Irregular Cycle Predictor
Most period calculators ask for one date and a cycle length you have to guess. This one asks for several real period dates and works out the rest — then shows you exactly how it got there.
Enter at least two period dates — one date on its own has no interval to measure.
Nothing you type here leaves your device. This calculator runs entirely in your browser — the dates are never sent to us, never stored, and vanish when you close the tab.
Cycles between 10 and 120 days count as cycles. The median is taken over your most recent 6.
Why one date and a cycle length is not enough
The standard calculator adds your cycle length to your last period date. That works when your cycles barely vary. When they do vary — with PCOS, in perimenopause, coming off hormonal birth control — it fails in a specific way: it asks you for the one number you do not reliably know, and then presents whatever you guessed back to you as a prediction.
Averaging is not the fix either. If your last two cycles were 45 and 28 days, the average is 36.5 — a cycle you have never had. The prediction will be about a week late for one and more than a week early for the other, and it will be wrong confidently, with a specific date.
What this calculator does instead
It takes the median of your recent cycles rather than the mean, so one unusual month cannot drag every later prediction with it. Before that, it looks for the interval that is roughly double the others — the fingerprint of a period you forgot to log, recorded as one long cycle instead of two normal ones — and leaves it out. It tells you which one it dropped and why, rather than quietly adjusting the answer.
And when there is not enough to go on, it says so. One period date has no interval to measure. A single interval gives a real answer but a thin one, and it is labelled as thin. At no point does it substitute 28 days — a population median wearing your name is not a prediction about you, and “still showing the default 28 days” is one of the most common complaints about every other app in this category.
This is the same code that runs inside Dawn Phase for logged-in users. Not a simplified demo of it — the identical prediction, from the identical file.
Your data stays on your device
The calculation runs in your browser. The dates you enter are never sent to a server, never written to a database, and are gone when you close the tab. There is no account, no email box, and no tracking pixel counting what you typed.
Note: These are estimates only and should not be used for contraception or medical decision-making. Consult a healthcare provider with any concerns.