Straight answers about
your cycle.
Two subjects, because they are the two this site can speak on honestly: what happens to the data you hand a period app, and what actually happens in a cycle. Every claim below is attributed to the NHS, ACOG, the NIH, a regulator or a peer-reviewed paper, with a live link on the page.
Privacy and your data
What period apps do with what you tell them, what the words in a privacy policy actually mean, and how to judge any tracker before you trust it with a year of your life.
Understanding your cycle
Straight answers about what a cycle is, what can be predicted, what cannot, and where the popular stories about the body came from.
When 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 7 min read · 6 sourcesThe 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 itHow this writing is done. Sources are limited to the NHS, ACOG, the NIH, the FTC, the ICO and peer-reviewed papers on PubMed Central. Every URL is opened and read before it is cited, and the citation carries the publisher, the year and the bare link so you can check it in one click.
What is not here, on purpose. No cycle-syncing protocols, no predicting a baby's sex, no "safe days", no diagnosis. Where an accurate sentence and a popular search phrase pull in different directions, the accurate sentence wins and the page finds another way to answer the question. Several widely repeated numbers were cut from these pieces during checking because no source supports them.
None of it is medical advice, and none of it replaces someone who knows your history.
Free cycle tools
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 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