When is my next period?

It is calendar math: your average cycle length is added to the first day of your last period, once for each upcoming cycle, to predict the next three start dates. Ovulation is then placed about 14 days before each predicted period, because the second half of the cycle (the luteal phase) is relatively fixed while the first half varies. The prediction is only as good as your average: if your cycles are regular, it is usually within a day or two, and it gets sharper if you recheck it against a couple of real cycles. If your cycles swing widely month to month, calendar predictions lose reliability and tracking apps or ovulation tests do better.

Under the FIGO 2018 classification that clinicians use worldwide, a normal cycle runs 24 to 38 days, counted from the first day of one period to the first day of the next. The textbook 28 days is just an average inside that range, not a requirement, and your normal can sit anywhere in it. Cycles consistently shorter than 24 days are classed as frequent and cycles longer than 38 days as infrequent, and either pattern is worth mentioning to a clinician. ACOG goes further and treats the menstrual cycle as a vital sign: a persistent change in your pattern is information, the same way a change in blood pressure is.

FIGO defines regularity by how much your cycles vary from shortest to longest over a year: a spread of more than 7 to 9 days counts as irregular (up to 9 days is allowed at ages 18 to 25 and 42 to 45, up to 7 days at 26 to 41). One odd cycle after stress, illness, travel, or a weight change is normal and not a diagnosis. But a persistently irregular pattern is worth a clinician conversation, because common, treatable causes like thyroid problems and PCOS sit behind many of them, and because irregular cycles make calendar predictions like this one unreliable — ovulation tests are the better tool there.

No, and please don't. This is a prediction built on your average cycle, and real ovulation moves with stress, illness, travel, and ordinary variation, while sperm can survive in the reproductive tract for up to five days. That combination means there is no reliably 'safe' day a calendar can promise you. Fertility-awareness methods that genuinely work for preventing pregnancy require daily tracking of basal temperature and cervical mucus, or ovulation testing, plus proper training, and even then they have meaningful failure rates. If you are trying to prevent pregnancy, use a proven contraceptive method and talk to a clinician about which fits you.

First, do not panic: a late period usually means ovulation happened later than usual that cycle, which stress, illness, travel, weight change, breastfeeding, and perimenopause can all cause. If you have had sex that could lead to pregnancy, take a home pregnancy test from the first day after your missed period, when results are reliable. If the test is negative and your period stays absent, retest in a few days. See a clinician if you go around three months with no period and no pregnancy, or if lateness is becoming your usual pattern — both are worth investigating, and most causes are treatable.

Completely. This calculator runs entirely in your browser: the dates you enter are never sent to a server, stored, or shared, and using it tells us nothing about you. Close the tab and everything is gone. In a post-Dobbs world we treat cycle and reproductive data as sensitive by default, so none of it leaves your device — there is no account, no cookie tracking your cycle, and nothing to delete. It is an educational estimate, not medical advice.