Period calculator: next period dates

How to calculate your next period

Add your average cycle length to the first day of your last period. A period that started on 5 January 2026, in a 28-day cycle, points to the next one on 2 February 2026, then 2 March and 30 March. The dates assume every cycle lasts the same number of days.

Cycle length runs from the first day of one period to the first day of the next, not the number of bleeding days. With five days of bleeding, the 2 February period would end on 6 February: the start counts as day 1, so add four days.

How to work out your average cycle length

Note the first day of each period for a few months, count the days between them and average the gaps. Cycles of 27, 30 and 29 days average 86 ÷ 3 = 28.7 days, so use 29. From a last period on 5 January 2026, the next is then due on 3 February.

Period dates by cycle length

For a last period starting on 5 January 2026, with five bleeding days and a 14-day luteal phase:

Each row follows three rules: next start = last start + cycle length, end = start + bleeding days − 1 and estimated ovulation = next start − luteal days. The window columns are the fertile window, explained below.

Cycles are counted in days, not months, so a 28-day cycle does not return to the same date each month: 2 February, 2 March, then 30 March.

What is a normal menstrual cycle?

ACOG gives 21 to 35 days as the typical adult cycle, with bleeding that usually lasts up to seven days. The 20- and 45-day rows above fall outside that range.

Exactly 28 days is less common than many people assume. In Bull and colleagues' 612,613 app-recorded cycles, 13% lasted 28 days and the mean was 29.3 days.

What cycle day am I on?

Cycle day is the number of days since your period started, plus one, so the first day of bleeding is day 1. The follicular phase begins with the period and ends at ovulation; the luteal phase runs from ovulation to the next period.

For a 28-day cycle with five bleeding days and a 14-day luteal phase, here is how different last-period dates read on 15 January 2026:

The phase names come from counting days, not from hormone levels. On day 29 of a 28-day cycle the next period is due that day, and each day after it counts as a day late.

How late can a period be?

A few days either way is within the usual variation. Bull and colleagues found that the standard deviation of each person's cycle lengths averaged 2.6 days, rising to 3.1 days at age 45.

So a period two or three days after the predicted date is common, and each later prediction is less certain than the one before. Recalculate from the first day of each new period.

If a period is late and pregnancy is possible, take a pregnancy test following its instructions.

Fertile days on a period calendar

The window columns run from five days before estimated ovulation to one day after: seven inclusive dates. Wilcox and colleagues found conception followed intercourse only in the six days ending on estimated ovulation, so the seventh date is a margin.

With a 28-day cycle and a 14-day luteal phase, estimated ovulation falls on cycle day 15. Calendar dates cannot confirm ovulation and are unreliable contraception on their own.

When to talk to a clinician

See a clinician about cycles that are regularly shorter than 21 days or longer than 35, bleeding that lasts more than seven days or is unusually heavy, and missed periods.

Equal-cycle predictions fit poorly with irregular cycles, on hormonal contraception, after giving birth and around perimenopause.

Settings worth checking

• Luteal phase length, from 10 to 16 days, moves the ovulation and fertile-window dates; the period dates stay the same.
• Period length sets only the end date of each period.

The ovulation calculator covers fertile days in more detail for anyone trying to conceive. If you are pregnant, the pregnancy due date calculator dates the pregnancy from the same first day of your last period.