Why period predictions are ranges, not dates

By the Twilune team, checked against the sources below · October 27, 2026 · 7 min read

A period prediction is an estimate, and a good app shows it as a range with a confidence level, not a single date. That is because cycle length changes from month to month, even in people whose cycles are regular. If your periods are irregular, the range is wider, and that is information too.

What is a normal cycle length?

The most quoted number is 28 days, and it is only an average. The NHS says periods usually come around every 28 days, but cycles commonly range from every 21 to every 35 days. The Office on Women's Health considers cycles regular if they come every 24 to 38 days. Cleveland Clinic gives both ranges in the same article. Sources differ a little, so treat the edges as soft.

Cycle length is counted from the first day of one period to the first day of the next. The period itself usually lasts around 5 days, though the NHS says anywhere from 2 to 7 days can be within the usual range.

Age shifts the pattern, too. The Office on Women's Health says cycles longer than 38 days are common for a few years after the first period, and become more regular within about three years. In your 20s and 30s cycles are usually regular. In your 40s, as the body moves towards menopause, they often become irregular, and periods may skip for a month or more.

How much does cycle length vary from month to month?

More than most people expect. A study of more than 600,000 cycles from users of a cycle-tracking app found these things:

• The average cycle was 29.3 days long.
• Only 13 percent of cycles were exactly 28 days.
• About two thirds (65 percent) of cycles fell between 25 and 30 days, 19 percent between 31 and 35, and 7 percent between 36 and 50.
• Within one person, the typical spread was around 2.6 days. That is one standard deviation of a person's own cycle lengths, so it means most of a person's cycles will land within a few days of their own average, and some will land further out.

Two caveats are worth stating. The study used data from a company's app users, and the company funded it, so the group may not represent everyone. It also looked only at cycles in which ovulation was detected. The pattern still agrees with how health sources describe cycles: they vary, and that is normal.

The study also found where the variation comes from. The part of the cycle before ovulation (the follicular phase) varied much more than the part after it. In plain words, most of the difference between a 26-day cycle and a 34-day one is that ovulation came earlier or later. The luteal phase, from ovulation to your next period, tends to be steadier. You can read more in the four phases of the menstrual cycle.

Why can a single predicted date be misleading?

If your cycles swing by a few days each way, a single "your period starts on Thursday" is right only some of the time. When it is wrong, it can feel like you did something wrong, or that your body is off. Usually it is neither. The prediction was just narrower than your real pattern.

A single date also hides information. Two people can both have an average cycle of 29 days, but if one has cycles that vary by a day and the other by five days, the honest answers are different. The first deserves a narrow window. The second deserves a wide one.

The same goes for phase estimates. Many apps and charts assume ovulation falls on day 14. In the large study above, even in cycles of exactly 28 days, ovulation did not fall on day 14 on average, and the authors state that the belief is not correct. Scientists who study the cycle say that phases should be confirmed with hormone measurements rather than by counting days, because phases are hard to predict in people whose cycles vary. A phone app cannot measure your hormones. So it should say "probably", not "definitely".

What does a good prediction look like?

Whatever app you use, look for these signs:

• A range, not a point. "Between the 12th and the 16th" is more honest than "the 14th".
• A confidence level. The range should widen when your history is short or your cycles vary a lot, and narrow as it learns your pattern.
• Your own data. Predictions based on your history beat a 28-day template.
• Plain language about limits. It should not claim to know your hormones.

Twilune works this way. It predicts your next period and your cycle phases from your own history and shows a range with a confidence level. With only one or two logged cycles, expect a wider range. As you log more, it can say more. Your logs stay on your phone, with no account needed. Twilune is not a medical device, and the range is an estimate, not a medical result.

If you only want a quick estimate without logging anything, the free cycle phase calculator uses your last period date and your usual cycle length. It is a rough guide for regular cycles. If you want to know how to pick an app that protects your data, read how to choose a private period tracker.

What counts as an irregular period?

There is no single line. The NHS says periods can change, for example by lasting longer or becoming lighter, and that this does not necessarily mean there is a problem, but it is worth getting checked. In practice, "irregular" usually means one or more of these:

• Cycle lengths that swing widely from month to month, well beyond a few days.
• Periods that come much earlier or later than your usual pattern.
• Missed periods.
• A big change in how long your period lasts or how heavy it is.

Compare to your own normal, not a textbook. A cycle that has always run 33 days is regular for you. A cycle that suddenly changes is the thing to notice.

What can make periods irregular?

The NHS lists these common causes:

• Puberty, when periods first start
• The start of menopause, usually between 45 and 55
• Pregnancy, where a missed period is often an early sign
• Some hormonal medicines, such as the progestogen-only pill, the injection and the intrauterine system
• Significant weight loss or gain
• Stress and anxiety
• Exercising too much
• PMOS (polyendocrine metabolic ovarian syndrome, formerly called PCOS) or thyroid problems

This is not a checklist for working out what is going on. Many of these look alike from the outside, and a doctor is the one to sort them out.

When should irregular periods go to a doctor?

The NHS advises seeing a GP (non-urgently) if:

• Your periods are irregular, or your usual pattern changes.
• Your periods last longer than 7 days.
• Irregular periods come with other symptoms, such as weight gain, tiredness, facial hair growth, or dry or oily skin.
• You bleed between periods.

Other NHS guidance on periods adds that bleeding after sex should be checked, and that if you have missed periods, have had sex and a pregnancy test is negative, you should see a GP once you have missed three in a row. Cleveland Clinic says to seek care if you have not had a period by age 16 or go three months or longer without one.

Heavy bleeding is its own reason to get help. The NHS describes it as changing a pad or tampon every one to two hours, needing two products at once, passing clots larger than a 10p coin, or leaking onto clothes or bedding. Severe period pain is also worth a visit.

A cycle log makes that visit easier. Dates, flow and symptoms across a few months give a doctor a clearer picture than memory does. Twilune lets you log your period, flow, symptoms, moods, energy and sleep, and you can export your data to bring with you.

Twilune is not a medical device, and this article is general information, not medical advice. If you have thoughts of harming yourself, contact your local emergency number or a crisis line straight away.

Frequently asked questions

This article is general information, not medical advice. If something worries you, or your symptoms are severe or getting in the way of your life, talk to a doctor. If you are thinking about harming yourself, call your local emergency number now.