How to predict your next period

The formula is one line: next period = last period's first day + your average cycle length. If your last period started on the 3rd and your cycles run 30 days, expect the next one around the 2nd. That is the whole trick — everything else is about getting "your average cycle length" right.

Step 1: count your cycle correctly

A cycle runs from the first day of one period to the day before the next one starts. Two mistakes are common: counting from the end of a period, and assuming everyone is 28 days. Real cycles between 21 and 35 days are normal, and your own can vary by a few days month to month.

Step 2: average your last few cycles

Write down the start date of your last three or four periods, count the gaps, and average them. Three gaps are enough to be dramatically better than the 28-day default. This is why the single most useful habit is boring: log the first day of every period. Three months of that one habit and predictions snap into focus.

Step 3: ovulation and the fertile window

Ovulation lands about 14 days before the next period — not 14 days after the last one. With a 30-day cycle, that is around day 16. The fertile window is roughly the 6 days ending the day after ovulation. Note the direction of the arithmetic: it counts backwards from the predicted next period, which is another reason the prediction needs to be yours, not a textbook's.

When the math isn't enough

Irregular cycles, stress, travel, illness and hormonal changes all move the date. Calendar math cannot see those coming — but it can tell you when your body went off its usual script, which is itself worth knowing. If your cycles swing wildly or a period is very late repeatedly, that is a conversation for a doctor, not an app.

Or let the arithmetic do itself

Everything above is exactly what a cycle tracker automates. Log the first day of each period in CycleCast and it maintains your rolling average, marks the expected period, ovulation day and fertile window on the calendar, and warns you two days before the predicted start. The math runs on your phone — no account, no server, and none of it is contraception-grade certainty: treat fertile-window estimates as planning help, not birth control.