01What this calculator tells you
This calculator answers one question directly: is my period actually late, and by how many days? Enter the first day of your last period and your usual cycle length, and it projects the date your next period was expected, then counts the days between that date and today. You get an exact number, a plain-language status from not due yet to missed period, the calendar date your period was due, and guidance on when a pregnancy test becomes reliable. You can browse our other everyday tools on the calculators home page.
The word “late” only means something against your own normal. A textbook cycle is 28 days, but the U.S. Office on Women’s Health notes that anything from about 21 to 35 days is a normal cycle length, and healthy cycles often drift a few days from month to month. That is why this calculator asks for your average cycle rather than assuming 28 — a period that looks late on a 28-day assumption may be right on time for a 32-day cycle.
02How late is late? Reading the result
Not every delay is a red flag. The bands below translate the raw day count into what it usually means, using the same thresholds clinicians and period-tracking apps use. They assume your cycles are normally fairly regular; if your cycle is naturally irregular, treat every band as a rough guide rather than a rule.
Body weight sits behind a lot of cycle changes: both very low and rapidly rising weight can disrupt ovulation and delay a period, which is one of the many causes Cleveland Clinic lists when it explains why a period runs late. If you are tracking weight changes, our BMI calculator gives you a quick reference point.
03Why a period runs late (besides pregnancy)
For anyone who is sexually active, pregnancy is the first thing to rule out with a late period — but it is far from the only cause. Psychological or physical stress, illness, long-haul travel and time-zone changes, intense exercise, and big shifts in weight can all delay ovulation and, with it, your period. So can hormonal conditions such as thyroid disorders and polycystic ovary syndrome (PCOS), starting or stopping hormonal birth control, breastfeeding, and the run-up to menopause. The UK’s NHS has a clear rundown of the common reasons for stopped or missed periods.
Energy balance matters too: heavy training loads or sharp changes in how much you eat can push a period back. If you are curious how your daily calorie needs stack up, our BMR calculator estimates the baseline your body burns at rest.
If pregnancy is possible, a home pregnancy test is the fastest answer. Most tests detect the hormone hCG from around the first day of a missed period, and MedlinePlus notes that testing is most accurate once your period is at least a week late. A negative test taken very early can be wrong, so if your period still hasn’t arrived after another week, test again with first-morning urine.
Book a clinician if you miss three periods in a row, if your cycles suddenly become much longer or stop, if you have severe pain, unusually heavy bleeding, or a positive pregnancy test, or if you are past 45 and your periods have changed. A late period is usually harmless, but a persistent pattern is worth investigating.
- It is only as good as your cycle length. If you don’t know your average, the 28-day default can make a normal long cycle look late. Track two or three cycles for an accurate number.
- Irregular cycles break the math. With PCOS, perimenopause or a naturally variable cycle, the “expected date” is a loose estimate, and the late/on-time labels mean much less.
- It is not a pregnancy test. A late result never confirms or rules out pregnancy — only a test or a clinician can do that.
- Recent birth control changes shift everything. Coming off the pill, or the first months on it, can delay or skip periods for reasons that have nothing to do with your usual cycle.
04Related calculators
Working through a related project? Try our Adjusted Age Calculator, Body Shape Calculator, and Protein Intake Calculator.
01The formula
Deciding whether a period is late takes two steps: project the date the next period was expected, then count how many days have passed since. The expected date is simply the first day of your last period plus your average cycle length, because a cycle is measured from the first day of one period to the first day of the next — the way menstruation is described in the NIH’s overview of the menstrual cycle.
Where:
- last period start= the first day bleeding began in your most recent cycle.
- cycle length= days from the start of one period to the start of the next (typically 21–35).
- today= the current date, or any “as of” date you choose.
- days late= days past the expected date; a negative value counts down to it.
02Worked example
Suppose your last period started on June 1, your average cycle is 28 days, and today is June 30. Work it in two steps:
Now change one number: if your average cycle is really 32 days, the expected date moves to July 3, so on June 30 you are 3 days early, not late at all. That is why the cycle-length field matters so much — the same calendar date reads as “late” or “not due yet” depending on it. Push the example further: if June 30 were instead July 8 on a 28-day cycle, you would be 9 days overdue, landing in the “notably overdue” band where a pregnancy test is the sensible next step.