Predict upcoming period start and end dates, ovulation days, and fertile windows for the next several cycles.
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Health — Fertility
Predicting upcoming periods helps with planning, identifying cycle irregularities, and understanding fertile windows. This calculator generates predictions for the next several menstrual cycles from your last period date, cycle length, and period duration.
Period calculators assume a regular cycle. From the last period start date, each subsequent period is predicted by adding one cycle length (default 28 days). Period end dates are estimated by adding the period duration (default 5 days). Ovulation is estimated at cycle day (cycle length − 14), and the fertile window spans roughly 5 days before to 1 day after ovulation.
Most people with regular cycles have periods that vary by 2–7 days month to month even when overall cycle length is consistent. This means predictions are most accurate for the next 1–3 cycles and less precise further out. Tracking your actual period dates over several months using this calculator or a dedicated app provides a progressively better average to work from.
Cycles shorter than 21 days or longer than 45 days are typically considered irregular. Common causes include PCOS, thyroid dysfunction, high stress, significant weight changes, perimenopause, and recent hormonal contraceptive use. Irregular cycles reduce the accuracy of period prediction tools significantly; if you are trying to conceive or concerned about irregularity, see a GP or gynaecologist.
Frequently asked questions
For regular cycles, predictions are typically accurate within 2–4 days for the next cycle. Accuracy decreases with longer prediction horizons. For irregular cycles, predictions may be off by a week or more.
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