Tracking your cycle helps you plan ahead and notice changes worth discussing with a doctor.
How to use the Period Calculator
- First day of last period
- Cycle length
- Period length
Results update as you type, and every field accepts the units shown in its menu.
How it works
Each future period is predicted by adding your average cycle length.
Inputs explained
| Field | What to enter |
|---|---|
| First day of last period | A date |
| Cycle length | In days |
| Period length | In days |
What each result means
- Next period = [First day of last period] + [Cycle length]
- Following period = [First day of last period] + 2×[Cycle length]
- Third period = [First day of last period] + 3×[Cycle length]
- Current period ends = [First day of last period] + [Period length] – 1
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Next period = LMP + cycle × nWorked example
Inputs — First day of last period: 2026-10-01; Cycle length: 28 days; Period length: 5 days.
Results — Next period = Thu, October 29, 2026; Following period = Thu, November 26, 2026; Third period = Thu, December 24, 2026; Current period ends = Mon, October 5, 2026.
Frequently asked questions
What is a normal cycle length?
Between 21 and 35 days in adults.
When should I see a doctor?
If periods stop, become very irregular, very heavy or very painful.
What formula does the Period Calculator use?
Next period = LMP + cycle × n.