Tracking your cycle helps you plan ahead and notice changes worth discussing with a doctor. A normal adult cycle is 21–35 days and a period usually lasts 2–7 days.
How to use the Period Calculator
- First day of your last period
- Average cycle length
- Period length
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the first day of your last period, your average cycle length and how many days your period lasts.
Inputs explained
| Field | What to enter |
|---|---|
| First day of your last period | A date |
| Average cycle length | In days |
| Period length | In days |
What each result means
- Next period starts = [First day of your last period] + [Average cycle length]
- Next period ends = [Next period starts] + [Period length] – 1
- Following period starts = [First day of your last period] + 2×[Average cycle length]
- Third period starts = [First day of your last period] + 3×[Average cycle length]
- Next ovulation (estimate) = [Next period starts] – 14
- PMS symptoms may start around = [Next period starts] – 7
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 × n
Period ends = start + period length − 1
Ovulation ≈ next period − 14 days
Worked example
Inputs: First day of your last period: 2026-10-15; Average cycle length: 28 days; Period length: 5 days.
Results: Next period starts = Thu, November 12, 2026; Next period ends = Mon, November 16, 2026; Following period starts = Thu, December 10, 2026; Third period starts = Thu, January 7, 2027; Next ovulation (estimate) = Thu, October 29, 2026; PMS symptoms may start around = Thu, November 5, 2026.
Key terms
- Menstrual cycle
- From the first day of one period to the day before the next.
Frequently asked questions
What is a normal cycle length?
Between 21 and 35 days in adults; it can vary by up to 7–9 days.
When should I see a doctor?
If periods stop for 3 months, become very irregular, very heavy, very painful, or bleed between periods.
Can stress change my period?
Yes. Stress, illness, weight change and travel can delay ovulation and the period.