Period Calculator

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

By Konstantin Iakovlev · Updated September 2026 · Source: Wilcox, Weinberg & Baird, N Engl J Med 1995 — timing of intercourse in relation to ovulation

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Use the Period Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only and is not medical advice. Results are estimates from standard formulas and the numbers you enter. Talk to a doctor or another qualified health professional about your own health before acting on them.

How It Works

Your typical cycle length is enough to project three things at once: when your next period should arrive, when you are most fertile, and which day ovulation likely falls on. Those dates support family planning, help you get ahead of period symptoms, and build a clearer picture of your body's rhythm. As a concrete example, a period that started on January 10th, 2026 paired with a 28-day cycle points to a next period around February 7th, 2026.

The method behind those projections is well established. The calculator takes your last period's start date and adds your average cycle length to find the next one. Ovulation is placed about 14 days before that predicted period, and the fertile window covers the 5 days leading up to ovulation, ovulation day itself, and the day after, for a 7-day span in total.

Treat all of this as prediction rather than certainty, since stress, diet, and other factors can shift any given cycle. The estimate is genuinely useful, but it does not replace medical advice or finer tracking tools like basal body temperature or ovulation predictor kits. Irregular cycles weaken the predictions in particular, so feed in an average drawn from several months for the steadiest results.

Example: Predicting Dates From a 30-Day Cycle

  1. 1 Your last period started on January 15, 2026, and your average cycle length is 30 days.
  2. 2 Next period: January 15 + 30 days = February 14, 2026. Ovulation is placed 14 days earlier, on January 31, 2026.
  3. 3 Fertile window: 5 days before ovulation through 1 day after, January 26 to February 1, 2026. The six days from January 26 to ovulation day are the span in which conception was observed in Wilcox et al.'s 1995 study; February 1 is the calculator's extra margin.
  4. 4 Predicted next period: February 14, 2026. Predicted ovulation: January 31, 2026. If your cycle length varies from month to month these dates move with it, so ovulation predictor kits or basal body temperature give a firmer read when timing matters.

Source: Wilcox, Weinberg & Baird, N Engl J Med 1995 — timing of intercourse in relation to ovulation · Last updated: September 2026

Frequently Asked Questions

How do you predict your next period?
Count the days from the first day of one period to the first day of the next (your cycle length). Add your cycle length to the first day of your last period to estimate the next one. Average cycle length is 28 days but 21-35 days is normal.
When is the fertile window in a menstrual cycle?
In a study of 221 women trying to conceive (Wilcox et al., New England Journal of Medicine, 1995), conception occurred only after intercourse in the six days ending on the day of ovulation; this calculator also shows the day after ovulation as a margin. Ovulation typically occurs 14 days before the next period starts. For a 28-day cycle, ovulation is around day 14.
What causes irregular periods?
Common causes include stress, weight changes, excessive exercise, hormonal imbalances (PCOS, thyroid issues), perimenopause, and certain medications. Cycles varying by more than 7-9 days are considered irregular.

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