How the math works

Two signals do the work. Your waking temperature, taken first thing in the morning before you get up, eat, or drink, while your body is still at rest. And your cervical fluid, which changes in a set order as fertility approaches: dry, sticky, creamy, egg white. Those are the only four words you need — the daily call is really just “fertile or not fertile”, and creamy or egg white means fertile.

Those two, plus menstrual flow, which is what marks the start of each cycle and so sets the day numbers everything else is counted in. Using temperature and fluid together, rather than either alone, is what makes the reading trustworthy. That combination is called the sympto-thermal method.

Everything else is recorded for your own reference and charted alongside them, but does not feed any calculation: cervix position, intercourse, test results, moods, symptoms, notes, and your own custom trackers. Cervix in particular is taught as the optional third sign, a cross-check you make yourself, so the rules below deliberately leave it to you.

Day 1 is the first day of real menstrual bleeding, meaning light, medium, or heavy flow following days that had none. Spotting on its own does not start a new cycle. From that Day 1, days are numbered 2, 3, 4 and on until the next period begins.

After an egg is released, progesterone raises your resting temperature by roughly 0.2 to 0.5°F. So your mornings fall into two groups: a run of lower readings before ovulation, and a run of higher ones after it. The jump between them is the thermal shift, and it is what confirms ovulation actually happened.

Finding it by eye is error-prone, so the rule is exact — and it runs in a fixed order: peak day, then temperature shift, then coverline. The shift is only looked for on or after your peak day; temperatures before it are not candidates at all. That ordering is what stops a fever or a broken night early in the cycle from being mistaken for ovulation.

At each candidate first high day, on or after peak day:

1. Look back at the six low readings just before it. Those six are the recent baseline.
2. Take the highest of the six and add 0.1°F. That value is the coverline, the bar a temperature has to clear to count as genuinely high.
3. Require three highs in a row. The shift is confirmed only when three consecutive readings all sit above the coverline.
4. Require the confirming temperature to clear the coverline by three boxes (0.3°F). Being above the line is not enough on its own — that height is what shows enough progesterone to prove ovulation is over. If it falls short, Lutaria waits one more day rather than closing your fertile window.

One warm morning on its own is not enough, because it could just be a bad night's sleep. Once the shift is confirmed, ovulation is placed about one day before the first high reading.

If a reading falls back to the coverline, the count restarts — but the coverline stays put. A dip below the line (or exactly onto it) partway through means the count of three begins again from the next high reading, measured against the same coverline. It was drawn from the six readings before the shift, and redrawing it from a later six would be a different rule than the one you were taught.

Peak day itself can move while the count is running. If fertile fluid returns before the three non-fertile days are complete, peak day moves to that later fertile day and the counts start again. Once both rules are fulfilled it stops moving: fertile fluid after that is still recorded, but ovulation has been confirmed and the question does not reopen.

Until a shift is identified there is no coverline at all. The coverline is defined as the six readings before the shift, so before there is a shift there is nothing to draw it from — and Lutaria says that rather than drawing a line from an arbitrary six and letting it look authoritative.

On a morning you slept badly, were ill, or took the reading much later than usual, you can flag the temperature as questionable. A flagged reading is left out of every temperature calculation. It never helps set the coverline and can never be one of the three highs that confirm a shift. It still appears on your chart, marked so you can see it, but it does not get a vote in the math. That is exactly how a careful person charting on paper would treat a reading they doubted.

Temperature confirms ovulation after the fact. Fluid does the opposite and warns you it is coming, which is why the two signals fit together so well. One looks forward, the other looks back.

The first fertile-quality day is the first day you record creamy or egg white. That is where the fertile window opens when you are charting to conceive. The peak day is the last day of anything fertile — creamy counts, not just egg white — before it dries up again. Ovulation happens right around it. You can also mark your own peak day by hand.

Because peak day is defined by the drying-up that follows it, it is only named in hindsight. While you are still seeing egg white, Lutaria reports no peak day yet rather than guessing that today is the one. And once both rules are fulfilled, peak day stops moving: fertile fluid appearing after that is still recorded, but ovulation has been confirmed and the question does not reopen.

