Period Calculator
Adjust the inputs below. Results update as you type.
How it works
Next period start = last period start + cycle length (average 28 days, range 21–35). Period duration typically 3–7 days. Fertile window centers around day 14 of a 28-day cycle. Cycle irregularity of ±2 days is normal; larger variation may warrant medical evaluation. Apps that track multiple months build more accurate personalized predictions than fixed-interval estimates.
Input guidance
- Use measured values (recent weight, height, activity level) rather than guesses.
- Recalculate every few weeks as your routine and body metrics change.
- Pair screening metrics with context such as sleep, stress, and medical history.
The formula
Predicted next period = first day of last period + cycle length. Future periods repeat by adding the cycle length each time.
Worked example
With a last period starting May 3 and a 28-day cycle, the next period is predicted around May 31, then June 28.
More examples to test
- Maintenance case: keep activity constant and observe expected baseline ranges.
- Adjustment case: change only one major variable to understand sensitivity.
How to interpret results
Use results for awareness and planning, then validate with qualified medical or nutrition professionals for individualized decisions.
When this can be inaccurate
Population formulas cannot account for all biological factors such as medical conditions, medication effects, body composition differences, and genetics.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Track fitness goals with a top app
General wellness resource.
Frequently asked questions
How does period prediction work?+
By adding your average cycle length to the start of your last period. Tracking several cycles improves the average and the prediction.
What is a normal cycle length?+
Anywhere from about 21 to 35 days is considered normal. Cycles can vary month to month, especially in the years after first menstruation.
Why is my cycle irregular?+
Stress, weight changes, exercise, and hormonal conditions can all affect timing. Persistent irregularity is worth discussing with a doctor.
Can this calculator diagnose a condition?+
No. It is a screening and education tool only, not a clinical diagnosis.
When should I seek professional advice?+
Seek professional advice when results are extreme, persistent, or conflict with symptoms, goals, or prior medical guidance.