Period Tracker & Next Period Calculator

Written by Cynthia Obinatu, community and editorial lead. Checked against published guidance from the National Health Service, the American College of Obstetricians and Gynecologists and peer-reviewed research. Not yet reviewed by a clinician. Educational, not medical advice.

How this calculator works

It does arithmetic, and it is worth knowing exactly which arithmetic, because that tells you where it can be wrong. You give it two things: the first day of your last period, and your usual cycle length. It adds your cycle length to that date to get the next first day, then repeats twice more. That is the entire method.

It does not measure anything about your body. It has no way of knowing that you travelled, fell ill, started a new job, lost weight or came off the pill, and every one of those can move a cycle. Nothing is sent anywhere either. The dates stay on this device.

If you are registering for antenatal care at a clinic in Nigeria, the first question you will be asked is the first day of your last period, because the due date is counted from it. Having the date written down instead of guessed is the single most useful thing this tool gives you.

What your result means, and what it does not

Read each date as the middle of a window. Cycles that vary by three or four days are ordinary, so your period can arrive three or four days either side of the date shown and nothing is wrong. The estimate gets better as you feed it a truer average. Tracking three or four months beats guessing once.

The result cannot confirm a pregnancy, so a late date on the screen tells you nothing that a test cannot tell you better. It cannot work as birth control either, because the fertile days it implies are an estimate of an estimate.

Most of us grew up hearing that a cycle is 28 days, repeated in school and by older women at home as though it were a rule. It is an average that fits some women. If yours runs 31 or 33 days every month, that is your normal, and counting against 28 will make you feel late every month for no reason.

How do I calculate my next period, and do I count 28 days from the start or the end?

Count from the first day of your last period. The National Health Service says the first day of a period is day 1 of the menstrual cycle, so the day bleeding stops has no part in the sum. Add your usual cycle length, and the day you land on is day one of your next period. If your last period started on 3 September and your cycle is 30 days, your next one is due on 3 October. The same sum works for any length, so for a 30-day cycle you add 30 days to the first day, and for a 28-day cycle you add 28.

This is where most people go wrong, because the bleeding ends somewhere around day five and it feels like the natural place to start counting. It is not. Every calculation on this page, and in every app you will use, starts at the first day of bleeding.

If you cannot remember the first day of your last period, work backwards from something you do remember, such as a trip, a payday or an event, and use that date as your best estimate. From now on, write down the first day of every period, because three or four months of dates will give you a real average to calculate from.

Your cycle length is the gap between one first day and the next. Count it over three or four months and take the average, because no two months are identical. Around 28 days is common, and anything from 21 to 35 days is normal according to the National Health Service. A 30-day cycle is not a long cycle. It is a 30-day cycle.

How do I calculate if my period is late?

Late means past the day you expected it, counted the way above. That only means something if you know your own average. That is the real reason to track. A woman who knows she runs 31 days can tell on day 34 that something is different. A woman who assumes 28 because that is the number everybody repeats will think she is six days late every single month.

A few days either side is ordinary. Illness, travel, a hard month at work, weight change and stress all move it. If your period has not come and there is any chance you are pregnant, take a test rather than counting again. Tests are sold over the counter in pharmacies here and do not need a prescription.

Can I get pregnant 1, 2 or 3 days after my period?

Yes. It is less likely than mid-cycle, but it is possible, and treating the days after your period as safe is how people get surprised. Sperm can survive for several days inside the body, so sex on the last day of bleeding can still meet an egg released later. The shorter your cycle, the sooner you ovulate, and the smaller that gap becomes.

Use this page to learn your own pattern. It cannot work as contraception. The ovulation calculator estimates your fertile window, and the fertile window guide explains what it estimates.

Are period tracker apps accurate for ovulation?

They are good at predicting your next period and much weaker at predicting ovulation, and the difference matters if you are trying to conceive. A calendar predicts. It does not detect. It is working from the pattern you typed in, so it cannot see the month your body does something else.

An analysis of 7,043 women's tracked cycles, published in the Journal of Medical Internet Research, found that ovulation does not sit reliably in the middle of the cycle. Most of the variation lives in the first half, before ovulation, while the second half is steadier. That is why assuming day 14 performs poorly for most women. A separate review in Frontiers in Public Health looked at apps advertising help with conception and found most gave no evidence for the method behind their predictions.

So use the date as a starting point and watch your own body alongside it. If you are trying to conceive and the months are passing, ovulation test strips or a scan at a clinic will tell you far more than any calendar can.

Can an app tell me if I am pregnant?

No. No app can, including the popular ones. An app knows the dates you gave it. It can tell you your period is late, which you can also see, and it can guess. Only a pregnancy test detects the hormone that confirms it. If your test is negative and your period still has not come after a week, take another one, and see a doctor if it stays that way.

When your cycle is worth taking to a doctor

The American College of Obstetricians and Gynecologists describes the menstrual cycle as a vital sign, in the same category as blood pressure. A cycle that changes is information about your health.

Worth a doctor's time: periods that suddenly become much heavier or much more painful than your normal, bleeding between periods, cycles that swing wildly from month to month, cycles consistently shorter than 21 days or longer than 35, or no period for three months when you are not pregnant. Take your tracked dates with you. A doctor can do far more with four months of real dates than with "it comes when it wants".

Common questions

Where this comes from

• Periods: how the menstrual cycle works and what is normal. National Health Service.
• Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. American College of Obstetricians and Gynecologists, 2015.
• Relationship between the menstrual cycle and timing of ovulation revealed by new protocols: analysis of data from a self-tracking health app. Journal of Medical Internet Research, via PubMed Central, 2017.
• Plausibility of menstrual cycle apps claiming to support conception. Frontiers in Public Health, via PubMed Central, 2018.
• Fertility in the menstrual cycle. National Health Service.

This page has not yet been reviewed by a clinician. It stands on the published guidance above, and we say so plainly rather than implying a review that has not happened. Read it alongside our medical policy.