How period apps estimate your next cycle
Most period apps put a date on the calendar and call it your next period. It looks exact. It is not. Behind that date is a menstrual cycle calendar built from the bleeds you logged, turned into an estimate of when the next one is likely. The useful question is what that estimate is doing, and what it is not allowed to claim.
Period apps estimate your next bleed from logged cycles, not from a fixed 28-day clock. The dates they show are a projection. They are not a diagnosis, and they are not a promise of the morning.
Average cycle length is a starting point, not a law
Cycle length is the number of days from one period start to the next. The NHS says periods often come around every 28 days, and that a range from about 21 to 35 days is common. ACOG uses the same day-one-to-day-one count, and describes a typical adult cycle as lasting between 21 and 35 days, with an average near 28.
| Idea people use | What it actually means |
|---|---|
| “I have a 28-day cycle” | Your recent gaps from start to start have often been about 28 days |
| Average cycle length | A typical gap across the cycles you measured, not a rule for every month |
| Textbook 28 | A round classroom number; useful as a placeholder when you have almost no history |
| Irregular | Your gaps vary enough that a narrow next-period window is less honest |
Twenty-eight is easy to remember. It is also why a frozen clock feels precise and still misses. If your last few cycles were 31, 33, and 30 days, counting 28 squares from the last start is ignoring the body you just lived in. How to know when your period is coming is the longer version of that trade-off: people want a day, and an honest app offers a window from your own starts.
What most period apps are computing
The basic loop is the same across serious trackers:
- You log period starts (and often how many days you bled).
- The app measures the gaps between those starts.
- It builds a typical length from recent cycles.
- It adds that length to your last start to place a next-period estimate.
- If your gaps disagree, a better app widens the window instead of hiding the scatter.
That is a menstrual cycle calendar with memory. A paper grid without memory only does steps you force with a pen. What a period calendar actually tracks separates the marked bleeds (your logs) from the highlight (the estimate). This post is about how the highlight gets made.
Many apps also place related spans from the same next-period estimate. A common approach is to treat ovulation as roughly two weeks before the next period, then draw a fertile window around that day. The NHS describes ovulation as about 12 to 16 days before the next period starts. That is still an estimate from timing, not a hormone reading. What an ovulation calendar actually shows explains why the fertile highlight is a span, not a single morning.
How Lua estimates (so you can read our dates)
Lua follows the calendar method on your iPhone. In plain terms:
| Lua rule | What it means for you |
|---|---|
| Uses your last six cycles | Older history fades; recent cycles weigh more |
| Removes clear outliers | One wild gap does not rewrite the whole picture alone |
| Holds a 28-day placeholder until you have two measured cycles | You need three logged period starts before the estimate is personalised |
| Keeps cycle length between 21 and 40 days | Extreme gaps are clamped so one bad log cannot invent an impossible rhythm |
| Places ovulation 15 days before the next expected period | A fixed luteal-length assumption, common in calendar estimates |
| Shows a fertile window from 5 days before that day to 1 day after | A span around the ovulation estimate, not a confirmed ovulation |
| Shows a day range only when enough cycles agree it should | Thin or very steady history may show a single day; more scatter can open a small range |
Every one of those dates is still an estimate. Lua does not read hormones. It does not know the morning. It projects from the starts you trusted enough to log.
When the estimate gets shaky
Irregular cycles are the honest stress test. If your gaps jump by a week or more, any app has less to stand on. The right response is a wider window and clearer uncertainty, not a more confident red dot. Tracking irregular cycles covers how a log still helps when the pattern is uneven.
Other ordinary reasons the next gap will not match the last average: starting or stopping hormonal birth control, illness, heavy stress, travel across time zones, the months after pregnancy, and the years around perimenopause. New logs should matter. Old averages should fade.
Missing logs hurt more than people think. If you skip marking a start, the app either invents less or leans harder on a default. Neither is magic. The estimate is only as good as the day ones you enter.
What those dates do not mean
A next-period date in an app is not:
- Proof you will bleed that morning
- A diagnosis of pregnancy, PCOS, or anything else
- A contraceptive schedule
- A replacement for a clinician if your bleeding suddenly changes
What a menstrual app is actually for puts the home-screen job in one sentence: estimate from your own cycles, and stay honest that it is an estimate. Colored squares that pretend to know more than the logs contain are a different product.
Keep the estimate close to the history
Lua: Period & Birth Control estimates your next period from the cycles you log, with the rules above. Log the first day of real flow, and the picture can move when your recent history moves. Your logs stay on your phone.
Sources
- American College of Obstetricians and Gynecologists. Amenorrhea: Absence of Periods.
- American College of Obstetricians and Gynecologists. Fertility Awareness-Based Methods of Family Planning.
- NHS. Periods.
- NHS. Trying to get pregnant (timing around ovulation).
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