How long is my cycle?
Enter the first day of your two most recent periods to find your exact cycle length, see whether it's in the typical range, and get your next three predicted periods.
The short answer: count from the first day of one period to the day before your next period starts. That number — usually somewhere between 21 and 35 days — is your menstrual cycle length. The calculator below does the counting for you.
Why these dates? The math we use
Same counting method as the MoodSwings app (algorithm v2.0.0): your next period is your last start plus your cycle length; ovulation is estimated by counting 15 days back from that next period, because the luteal phase is more consistent than the first half of the cycle. The fertile window shown is the deliberately-wide estimate we give a brand-new user (6 days before ovulation to 4 after) — with only one cycle to go on, honesty beats false precision. As the app learns your real pattern it narrows the window. For cycles under 20 days a meaningful ovulation estimate isn't possible from dates alone, so we don't show one.
Stop recalculating by hand
Log periods as they happen and MoodSwings keeps your average, variation and pattern type up to date automatically.
Estimates for education, not medical advice — and not a contraception method. Cycles shift with stress, sleep, travel and health.
How to calculate your menstrual cycle length
Your menstrual cycle is measured from the first day of one period to the day before the next one starts — not the length of your bleed. That distinction matters, because counting from the day your period ends is the single most common way people get the number wrong. Here is the count, step by step.
- Find day 1 of your most recent period. Day 1 is the first day of real bleeding — not the day you spotted beforehand. Write that date down.
- Find day 1 of the period before that. Go back one period and note the first day of full bleeding again. You need two consecutive period starts, with no skipped period in between.
- Count the days between the two start dates. Count from day 1 of the earlier period up to the day before the later period began. Counting the earlier start as day 1 and stopping the day before the next start gives you the number of days in that cycle.
- Repeat across a few cycles for your typical length. One cycle is a snapshot. Do the same count for three to six cycles and take the average to get the number that actually describes your body.
A worked example. Say your last period started on Monday 1 June, and the one before it started on Sunday 4 May. Count from 4 May as day 1 and stop the day before 1 June. That is 28 days, so your cycle length for that month was 28. If your next period then arrives on 27 June, that cycle was 26 days — and both are perfectly normal. Doing this by hand is fiddly, which is exactly why the calculator above exists.
Whether you call it your menstrual cycle, your period cycle, your monthly cycle, or your menses cycle, it is the same count: start to start.
What counts as a normal cycle length?
A typical cycle runs 21 to 35 days. Twenty-eight is the number most often quoted, but it is an average across a lot of people rather than a target any individual body is meant to hit — most people never have a 28-day cycle at all, and that is not a problem. A perfectly healthy cycle can sit anywhere in that 21-to-35 window and shift by a few days each month.
Knowing your number is useful because almost everything else is anchored to it: ovulation usually happens about 14 days before your next period, your fertile window sits in the days around it, and PMS tends to show up in the week before bleeding. Once you know your cycle length you can estimate all of those — which is what the prediction above does, and what MoodSwings keeps doing for you automatically.
Why your cycle length changes from month to month
Some variation is completely normal. Most people see their cycle move by a few days in either direction, and a month that lands three or four days off your usual number is not a sign that something has gone wrong. Stress, broken sleep, travel across time zones, illness, a hard training block, and changes in weight all nudge the timing — often by delaying ovulation, which pushes the whole rest of the cycle back with it.
What is worth noticing is a pattern: cycles that swing by more than seven to nine days between months, cycles that regularly come in under 21 or over 35 days, or a run of skipped periods. One odd month is noise. A trend is signal, and the easiest way to tell them apart is to have the dates written down somewhere.
How to calculate your cycle length when your periods are irregular
If your cycle is 26 days one month and 39 the next, a single average is close to useless — it describes a month you never actually have. Calculate the length of your last three to six cycles, then look at three numbers rather than one: your shortest, your longest, and the average in between. That range is the honest answer, and it is far more useful than a confident date that keeps being wrong.
