Prediction is simple arithmetic — the difficult part is knowing your own cycle length, because almost nobody runs to a textbook 28 days. Here is how to work out your average, forecast ahead, and read the variation.
next period = first day of last period + cycle length
cycle length = day 1 of one period → day 1 of the next
Two details people get wrong: day 1 is the first day of proper flow, not spotting the evening before, and the cycle is measured start-to-start — it does not include the length of the bleed as a separate figure.
Finding your average
Record the start date of the last three to six periods and count the days between each pair:
Period start
Days since previous
4 April
—
2 May
28
3 June
32
2 July
29
1 August
30
Average: (28 + 32 + 29 + 30) ÷ 4 = 29.75, round to 30 days. Next period due around 31 August, with a realistic window of roughly 29 August to 2 September.
under 21 days worth checking21–35 days normal rangeover 35 days worth checking
Bleeding typically lasts two to seven days. Cycles are often irregular for the first couple of years after periods begin and again approaching menopause — both are expected rather than a problem.
The two halves of a cycle
Phase
When
Length
Menstrual
Day 1 onward
2–7 days
Follicular
Day 1 → ovulation
Variable
Ovulation
~14 days before next period
~24 hours
Luteal
Ovulation → next period
12–16 days
The key asymmetry: the luteal phase is fairly fixed, so almost all cycle-length variation happens in the follicular phase before ovulation. A cycle that runs long usually means ovulation came late — not that the second half stretched. This is also why the fertile window is easier to locate by counting back from the next expected period.
Why a period arrives late
Stress and illness — the most common cause, acting by delaying ovulation.
Weight and exercise changes — significant shifts in either direction can disrupt cycles.
Travel and shift work — disrupted light exposure and sleep affect hormone timing.
Contraception changes — starting, stopping or switching methods commonly unsettles cycles for a few months.
Pregnancy — worth ruling out with a test if you are sexually active and a period is more than a week late.
When to speak to a clinician
Worth getting checked: cycles consistently shorter than 21 days or longer than 35; bleeding lasting more than 7 days; soaking through a pad or tampon hourly for several hours; bleeding between periods; three or more missed periods without pregnancy; or a sudden change from a previously regular pattern. This article is general information, not medical advice.
Tracking a few cycles also makes everything downstream more accurate — fertile window timing, and due-date estimation if you do conceive, both depend on knowing your real cycle length rather than assuming 28 days.
Add your average cycle length to the first day of your last period. If your cycle is 30 days and your last period started on 3 August, the next is due around 2 September. Day 1 is the first day of full flow, not spotting.
What is a normal cycle length?
Anywhere from 21 to 35 days in adults, and up to about 45 days in the first few years after periods begin. The average is around 28 days, but that is a population figure — a consistent 24-day or 33-day cycle is equally normal.
How much can cycles vary month to month?
Variation of a few days either side of your average is typical and not a concern. Swings of more than seven to nine days between your shortest and longest cycle are worth mentioning to a clinician, particularly if the pattern is new.
Why is my period late?
Common causes include stress, illness, significant changes in weight or exercise, travel across time zones, shift work, and starting or stopping hormonal contraception. Pregnancy is also possible if you are sexually active. A single late period is rarely significant; a pattern is worth investigating.
How many cycles should I track before predicting?
Three to six gives a useful average. One cycle tells you almost nothing about your pattern, and older data can mislead if your cycles have changed — so a rolling average of the last six is generally the most accurate basis for prediction.
Sources
Primary references used for the figures and rules on this page.
