How this calculator works
Enter the first day of your last period, your average cycle length, and your average period length, and this calculator projects your next period and the five that follow, assuming both lengths stay roughly constant.
Menstrual cycles are rarely perfectly regular - stress, illness, travel, medication, and many other factors can shift timing from month to month. This calculator's projections use simple arithmetic from averages you provide; they are planning estimates, not guarantees, and become less reliable the further into the future they project. This tool is not a reliable method of contraception or a way to avoid pregnancy - if you are trying to avoid pregnancy, use a method your healthcare provider has confirmed is effective for you.
How each period is projected
Period N's start date = last period's start + (N × cycle length) daysPeriod N's end date = period N's start + (period length − 1) daysCycle length is measured from the first day of one period to the first day of the nextWorked example: last period January 1, 28-day cycle, 5-day period
- Next period start: January 1 + 28 days = January 29 (January has 31 days, so no month rollover yet).
- Next period end: January 29 + 4 days = February 2 (a 5-day period runs from day 1 through day 5, so the end date is 4 days after the start).
- Second projected period start: January 29 + 28 days = February 26.
- Each subsequent row simply adds another 28 days to the previous start date.
Frequently asked questions
How far in advance are these predictions reliable?
The further out a prediction reaches, the more small month-to-month variations can accumulate. Treat the next 1-2 predicted periods as reasonably useful and later rows as rougher estimates, especially if your cycles are not very regular.
What counts as the start of a period for cycle length?
The first day of full flow (not spotting) is the conventional start day, both for the cycle length you enter and for the last-period date.
My periods vary by several days each month - what should I enter?
Use your average or most typical cycle length over the last few months. If variation is large, consider the projected dates a rough window rather than exact days.
Why does my period length matter here?
It only affects the projected end date of each period; the start-date projections depend solely on your cycle length. A longer or shorter typical period simply shifts the end date shown for each row.
What would my next period date be if my cycle is 30 days instead of 28?
It would start on January 31 rather than January 29, for a last period beginning January 1. Each projected start date is the last period's start plus the cycle length in days, so a 30-day cycle simply adds 30 days instead of 28 - and every row in the table shifts by that same 2-day difference.