Predict your next period, fertile window and ovulation date from your last period and cycle length — an estimate for planning, not a contraceptive method.
Estimates only — not medical advice, and never use period predictions as a contraceptive method.
Cycles vary naturally; predictions get less reliable the further ahead they go, especially with irregular cycles.
next period starts
How to use this tool
Use the calendar picker for the date your last period started.
Count from the first day of one period to the first day of the next; 28 days is typical, 21–45 is the accepted range.
The big result is your next expected period; ovulation and the fertile window are shown below.
Under Advanced options, show up to 6 future cycles — but remember predictions get less reliable over time.
Formula & worked example
With a last period starting 2026-01-01 and a 28-day cycle, the next period is expected 2026-01-29, ovulation around 2026-01-15, and the fertile window 2026-01-10 to 2026-01-15.
Limitations — read this before you rely on it
These are averages, not certainties: stress, illness, travel and natural variation shift real dates, and irregular cycles make predictions unreliable. Results are estimates, not medical advice, and must never be used as a contraceptive method.
Frequently asked questions
Ovulation usually happens about 14 days before the next period starts, so it is estimated as last period start + (cycle length − 14) days.
The days when conception is possible: roughly the 5 days before ovulation (sperm can survive that long) plus ovulation day itself.
No. Period predictions are estimates and cannot reliably prevent pregnancy — use a proper contraceptive method.
28 days is only an average. Healthy cycles commonly range from 21 to 35 days, and they vary from month to month.
Predictions become much less reliable. Track several cycles and use your own average, and see a clinician if cycles are very irregular or suddenly change.