Free cycle tool

Is My Period Late? A Calculator

Late compared with what, exactly. Give this your last period, your usual cycle length and — if you know it — how much your cycles normally move, and it will tell you how far past your own average today is, rather than how far past somebody else's 28 days.

Runs entirely in your browser. No account, no email, and nothing you type is sent anywhere.

Your last period
If you know how much yours move

Optional, and it is the pair of numbers that makes this page about you. With them, "late" starts when today passes your own longest cycle. Without them, the page assumes a few days either side and says so.

How late is this, for you
0
days past your expected date

Put the first day of your last period in the box, and your usual cycle length if you know it. This will count the days and then tell you whether that count is inside your own normal spread or past it.

This page cannot tell you whether you are pregnant — nothing that works from dates can. Only a test can, and when one would be worth taking is worked out below. It is not contraception, and it cannot diagnose anything.

Your own range, and where today falls

The band is the stretch your period would arrive in if this month behaves like your others. The dot is today.

Your expected range drawn as a band, with today marked against it. Fill in the dates to draw it.
the range your own cycles give your expected date today

When a pregnancy test would mean something

These dates are the NHS's rule applied to your own numbers, not a brand's claim about its box.

Most tests can be used from
add your dates
the first day of a missed period — which is your expected date, once it passes
If your cycles run long
add your dates
the day your period would be due even on the longest cycle you have given, so a test from here is not an early one

If you do not know when your period was due, the NHS gives a second rule that needs no cycle at all: do the test at least 21 days after you last had unprotected sex.

And the part nobody is told: the two results are not equally reliable. A positive result is almost certainly correct. A negative one is less reliable and can be wrong if the test was taken too early — so if your period still has not come, test again in a few days.

Work out the test dates properly, from your ovulation, conception or last period

What ordinarily does this

The NHS lists these as the common causes of a missed or late period. It is a list of what can move a cycle, not a checklist about you — nothing on this page can tell you which of them, if any, is what happened this month.

  • Being pregnant
  • Stress
  • The start of the menopause, called perimenopause, usually between the ages of 45 and 55
  • Polyendocrine metabolic ovarian syndrome (PMOS) — the hormone condition the NHS used to call polycystic ovary syndrome, or PCOS, and one of whose main symptoms is irregular periods or long gaps between them
  • Sudden weight loss
  • Being overweight
  • Doing too much exercise
  • Using certain hormonal methods of contraception
  • Breastfeeding

Stress is on that list with a mechanism behind it rather than only folklore: researchers report that prolonged or chronic stress can block, inhibit or delay the surge of luteinising hormone that triggers ovulation. Delay the surge and everything after it moves, the period included. It also fits what the large cycle datasets show — differences in cycle length come predominantly from differences in the first half of the cycle, before ovulation, rather than the second. A late period is usually a late start, not a long finish.

When the NHS says to see a GP

Not because a late period is likely to be serious. Because these are the points at which a clinician can do something with it, and a calculator cannot.

  • If you have missed your period three times in a row. One late month is a data point. Three is a pattern, and a pattern is what an appointment can work with.
  • If your periods are irregular, or your normal pattern changes. The NHS counts periods as irregular when the gap between them is under 21 days or over 35, and notes this is most likely when periods first start and when nearing the menopause.
  • If your periods have not started by the time you are 15.

Take your own dates with you if you go. The average, the shortest, the longest and how many cycles they come from will get further in ten minutes than "it has been all over the place lately" — and the cycle length calculator on this site will work them out from a handful of period start dates.

This page cannot tell you whether you are pregnant, and it is not contraception. It counts days between dates you typed in and compares that count with your own usual spread. It cannot see your body, it cannot diagnose anything, and it has no way of knowing why this month is running differently. Only a test can answer the pregnancy question, and if you have a regular cycle the NHS calls a missed period the earliest and most reliable sign there is.

Every date here is an estimate, and the range around it is the honest part. Your expected date is your last period plus a cycle length, which assumes this month repeats the last one — and cycles routinely do not. Across 612,613 tracked cycles the average person's own cycle length moved by about 2.6 days from one month to the next, so a few days either side of the expected date is the ordinary width of the thing, not a sign of anything.

If it stays late, count rather than search. The NHS says to see a GP if you have missed three periods in a row, or if your periods have become irregular when they were not before. That is a clearer threshold than anything you will find in a forum at two in the morning, and it is the one worth using.