Irregular cycles are common, and there are plenty of benign reasons for them — the years after your first period, the run-up to menopause, coming off hormonal birth control, and conditions like PCOS or thyroid changes. If yours keep moving, our guide to tracking an irregular period goes deeper on what to log and how predictions adapt to a cycle that refuses to settle.
How to calculate your next period from your cycle length
Once you have your number, the projection is simple arithmetic: take the first day of your most recent period and add your cycle length. That is your next estimated start date. Add it again for the month after. The calculator above shows your next three predicted periods this way, and the honest caveat is that a prediction built from one cycle is only as steady as that cycle was — the estimate sharpens as you feed it more history.
If you want the projection with your PMS window marked as well, the period calculator does that. If your period is already overdue and you want to know by exactly how many days, the late period calculator answers that directly.
How to calculate ovulation from your cycle length
Ovulation is not fixed to day 14. It usually happens around 14 days before your next period starts, so the day it lands on moves with your cycle length: on a 28-day cycle that is roughly day 14, but on a 34-day cycle it is closer to day 20. Estimate your next period first, then count back about two weeks.
Treat that as an estimate, not a certainty — the timing of ovulation is the part of the cycle that moves most, and it is not a method of birth control. The ovulation calculator works the window out for you, and the cycle phase calculator shows which phase today falls in.
When a cycle length is worth a closer look
Cycles that are regularly shorter than 21 days or longer than 35, or that swing wildly from month to month, can point to things like stress, PCOS, thyroid changes, or perimenopause. Periods that stop for three months or more, bleeding between periods, or bleeding heavy enough to soak through a pad or tampon every hour are all worth raising sooner rather than later.
A single odd cycle is rarely a problem, but a pattern is worth tracking and bringing to a clinician — and arriving with a few months of real dates makes that conversation dramatically faster than trying to reconstruct it from memory. This page is general education, not medical advice.
Questions people ask
How do I calculate my menstrual cycle length?
Count from the first day of one period (day 1) to the day before your next period starts. The number of days in between is your cycle length. Enter the start dates of your two most recent periods above and the calculator does the count for you.
How do I count my menstrual cycle — from the start or the end of my period?
From the start. Your cycle is measured start-to-start, so day 1 is the first day of bleeding, and the cycle runs until the day before your next period begins. The length of your bleed does not change the count. This trips a lot of people up, because it feels natural to count from the day your period ends.
How long is my cycle if my periods are irregular?
With irregular periods a single number is misleading. Calculate the length of your last three to six cycles, then look at the shortest and the longest as well as the average. That range is a more honest answer than one average, and it is what a good tracker should show you instead of a falsely precise date.
What is a normal menstrual cycle length?
A typical cycle runs 21 to 35 days, with 28 days often quoted as the average. Anything in that range is considered normal. Cycles a little shorter or longer happen, and they naturally shift by a few days from month to month.
Why is my cycle length different every month?
Some variation is completely normal — most people see their cycle move by a few days either way. Stress, sleep, travel, illness, exercise, and weight changes all nudge it. A swing of more than 7–9 days between cycles, or cycles regularly under 21 or over 35 days, is worth tracking and mentioning to a clinician.
Can I predict my next period from my cycle length?
Yes — add your cycle length to the first day of your last period to estimate the next one. This tool shows your next three predicted periods. The more cycles you track, the more accurate the prediction, which is exactly what MoodSwings does automatically. Free to try on iPhone.
How do I calculate ovulation from my menstrual cycle length?
Ovulation usually happens about 14 days before your next period starts — not on day 14 of every cycle. Estimate your next period first, then count back roughly 14 days. It is an estimate that moves with your cycle, and it is not a method of birth control.
What if I skipped a period — can I still calculate my cycle?
Not from those two dates. If a period was skipped or you cannot remember a start date, the gap covers two cycles and the count will come out roughly double. Use your two most recent consecutive period starts instead, or start logging from today and let the count build.
Stop counting on your fingers
MoodSwings learns your real cycle length from your logged periods, predicts every upcoming period and fertile window, and gives you an honest range instead of a falsely confident date when your cycle is irregular.