Everyday
Period Calculator
Estimate your next three period start dates from your usual cycle length. Change the inputs below and the estimate updates immediately.
Women’s health · calendar estimates
Cycle timing varies. These dates cannot confirm ovulation and must not be used to prevent pregnancy.
Your calculator inputs stay in this page and are not saved or sent to us.
Input
Your numbers
Calculated result
Predicted period start 1
October 12, 2026
Calendar estimate; actual timing may vary.
Predicted period start 2
November 9, 2026
Calendar estimate; actual timing may vary.
Predicted period start 3
December 7, 2026
Calendar estimate; actual timing may vary.
Estimate only. Verify important decisions with an appropriate professional or official source.
How to use it
A quick estimate, without the spreadsheet.
Inputs and units
Enter the first day of your last period and your usual cycle length in whole days. Cycle length runs from the first day of one period to the first day of the next; it is not the number of bleeding days. This tool supports cycle lengths of 21–45 days as a calculation range, not a diagnosis of what is normal.
Formula
Predicted period start = last period start + cycle length × cycle number. The next three starts assume your cycle length stays the same.
Worked example
For a last period starting January 1, 2026 and a 28-day cycle, predicted starts are January 29, February 26 and March 26, 2026.
Assumptions and limitations
Calendar estimates assume a regular cycle. Irregular cycles, breastfeeding, hormonal contraception and conditions affecting ovulation can make predictions unreliable. This calculator cannot confirm ovulation or pregnancy and must not be used to prevent pregnancy. Predictions are relative to the period you enter; an old entry can produce dates already in the past.
Health references
Keep calculating