bleeding in the following period.
Why does the calculation subtract one day for the end date?+
Menstrual flow includes the start day itself. For example, a 5-day period starting on a Monday runs through Friday (Monday, Tuesday, Wednesday, Thursday, Friday), which is start date + 4 days.
Can this calculator predict ovulation or fertile windows?+
No, this calculator only projects the start and end dates of menstrual bleeding based on the entered cycle length and period length.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.