Work out menstrual cycle dates instantly with clear inputs, formula shown and shareable results.
Cycle tracking projects forward by the average cycle length from the first day of the last period, counted as cycle day one. With a 28 day cycle starting 20 August 2026, the next period is expected on 17 September and the one after on 15 October. Real cycles vary by several days month to month, so the projection is indicative.
Next period
next start = last start + cycle length
Period end
end = next start + period length - 1
Cycle predictions from average length are estimates only and must not be relied on for contraception or fertility decisions. Cycles vary between months and between individuals. Consult a healthcare professional for contraceptive, fertility or menstrual health advice.
It is the average, but anything from 21 to 35 days is considered normal in adults, and variation of a few days between cycles is typical.
No. Calendar methods have high failure rates because ovulation timing varies. Use a recognised contraceptive method.