Period Calculator
First day of your last period
Usual cycle length
days
Counted from the first day of one period to the first day of the next.
How much it usually varies
days
If your cycles run 26 to 31 days, that is about 3 either side of the middle.
How many days it lasts
days
Today
Next period starts
Mon, October 12, 2026
Show the work
Period Calculator
by Rule Calculator