How to Predict Your Next Period

How do you work out when your next period is due?

Predicting a period is mostly arithmetic. You need two things: the date your last period started, and how long your cycles usually last.

1. Find day 1. Your cycle starts on the first day of your period. The US Office on Women’s Health (OWH) counts a cycle from the first day of one period up to the first day of the next.
2. Collect a few cycle lengths. Count the days between the start dates of your last few periods. The NHS notes that an app or a diary makes this easier.
3. Average them. Add the lengths and divide by how many cycles you have.
4. Add the average to your last start date. That date is your best single estimate.

Here is a worked example. Say your last five cycles were 27, 29, 31, 28 and 30 days. They add up to 145, so your average is 29 days. If your most recent period began on March 3, adding 29 days gives April 1. To look further ahead, keep adding the average: the period after that would be due around April 30, then May 29. Each step adds a little uncertainty, so the near prediction is always the most reliable one. The period calculator does the same sum for you and also projects the next few months.

Why is a range better than a single date?

A single date looks precise, but most cycles vary a little from month to month. OWH puts it plainly: some people are so regular they can predict the day their period starts, while others who are still regular can only predict it within a few days.

Big datasets agree. In a study of 32,595 people who used a connected home ovulation test (funded by the test’s maker), 52% had cycles that varied by 5 days or more. The same study, by Soumpasis and colleagues, found that 25.3% of users believed they had a 28-day cycle, but only 12.4% actually did. What we remember about our cycles and what our dates show are not always the same.

A useful habit is to build a window from your own extremes. In the example above, the shortest recent cycle was 27 days and the longest was 31. So instead of circling April 1, you would expect your period between March 30 and April 3. If you plan travel, events or supplies around that window, a few days’ drift will not catch you off guard.

How many cycles do you need for a good estimate?

One cycle tells you very little, because it cannot show how much you vary. Three cycles give a first rough picture, and six or more give a steadier average. OWH suggests marking each start date on a calendar and, after a few months, looking at whether your cycles are regular or differ from month to month.

Use recent cycles rather than your whole history. Cycle length changes slowly over the years. In the 2019 study by Bull and colleagues of more than 600,000 cycles, the average shortened by about 0.18 days per year between ages 25 and 45. That is a small amount, but it adds up over a decade.

Can tracking ovulation make the prediction more accurate?

Yes, if you already track it. The days from ovulation to your next period tend to be steadier than the days before ovulation. Bull and colleagues found the luteal phase, the stretch after ovulation, averaged 12.4 days and barely changed across age groups. Most of the difference between short and long cycles came from the phase before ovulation.

That gives you a second clue. The NHS says ovulation usually happens 10 to 16 days before the next period. Ovulation tests detect luteinizing hormone (LH), which OWH says starts to surge about 36 hours before ovulation and peaks about 12 hours before it. Putting those together, a positive test suggests your period is roughly 11 to 17 days away. This is still a range, but it often narrows the estimate in a cycle that started later than usual.

Basal body temperature, measured right after waking, rises slightly around ovulation and can serve the same purpose for people who chart it.

What can make a period arrive earlier or later than predicted?

A prediction assumes this cycle will look like your recent ones. Sometimes it doesn’t. The NHS lists common reasons for a change in timing:

• puberty, and the years leading up to menopause (usually between ages 45 and 55)
• hormonal contraception, including the progestogen-only pill, the contraceptive injection and the hormonal coil (IUS)
• losing or gaining a lot of weight
• stress and anxiety
• exercising a lot
• pregnancy

A single shifted cycle usually doesn’t change the bigger picture. Log the real start date, and your average will adjust.

How do period tracking apps make their predictions?

Most apps do what is described above, automatically. Bull and colleagues describe calendar-based apps as working from the length of your cycles and assuming average phase lengths. Some apps add signals such as temperature or ovulation tests on top.

Lunation predicts your next period and fertile window from the periods you log. Until it has some of your own history, it starts from the cycle length you enter (28 days if you skip it), so its estimates get more personal as you add cycles. It works without an account and never shares or sells your health data.

Whichever tool you use, the rule is the same. Predictions come from your past dates, so logging the first day of every period is what makes them accurate.

When is it worth talking to a doctor?

Unpredictable timing is common now and then. ACOG suggests talking to a doctor if your cycles are regularly shorter than 21 days or longer than 35 days, or if their length varies by more than 7 to 9 days. The same applies if you have no period for 3 to 6 months, bleeding that lasts more than 7 days, or bleeding between periods. The NHS also suggests seeing a GP if your periods become irregular or your usual pattern changes. Your tracked dates will be useful at that appointment.