How this calculation works
This calculator uses: Next Period = Last Period + Cycle Length | Ovulation = Next Period − 14 days. Enter your values above to get an immediate result. Results are rounded for readability.
Using the period calculator
Start by entering values in the units shown. The result updates immediately, and the shareable link preserves your inputs without creating a separate indexable page. Use this as a clear estimate and check important assumptions before relying on it.
Common questions
How accurate is the period prediction?
For women with regular cycles, predictions are fairly accurate (within 1-3 days). For irregular cycles, accuracy decreases. Track your actual period dates for 3-6 months to find your true average cycle length for best results.
What is the fertile window?
The fertile window is the days when conception is most likely. It spans about 5 days before ovulation and 1 day after because sperm can survive up to 5 days inside the reproductive tract. Ovulation typically occurs 14 days before your next period.
What is a normal cycle length?
A normal menstrual cycle is 21-35 days, with 28 days being the textbook average. However, many women have cycles of 24-32 days. Occasional variation of a few days is normal. Consistent variation of more than 7-9 days may need medical evaluation.
Can PCOS affect my period calculator results?
Yes. PCOS (Polycystic Ovary Syndrome) often causes irregular, infrequent or prolonged periods. If your cycles are irregular due to PCOS or other conditions, this calculator's predictions will be less accurate. Consult a gynecologist for personalized guidance.