Period Calculator
Predict your upcoming period dates based on the start of your last period and your average cycle length.
How to use the Period Calculator
- Enter your inputs into the Period Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The period prediction
Next period = Last period + Cycle length · · · Subsequent periods stack at cycle-length intervals
Track cycle length over 3–6 months to get a reliable average. Most cycles are 21–35 days; significant variation often signals hormonal changes worth discussing with a clinician.
Worked example
Last period started April 3 with a 30-day cycle: next period May 3, then June 2, July 2, August 1, August 31. Cycle length of 28 days would shift these earlier by 2 days each.
Frequently asked questions
How regular should my cycle be?
Most adult women have cycles 21–35 days long, with month-to-month variation of ±3 days. Sudden changes can be caused by stress, illness, weight changes, perimenopause or pregnancy.
How do I track cycle length?
Count from day 1 of one period to day 1 of the next. Track over at least 3 months for an average. Smartphone tracking apps can simplify this.
When should I see a doctor about irregular cycles?
If cycles vary by more than 7–10 days month to month, are consistently shorter than 21 days or longer than 35 days, or change suddenly after years of regularity.