Period & Cycle Calculator

Estimate your next period, fertile window and ovulation day from your cycle length.

Next period (estimated)

These inputs don't produce a valid result. Check for zeros or values that can't go together.

Educational estimate only — not medical advice. Talk to a healthcare professional about your health.

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How to calculate period & cycle

Next period = last period + cycle length · Ovulation ≈ next period − 14 days

Worked example. Next period (estimated): Thu, Jan 15, 1970.

Example inputs — First day of your last period: 1969-12-18; Cycle length (days): 28; Period length (days): 5. Starting values are illustrative.

Expected to last about 5 days, until Mon, Jan 19, 1970

From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.

How it works

Cycles are counted from the first day of one period to the first day of the next. Ovulation is estimated at 14 days before the next period, and the fertile window covers the five days before it plus the day after.

Assumptions & limitations

Results are rounded for display.

Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators

Frequently asked questions

How accurate is this?

Cycles vary, so treat dates as estimates. Tracking several cycles improves accuracy.