Ovulation. If the temperature shift is confirmed, that is the stronger signal, so ovulation is placed one day before the first high reading. With no confirmed shift, the peak day becomes the estimate. With neither, Lutaria says it cannot estimate ovulation yet rather than inventing a day.

The fertile window opens according to what you are charting for. If you are avoiding pregnancy, it opens on cycle day 1: there is deliberately no calculation rule here. Sperm can survive long enough to meet an egg released later than expected, so the whole pre-ovulatory phase is treated as fertile until both closing rules prove otherwise. If you are trying to conceive or just observing, it opens at your first day of fertile-quality fluid— creamy, watery or egg white — because “you are fertile from day one” is true but useless when you are timing intimacy.

It closes only when both rules are fulfilled, never on one alone, on the evening of whichever lands later. The temperature rule needs three readings above the coverline with the confirming one at least 0.3°F above it. The cervical fluid rule needs Peak+3: three consecutive non-fertile days — dry or sticky — after your peak day. If only one rule is met, the window stays open and Lutaria tells you which one is still outstanding. If fertile fluid comes back during the count, the count restarts and your peak day moves with it.

A day you charted but left the fluid field blank counts as non-fertile. That is how this method reads a real chart — “nobody even bothers to record dry, I just assume that if nothing’s recorded, that was a dry day” — and insisting on an explicit “dry” would be stricter than the method itself, leaving the count unfinished and the window open forever. A day you never charted at all is different: it still blocks the rule, because a day you skipped is not an observation.

Until one of those counts finishes, the window is open — which is not the same as absent. A peak day is only named once the fluid dries up, so a cycle in the middle of its fertile phase usually has no closing day yet, and that is exactly when the shading matters most. An open window is still drawn: from its opening day through the last day you have logged, with an unfinished edge. The chart band fades out instead of stopping in a hard line, the ring on your home screen ends flat rather than rounded, and the calendar shades those days lighter. The shading stops where your log stops, not where the window ends.

Confidence is graded so you know how much weight to give the reading. High means both signals point at the same day: the shift is confirmed, a peak day is established, and the two land within two days of each other. Medium means only one signal is present, or that both are but they disagree by more than two days, which is a chart worth a second look rather than a confident reading. Low means neither is established yet, which is normal early in a cycle.

The answer is a date range, not a single day, and there are two ways to reach it. The stretch from ovulation to the next period, called the luteal phase, stays fairly constant for a given person, usually between about 10 and 16 days, while the part that varies is the stretch before ovulation. So once ovulation is known:

next period = estimated ovulation day + your usual luteal phase

The other way is the count most people already do by hand, and it needs no ovulation estimate at all — which is exactly why it is worth having:

next period = cycle day 1 + your usual cycle length

Which one Lutaria uses, in order:

1. A cycle length you set yourself, under Settings → Period prediction. If you know your cycles run 30 days, say so; your number beats a measured one.
2. The ovulation count, once your temperature shift is confirmed. A confirmed shift means ovulation was observed rather than guessed, and an observation from this cycle beats an average taken from other ones.
3. The day-1 count, using your own average cycle length. This is what runs while the shift is still unconfirmed, which is most of the first half of a cycle.
4. The ovulation count from a fluid peak alone, only if you have no completed cycles yet to average. Lutaria labels this one as a guess that will move.

That order matters. Before it existed, an early patch of creamy fluid that dried up could set a peak day around day 7 and push the prediction nearly a week early, on a chart whose owner already knew roughly when her period was due.

The rangecomes from how much your own cycles actually vary: the shortest and longest of your recent completed cycles set its edges. Cycles that run 27 to 28 days give a tight range; cycles that run 24 to 35 give an honestly wide one. It is never narrower than five days, because a period predicted to the exact day claims a precision this method does not have — and never wider than nine. When your cycles have varied more than that, no range is drawn at all: a fortnight of “your period might start here” is not a prediction. You still get the single most likely day, and the "why these numbers" panel names your real spread and points at the cycle length you can set yourself, which is the better number when a history varies that much. Unusually short or long cycles are set aside before the spread is measured, so one odd month does not widen the band for every cycle after it — and a cycle length you set yourself always gets the plain five-day band.

