Estimate your next period from your last period date and average cycle length, with a window of at least 3 days. You also get two later start dates, your current cycle day, late status, and suggested pregnancy-test timing if late.
Calendar estimates cannot confirm pregnancy or diagnose a condition. For personal advice, speak with a clinician.
Methodology
The next period date is your last period start date plus your average cycle length. The following two starts add another one and two cycle lengths. Cycle day counts the entered start date as day one. The optional period-length input provides bleeding context and does not change these dates.
The prediction window extends at least one day either side of the expected start, giving a minimum 3-day window. Higher variability widens it by the number of days you select in each direction. This calendar window is not a measured confidence interval; irregular cycles can fall outside it.
Late status begins after the window ends. Later starts then become a hypothetical regular-cycle schedule, not a record of periods that happened. Future start dates remain available for planning. The calculator accepts cycle lengths of 21–90 days, variability of 0–30 days and period lengths of 2–14 days; accepting a value does not mean it is typical.
The suggested test date is the day after this calendar window ends, not a prediction of the earliest reliable positive result. NHS pregnancy-test guidance recommends testing from the first missed period, or at least 21 days after the last unprotected sex if you do not know when your period is due. Follow your test’s instructions.
These are calendar estimates rather than measurements of ovulation or hormones. For background on cycle variation and bleeding, see the NHS guide to periods.
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 status appears after your predicted window ends. The suggested test date is a calendar prompt, not the earliest day a test can work.
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?
You can use most home tests from the first day of a missed period. If you do not know when it is due, test at least 21 days after the last unprotected sex. Follow the test instructions.