How accurate are period tracker apps?

Why next-period predictions are usually close but rarely exact, why calendar ovulation dates are often wrong, and how to read any app’s dates, Bloom’s included.

Last reviewed 16 October 2026Checked against the sources listed below

Your app said your period would start on the 12th and it came on the 17th. Or it showed a fertile window that turned out to mean nothing. This guide is for anyone wondering how much to trust a period tracker: what apps predict well, what they predict badly, what research has found, and how to read any app’s dates, Bloom’s included.

What a period app is actually doing

Most period trackers do the same basic thing: they look at the dates you have logged and project them forward. If your last few cycles were around 30 days, the app expects the next one in about 30 days. Some then estimate ovulation and a “fertile window” by counting back from the expected period. Unless you also log something your body measures, such as ovulation test results or temperature, every date after your period is a calculation from the calendar, not an observation.

That makes two very different kinds of prediction, and they deserve different amounts of trust.

Next-period predictions: usually close, rarely exact

A prediction of your next period can only be as steady as your cycles are. Cycles vary for nearly everyone: in an analysis of more than 600,000 cycles from app users, the average was 29.3 days, only about 13% of cycles were exactly 28 days long, and most of the variation came from the first half of the cycle, before ovulation.^[1] So an app that predicts a date is really predicting the middle of a range. If your cycles usually vary by four days, a prediction that is three days off is the app working normally.

Predictions get worse when cycles are irregular, when you have only logged a cycle or two, or when your body has recently changed, for example after pregnancy or stopping hormonal contraception.

Ovulation and fertile windows: the weak spot

Calendar-based ovulation dates are where apps go most wrong. In a study of how women’s fertile window actually falls, ovulation happened exactly 14 days before the next period in only about 10% of cycles, and the gap from ovulation to the next period ranged from 7 to 19 days.^[2] An editorial on that study concluded that fertile days can fall at almost any point in the cycle, even for women whose cycles are regular.^[3]

When researchers tested ten period tracker apps with simulated users, the apps’ ovulation predictions were mostly wrong: of 36 predictions for four of the user profiles, only 3 were exactly right, and most fell two to nine days early.^[4] The authors concluded that predicting the fertile window needs a real sign of ovulation, such as temperature, ovulation tests or cervical mucus, not the calendar alone.^[4]

Even those signs have limits. ACOG explains what ovulation tests and cervical mucus can and cannot tell you about your fertile days.^[5]

How Bloom’s predictions compare

Bloom’s dates have the same limits, and it is better to say so. Here is what it does to keep them as honest as it can:

• It predicts from your recent cycles, using the median of your last six, so one odd month doesn’t pull every prediction with it.
• It labels its first estimates as estimates and tells you how many more cycles it needs before it shows the rhythm it has seen in your logs.
• It doesn’t pretend a late period has arrived. It waits and tells you, and calls a period late only when it is outside your own usual variation.
• Its ovulation estimate starts as a calendar estimate, counted back 14 days from your next expected period, which is exactly the assumption the research above questions. If you log ovulation tests or cervical fluid, Bloom uses them to place your phases, and after two cycles where they line up it uses your own luteal length instead.

Bloom cannot confirm ovulation. It is not a fertility tool or a method of contraception, and its fertile-time estimates should never be used to avoid or plan a pregnancy. There is more on how it reads your phases in which phase of your cycle you’re in.

How to read any app’s dates

• Treat a predicted date as the middle of a window, about as wide as your cycles usually vary.
• Give it time. Predictions from one or two cycles are guesses; from six, they are a pattern.
• Don’t rely on calendar fertile windows for avoiding or planning pregnancy. Talk to a doctor or nurse about methods that are tested for that.
• Use the record, not just the forecast. Your logged history of cycle lengths and symptoms is often more useful than the prediction, especially at a doctor’s appointment. If you are wondering whether your own numbers are typical, see is my cycle length normal?

Bloom is a wellness and planning app. It is not a medical device, it does not diagnose or treat any condition, and it is not a method of contraception. This page is general information, not medical advice. If something about your cycle worries you, talk to a doctor, nurse or midwife.