How this is worked out

  1. Your expected date is your last period plus your usual cycle length. Today's distance from that date is the number this page leads with, and it is a count of days rather than a verdict.
  2. That count is then read against your own variability. If you know your shortest and longest cycle, "late" starts when today passes your longest; if you do not, the page assumes a few days either side and says so, because across 612,613 cycles the average person's own cycle length moved by about 2.6 days from one month to the next.
  3. When a pregnancy test would be reliable is worked out from the NHS's own rule rather than from a brand's: most tests can be used from the first day of a missed period, and if you do not know when your period was due, at least 21 days after unprotected sex.
Questions

The short answers

There is no universal number, which is why this page asks about your cycles rather than telling you about everyone's. A few days either side is ordinary: in a dataset of 612,613 cycles the average person's own cycle length moved by about 2.6 days between months. A useful working line is your own longest cycle — until today passes that, this month is still inside your normal range. Clinically, the NHS calls periods irregular when the gap between them is under 21 days or over 35, and says to see a GP if you have missed three in a row.
Usually because ovulation was late, and ovulation is the part of the cycle that moves. Across 612,613 cycles, differences in cycle length were caused predominantly by differences in the follicular phase — the stretch before ovulation — while the stretch after it stays comparatively fixed. So a late period is generally a late start rather than a long finish. The NHS lists the common causes as pregnancy, stress, the start of the menopause, polyendocrine metabolic ovarian syndrome (which it used to call PCOS), sudden weight loss, being overweight, doing too much exercise, some hormonal contraception, and breastfeeding.
Yes, and there is a mechanism rather than only folklore. The NHS names stress among the common causes of a missed or late period, and the research describes how: prolonged or chronic stress has been reported to block, inhibit or delay the LH surge that triggers ovulation. Delay the surge and you delay everything downstream of it, including the period. What no page can tell you is whether that is what happened to you this month.
From the first day of your missed period, per the NHS — which is what most tests are designed for. If you are not sure when your period was due, the NHS's other rule is the one to use: at least 21 days after you last had unprotected sex. A positive result is almost certainly correct. A negative one is less reliable and can simply mean you tested too early, so if your period still has not come, test again. The pregnancy test calculator on this site will work the date out from whichever of those you know.
Two things, in this order. Test again in a few days if your period still has not arrived, because an early negative is the least reliable result a test gives. And if it stays late, count: the NHS says to see a GP if you have missed three periods in a row, or if your periods have become irregular when they were not before. One late month is a data point. Three is a pattern, and a pattern is what a clinician can work with.
No. Nothing on this page can, and nothing anywhere can except a test. This is arithmetic on dates you typed in: it can tell you how far past your own average today is and when a test would mean something, and that is the whole of what it does. It is also not contraception and cannot diagnose anything.

Where this comes from

  1. NHS (2026). Missed or late periods. https://www.nhs.uk/symptoms/missed-or-late-periods/
  2. NHS (2026). Missed or late periods (common causes). https://www.nhs.uk/symptoms/missed-or-late-periods/
  3. NHS (2025). Irregular periods. https://www.nhs.uk/symptoms/irregular-periods/
  4. NHS (2026). Polyendocrine metabolic ovarian syndrome (PMOS). https://www.nhs.uk/conditions/polycystic-ovary-syndrome-pcos/
  5. NHS (2022). Doing a pregnancy test. https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/
  6. NHS (2022). Signs and symptoms of pregnancy. https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-of-pregnancy/
  7. npj Digital Medicine (PubMed Central) (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/
  8. Frontiers in Global Women's Health (PubMed Central) (2022). Chronic Stress and Ovulatory Dysfunction: Implications in Times of COVID-19. https://pmc.ncbi.nlm.nih.gov/articles/PMC9168655/

Every link above was checked when this page was last updated. Athena is not affiliated with any of these organisations, and none of them has reviewed this page. Nothing here is medical advice.

Go deeper

The reading behind this page

Understanding your cycle · 6 min read

Why Is My Period Late? The Ordinary Explanations, and the Ones to Check

A late period is nearly always a late ovulation, which is why the delay sits in the first half of the cycle. The ordinary explanations, in prose, for somebody who wants to understand rather than count.

Read it Understanding your cycle · 6 min read

Stress and Your Cycle: What Stress Can Actually Do to a Period

Stress is the explanation everybody reaches for, and it is both real and over-used. It moves the timing of a cycle; the studies that show that also show what it does not move.

Read it Understanding your cycle · 7 min read

Irregular Periods: When Normal Variation Is Worth a Conversation

Almost everybody thinks their periods are irregular, and most are describing ordinary movement. Here is what the word means to the NHS, to ACOG and to NICHD, and where each of them draws the line.

Read it
Next door

Pregnancy Test Calculator

Three ways in — ovulation, conception, or your last period — and two dates out: the earliest a test is worth taking, and the day its answer can be trusted.

Open it

Cycle Length Calculator

Log a few period start dates and read your own numbers: average, shortest, longest, and how much the whole thing moves.

Open it

Period Calculator

Your last period plus your usual cycle length, and the next few months laid out on the moon ring — with the spread you should actually expect.

Open it
The app this site is for

Athena keeps all of this on your phone.

The calculators here forget you the moment you close the tab. Athena is the same arithmetic living on your device — the moon ring, an honest calendar with ranges instead of false precision, and a database that never leaves the phone because there is no account and no server behind it.

In development for iPhone and Android. Not released yet — when it is, it will be on this site.

What Athena is