Period Calculator

What counts as a cycle

Cycle length is measured from the first day of bleeding to the day before the next bleed starts — not from when bleeding stops. A typical adult cycle runs 21 to 35 days and a typical period lasts 2 to 7 days. Both vary between people and, importantly, within the same person from month to month.

Normal variation is larger than most people expect

Cycles that vary by a few days either side of your average are entirely normal. That is why the two scenario rows above exist: a three-day swing, which is unremarkable, moves a prediction six months out by more than two weeks. Treat the later cycles in the table as a rough guide rather than fixed dates.

When variation is worth mentioning to a clinician

Consistently short cycles under 21 days, long cycles over 35 days, bleeding longer than seven days, very heavy bleeding, bleeding between periods, or a sudden change from your own established pattern are all worth raising with a healthcare professional. A calendar is useful evidence to bring to that conversation — irregularity is much easier to describe with dates than from memory.

What shifts a cycle, and what a calendar cannot see

Illness, significant stress, travel across time zones, heavy training, substantial weight change, and starting or stopping hormonal contraception can all move a cycle. Perimenopause makes cycles progressively less predictable by design. A calendar projects your average forward and cannot account for any of that, which is why a prediction six cycles out is a planning convenience rather than a forecast. Tracking actual start dates over several months gives a far better average than a single remembered cycle, and the spread of those dates tells you how much confidence the prediction deserves.

Frequently Asked Questions