How the expected date is worked out
Your next period is expected roughly one cycle length after the first day of your last period. So if your last period started on a given day and your cycles run about 28 days, the next one is due around 28 days later. The calculator counts the days from that expected date to today: a positive number means your period is late, and a negative number means it is not due yet.
A period that is a day or two off its expected date is completely normal - cycle length naturally shifts from month to month. Most guidance treats a period as "late" once it is more than about 5 days overdue.
What a late period can mean
A late or missed period has many possible causes beyond pregnancy. Common ones include stress, illness, travel, significant weight change, intense exercise, thyroid issues, PCOS, certain medications, breastfeeding, and the transition into perimenopause. Of course, if you could be pregnant, a late period is also one of the earliest signs - so a test is the quickest way to know.
If there is any chance of pregnancy, our pregnancy test calculator shows the most reliable day to test. To understand non-pregnancy causes, see late period reasons beyond pregnancy.
If your cycles are irregular, remember that "late" is fuzzy - the expected date is only as steady as your typical cycle length, so use it as a guide rather than a hard deadline.
Frequently asked questions
When is a period officially late?
A period is generally considered late once it is more than 5 days past its expected date. Because cycle length varies from month to month, most clinicians don't treat a single late period as a concern until it is at least a week overdue, or until you have missed it entirely.
How late can a period be before I should worry?
An occasional period that is a few days to a week late is common and usually nothing to worry about. If you are not pregnant and miss three or more periods in a row, or your cycle suddenly becomes very irregular, ACOG advises checking in with a healthcare provider to look for an underlying cause.
Why is my period late if I'm not pregnant?
Plenty of things can delay a period besides pregnancy: stress, sudden weight change, intense exercise, illness, travel across time zones, thyroid problems, polycystic ovary syndrome (PCOS), some medications, breastfeeding, and approaching perimenopause. The NHS notes that stress and lifestyle shifts are among the most common reasons for an occasional late or missed period.
When should I take a pregnancy test?
If there is any chance you could be pregnant, the most reliable time to test is the day after your missed period, when hCG levels are usually high enough to detect. Some early-detection tests work a few days before your expected date, but a negative that early can still turn positive later, so retest in a few days if your period hasn't arrived.
What if my cycles are irregular?
With irregular cycles, the idea of "late" is fuzzy - the expected date is only as reliable as your average cycle length. If your cycles regularly vary by more than a week, treat the calculator's expected date as a rough guide rather than a fixed deadline, and track several cycles to find your typical range.
Related tools: period calculator and why is my period late?
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.