How accurate is a period calculator?

It's an estimate based on your average cycle, so it's most accurate for regular cycles. If your cycles vary, expect your period within a few days either side of the predicted date, and use the average of your last three cycles for the best guess.

Does the calculator work for irregular periods?

It can still give a rough guide. Work out your cycle lengths for the last three months (first day of one period to the first day of the next), average them, and enter that number. If your cycles are often less than 24 or more than 38 days apart, that's worth mentioning to a doctor.

When am I most fertile in my cycle?

Usually in the six days ending on ovulation, which happens about 14 days before your next period. The calculator shows that window for your current cycle, and the ovulation calculator gives more detail on peak days.

Period Calculator: When Is My Next Period?

Q: How late can a period be?

A few days either way is common, because cycle length naturally varies from month to month. Mayo Clinic describes typical cycles as anywhere from 24 to 38 days apart. If you could be pregnant, a test is reliable from the first day of a missed period. If you're not pregnant, it's worth seeing a doctor when periods stop for more than 90 days or you miss three in a row.

Q: My period is late but my pregnancy test is negative. What now?

The most common reason is testing a little early, especially if you ovulated later than usual that cycle. The NHS suggests waiting a few days and testing again, and seeing a GP if a second test is negative and your period still hasn't come. Stress, weight change, intense exercise, PCOS, thyroid problems, breastfeeding and hormonal contraception can all delay a period too.

Q: Can Plan B make your period late?

Yes. Mayo Clinic notes that the morning-after pill may delay your period by up to a week. If your period hasn't come within three weeks of taking it, take a pregnancy test.

At a glance

Formula used

The first day of your last period plus your average cycle length gives the next one; each later period adds another cycle.

What's typical

Mayo Clinic describes periods anywhere from 24 to 38 days apart, lasting up to 8 days, as typical. Cycles also vary by a few days month to month.

If it's late

A pregnancy test is reliable from the first day of a missed period. If it's negative, the NHS suggests testing again a few days later.

Talk to a provider

If periods stop for more than 90 days when you're not pregnant, you miss three in a row, or your cycle suddenly becomes irregular.

How the period calculator works

Your next period is the first day of your last period plus your average cycle length. A cycle is counted from the first day of one period to the first day of the next, so a period that started on March 1 in a 28-day cycle points to the next one around March 29. Each later period adds another cycle.

The calculator also shows your fertile window. Ovulation usually happens about 14 days before the next period, and the fertile window is the six days ending on ovulation day. For more detail on peak days, use the ovulation calculator.

How late can a period be before you should worry?

A period a few days later than expected is common and usually nothing to worry about. Cycle length naturally shifts from month to month, and Mayo Clinic describes typical cycles as anywhere from 24 to 38 days apart, with bleeding lasting up to 8 days.

If there's any chance you're pregnant, a home pregnancy test is reliable from the first day of a missed period. If you're not pregnant, check in with a doctor if:

your periods stop for more than 90 days
you miss three periods in a row
your periods become irregular after being regular
your cycles are often less than 24 or more than 38 days apart

Late period but a negative pregnancy test?

The most common reason is testing slightly too early. If you ovulated later than usual this cycle, your period, and the hCG a test looks for, both arrive later too. The NHS suggests waiting a few days and testing again, and seeing a GP if a second test is negative and your period still hasn't come. Our pregnancy test timing calculator shows the earliest reliable day for each type of test.

Plenty of things besides pregnancy can delay a period. The NHS lists stress, sudden weight loss or being overweight, doing a lot of exercise, polycystic ovary syndrome (PCOS, now also called PMOS), some hormonal contraception, breastfeeding, an over- or underactive thyroid, and the start of perimenopause, usually between 45 and 55.

Can Plan B make your period late?

Yes. According to Mayo Clinic, the morning-after pill may delay your period by up to a week. If your period hasn't come within three weeks of taking it, take a pregnancy test. The calculator can't account for emergency contraception, so treat its date as a rough guide that cycle.

Using a period calculator with irregular periods

For irregular cycles, enter an average rather than your most recent cycle. Note the first day of each period for three months, count the days between them, and average the gaps. The predicted dates will still be approximate, so expect your period within several days either side.

If your cycle length changes a lot, the timing of ovulation changes with it, which matters if you're trying to conceive. Ovulation tests (LH strips) or tracking your temperature give a more direct signal than any calendar-based estimate.

Frequently asked questions

Add your average cycle length to the first day of your last period. With a 28-day cycle, a period that started on March 1 points to the next one around March 29. The calculator above does this for your next six periods and shows how long each is likely to last.

Sources and medical references

The cycle ranges and late-period advice on this page come from Mayo Clinic and the NHS.

Period calculator