Period Calculator
Estimate period from the last period date and cycle length. Use this free period calculator for fast, clear results with no signup required.
Period Calculator guide
What it is for
The Period Calculator provides a planning estimate from the information entered.
How it is calculated
Next period start = last period start + entered cycle length. This is a calendar projection using one constant cycle length, not a diagnosis of menstrual irregularity.
Assumptions
It assumes the date and cycle length are known; real cycles and dates vary.
Example
For a start date of January 1, 2026 and a 28-day cycle, the projected next start is January 29, 2026.
Limitations
This is an educational estimate, not a diagnosis, treatment, or dosing instruction. Consult a licensed healthcare professional, particularly during pregnancy, with symptoms, or before changing health behavior.
Public source: ACOG: Due Date Calculator