Free cycle tools,
kept on your side.
Five instruments for the questions a calendar can answer, and honest about the ones it cannot. Every one of them runs in this browser: nothing you type is uploaded, stored on a server or attached to a name, because there is no server and there is no name.
Period Calculator
Your last period plus your usual cycle length, and the next few months laid out on the moon ring — with the spread you should actually expect.
Open itCycle Length Calculator
Log a few period start dates and read your own numbers: average, shortest, longest, and how much the whole thing moves.
Open itLuteal Phase Calculator
The days from ovulation to your next period — counted exactly when you know the date, and estimated with the uncertainty shown when you do not.
Open itPrintable Period Tracker
Build a paper tracker for any month or a whole year, then print it. Flow key, symptoms, an optional temperature row — and a gentler version for a first tracker.
Open itPrintable BBT Chart
Graph paper for a cycle: a temperature grid in °C or °F, a row for the coverline, and space for the notes that make a chart readable later.
Open itWhat these tools are, exactly. They do arithmetic on dates you type in. A cycle is a rhythm, not a timetable, so every date they produce is an estimate with a range around it — and each page prints the range rather than hiding it behind a confident-looking number.
None of them is contraception, none can tell you whether you are pregnant, and none can diagnose anything. If a number here worries you, or your cycle has changed and you do not know why, the person to ask is a doctor, nurse or midwife. Every page names the NHS, ACOG, NIH or peer-reviewed papers it stands on, with live links.
Longer answers
The Most Private Period Tracker: What Actually Protects You
Most privacy claims are promises. A few are architecture. This is how to tell which one you are being offered — and the seven questions that separate them.
Read it Understanding your cycle · 8 min readWhen Will I Get My Period? What a Quiz Can and Can't Tell You
Every quiz that promises a date is guessing. Here is what the signs actually mean, what the timelines usually look like, and an estimator that gives you a range and admits it.
Read it Understanding your cycle · 7 min readThe Moon and Your Menstrual Cycle: What the Research Actually Found
Two rhythms of about twenty-nine days, and a very old story about why. Here is what the studies actually found — including the ones that found nothing.
Read itAthena keeps all of this on your phone.
The calculators here forget you the moment you close the tab. Athena is the same arithmetic living on your device — the moon ring, an honest calendar with ranges instead of false precision, and a database that never leaves the phone because there is no account and no server behind it.
In development for iPhone and Android. Not released yet — when it is, it will be on this site.
What Athena is