Period Calculator (Next Period Dates)
Enter your last period’s start date and cycle length to predict the next ones.
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Period Calculator (Next Period Dates) by TheCalcTools
How the Period Calculator (Next Period Dates) works
Each predicted period starts one cycle length after the previous start. Cycle length is counted from the first day of one period to the first day of the next; 21 to 35 days is typical for adults.
Track several cycles and use their average for better predictions. See a doctor if periods are very irregular, very heavy or stop unexpectedly.
The formula
Worked example
Using the default values shown in the calculator:
| First day of your last period | 2026-10-03 |
|---|---|
| Average cycle length (days) | 28 |
| Period length (days) | 5 |
- Starting 3 October 2026 with a 28-day cycle: next period 31 October 2026 to 4 November 2026.
Result:
| Next period | 31 October 2026 to 4 November 2026 |
|---|---|
| Period after that | 28 November 2026 to 2 December 2026 |
| Third period | 26 December 2026 to 30 December 2026 |
| Next ovulation (estimate) | 17 October 2026 |
Frequently asked questions
Is a late period always pregnancy?
No. Stress, illness, weight change, exercise and some conditions can shift a cycle. A pregnancy test is reliable from the day the period is due.
Sources and further reading
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This calculator gives general estimates for information and education. It is not medical advice and does not diagnose anything. Talk to a qualified health professional about your own situation. See our disclaimer.