Predict your next period with a 3-day window from your last period date and average cycle length. You also get the next two start dates, today’s cycle day, a late flag, and the earliest reliable test day if late.
How this calculator works
- Inputs: last period start date, average cycle length (21–90 days), optional variability (±0–30 days), optional period length (2–14 days).
- Outputs: predicted next period date with a 3-day window, the following two predicted start dates, today’s cycle day, “Late?” status once past the window, and a suggested earliest reliable test day if late.
Methodology & sources
The next period date is estimated from your last period start plus your average cycle length. The 3-day window widens with the variability you set. “Late?” shows after the window end. The suggested first reliable test day is the day after that window ends.
This reflects standard cycle tracking concepts used in gynaecology education. It does not use hormone data and cannot diagnose conditions. For personal advice, see a clinician.
Methodology and notes
How dates are calculated
Next period is the last period start plus your average cycle length. The window is at least three days wide, or wider if you set a higher variability. Following starts add one and two cycle lengths.
Cycle day and late status
Cycle day counts from the last period start as day one. Late status turns to late once today is after the window end. The suggested test day is the day after the window end.
Frequently asked questions
How accurate is the 3-day window?
It depends on how stable your cycles are. The window widens if you add more variability.
What does cycle variability mean?
It is how much your cycle length usually shifts. For example, ±2 means your cycle is often 2 days shorter or longer.
When am I considered late?
“Late?” shows after the end of your predicted window. That is when the tool suggests a first reliable test day.
Does period length change the prediction?
It does not change the next start date. It helps show timing context only.
When should I take a pregnancy test?
Most home tests are more reliable after your period is due. The tool suggests a day to test once you are late. Follow the test pack instructions.