Menstrual Cycle Calculator

No. Day 1 is the first day of regular bleeding. Light spotting before your flow starts does not count. Counting from the spotting can make your cycle look a few days longer than it is, and it can move your ovulation estimate too. If a clinic gives you different instructions, follow your clinic.

Add your average cycle length to the first day of your last period. With a 28-day cycle that started on 1 March, your next period is expected on 29 March. Most cycles land within about 4 days either side of the average, so think of it as a window, not an exact day. If your cycles vary a lot, enter your last few periods instead and the calculator gives you a range.

It depends on your cycle length, not on how many days your period lasts. Ovulation usually happens about 14 days before your next period starts. In a 28-day cycle that is around cycle day 14 or 15 (sources count it differently), which is roughly 9 to 10 days after a 5-day period ends. In a 35-day cycle it is later, around cycle day 21 or 22. For every cycle length from 21 to 35 days, see ovulation day by cycle length.

Most sources say 21 to 35 days is normal for adults, counted from the first day of one period to the first day of the next. FIGO's 2018 guidance uses 24 to 38 days. FIGO calls cycles regular when the gap between your shortest and longest cycle is 7 days or less if you are 26 to 41, and 9 days or less if you are 18 to 25 or 42 to 45. In the teen years, cycles are often longer and vary more.

Averaging helps, but a single date will often be wrong. Choose "My cycles are irregular" and enter the start dates of your last 3 to 6 periods. The calculator works out your average, shortest and longest cycle, then shows a window from your shortest to your longest cycle instead of one guess. The more periods you add, the better the window. Cycles that are very irregular, or regularly longer than 38 days, are worth mentioning to your provider.

A period is usually called late once it is more than a few days past the expected date, since cycles normally vary by up to about 4 days either side. If you could be pregnant, the NHS says most pregnancy tests are reliable from the first day of a missed period. If you do not know when your period is due, wait at least 21 days after you last had unprotected sex. A negative test is less reliable than a positive one, so test again after a few days if your period has not come.

Common reasons include late ovulation, stress, big changes in weight or exercise, hormonal birth control, breastfeeding, and conditions such as PMOS (previously called PCOS) or thyroid problems. A test taken too early can also be negative, and Ireland's HSE notes you can get pregnant in the days after your period is normally due if ovulation was delayed. If you still think you could be pregnant, wait a few days and test again. See your GP or doctor if a second test is negative and your period still has not arrived, and if you miss 3 periods in a row.

No day is completely safe, and a period calculator is not a birth control method. Calendar methods only suit people whose cycles are regularly between 26 and 32 days, and even then about 11 to 14 in 100 people using the Standard Days method get pregnant in a year with typical use. An IUD or implant fails in fewer than 1 in 100, and the pill in about 7 in 100. Sperm can survive for up to 5 days (the NHS says up to 7), and ovulation can shift. If you want to avoid pregnancy, a method that does not depend on predicting ovulation is more reliable. To compare methods, use the contraceptive effectiveness calculator.

It is as accurate as your cycle is regular. Even with regular cycles, a few days either side is normal. In one study, almost half of women who believed their cycles were regular had a gap of 7 days or more between their shortest and longest cycle. That is why this calculator shows a window and not only one date. A calculator cannot see your hormones, so it cannot tell you when you actually ovulate.

Yes. If you think you might be pregnant, the first day of your last period is used to date a pregnancy, and the due date is 280 days (40 weeks) after it. This page predicts periods, ovulation and fertile days, not pregnancy dates. For a due date, use the pregnancy due date calculator.

Count the first day of your last period as day 1. In a typical 28-day cycle, your period is roughly days 1 to 5, the follicular phase runs from day 1 to about day 13, ovulation happens around day 14 or 15, and the luteal phase runs from about day 15 until your next period. If your cycle is longer or shorter, use the ovulation date this calculator gives you: before it you are in the follicular phase, and after it you are in the luteal phase. Timings vary from person to person, so treat this as an estimate.

It may not match what your body does. Hormonal methods such as the pill, the injection or a hormonal IUD (IUS) can make periods irregular, or stop them for a while, and this is not usually a cause for concern. A date worked out from a cycle length may then be off. If you recently stopped hormonal birth control, your cycle can take time to settle, and a doctor can advise if your period has not returned within about 3 months. To see how ovulation returns, read ovulation after stopping birth control.

Speak to a doctor or nurse if you miss 3 periods in a row and are not pregnant, if your periods become irregular or change suddenly, if they last longer than 7 days, if you bleed between periods, or if you have severe pain or much heavier bleeding than usual. Conditions such as PMOS (previously called PCOS) and thyroid problems can also cause irregular periods, and a doctor can check for them. If you could be pregnant, take a pregnancy test first.

A period tracker, such as an app or a calendar, is a record of the periods you have already had. A period calculator, like this one, uses the dates you enter to estimate your next periods, ovulation and fertile window. Nothing you enter here is saved, so keep your own record of your start dates and enter the last few whenever you want a fresh estimate.