The range is a range of start days, not a claim about how long your period will last. On the calendar it is drawn as outlined days, with the single most likely first day filled in.

When the two counts disagree by three days or more, both are shown, along with which one was used and why. A wide gap usually means this cycle ovulated earlier or later than your others, which is information rather than an error.

Your usual numbers come from your own history, not a textbook. As you complete cycles, each one's length and luteal phase are measured and averaged, then the current cycle is analysed again using your numbers. Until there is history, the luteal phase starts from a 14-day default and says so. Lutaria does not invent an average cycle length: with no completed cycles and no confirmed shift, it says it cannot predict yet.

Here is a cycle with all of it in one picture. Temperatures run low for eight days, then jump and stay up. Fluid builds from creamy on day 4 to egg white on days 7 and 8.

• The six lows before day 9 top out at 97.5°F, so the coverline is 97.5 + 0.1 = 97.6°F.
• Days 9, 10 and 11 all sit above it, three in a row, and day 11 at 98.0°F clears the coverline by 0.4°F — more than the required 0.3°F — so the temperature rule is fulfilled.
• Ovulation is estimated at day 8, one day before the first high reading.
• The fertile window opens on day 1 if you are charting to avoid pregnancy, or on day 4 (the first creamy day) if you are charting to conceive. It closes on day 11, where both rules happen to land: the temperature rule, and Peak+3 counting from the day-8 peak.
• The peak day is day 8, the last egg white day before the fluid turns sticky on day 9. It lands on the same day as the temperature estimate, so confidence is high.
• If this person's usual luteal phase is 14 days, the next period is predicted around cycle day 22, and because the temperature shift is confirmed, that ovulation count is the one used. If her recent cycles have run 27 to 31 days, the range drawn around it runs to cycle day 31 — and since the day-1 count lands about a week later than the ovulation count, both are shown, with a note that this cycle appears to have ovulated early.

Every one of those numbers traces back to a rule on this page and to the raw observations you logged. None of it is a black box.

There is more than one sympto-thermal tradition, and they disagree on details. Lutaria follows the method as taught by Asya Mishory in her Unlocked Fertility workshop. This app exists to digitize that curriculum, so if you learned to chart in her workshop or bootcamp, what you see here should match what you were taught. Where her teaching differs from other published systems, hers is what the app implements.

Because that matters, here is where Lutaria deliberately differs from what you may read elsewhere:

• Creamy fluid is fertile. Peak day is the last day of anything fertile, egg white or creamy. Some systems only count the most slippery qualities, which sets peak day earlier and shortens the count.
• The confirming temperature has a height requirement. Three readings above the coverline is not enough on its own; the confirming one must clear it by 0.3°F. Many charting guides stop at three-in-a-row.
• There is no calculation rule for opening the window.Rules like “shortest cycle minus 20” are common elsewhere. This method does not use one: when you are avoiding pregnancy you are treated as fertile from day 1 until both closing rules are met.
• Both rules always close the window together. A confirmed temperature shift on its own never closes it, even when there is no fluid logged to cross-check.

Lutaria is a charting tool, not a teacher and not a contraceptive. It shows you what the rules say about the observations you entered. Learning the method itself, and deciding what to do with a given chart, is between you, your instructor, and your own judgment.

On your device. The calculations above are fixed rules in the app itself, not a service call, so your daily log does not have to go anywhere for your chart to be drawn.

The same goes for the explanations. Alongside every number, the engine records which rule produced it and why, and shows you that directly under Why these numbers on your chart, in your cycle history, on the provider report, and on any chart you share with an educator. No account, connection, or AI is involved in reading your own chart.

Optionally, an assistant can reword that same explanation in more conversational language. It receives only the summary those rules already produced, never your raw entries, it adds nothing to the reasoning, and it cannot invent a number. It can also be switched off entirely in Settings. The what we can and can't see popup has the full detail.

Lutaria is an educational tracking tool. It is not a medical device, not medical advice, and not a contraceptive. Charts and predictions are estimates built from data you enter. If you are making decisions about preventing pregnancy, work with a healthcare provider or certified educator on a method appropriate for you. If you have been trying to conceive without success for a year, or six months if you are 35 or older, talk with a doctor.