Flo is the most popular period tracker on Android, and tracking beats guessing. But not everyone wants an account, a subscription pitch, or cycle data sitting on someone else’s server. Period trackers as a category have faced regulatory scrutiny over data sharing, so wanting to keep this information to yourself is a reasonable position. We built a simpler option: a free cycle calculator that runs entirely in your browser, needs no signup, and keeps every date you enter on your own device.
Use the cycle calculator
Enter the first day of your last period and your averages, and the predictions update as you type.
How to use it
Set three things: the first day of your last period, your average cycle length, and how long your period usually lasts. The calculator starts with common defaults (28-day cycle, 5-day period), so you see a projection immediately and only need to adjust the numbers to match your own pattern. There is no calculate button. Every change updates the results live.
You get the days until your next period as the headline, your next three predicted periods with dates, and the estimated ovulation day and fertile window for each cycle. A three-month calendar marks period days in indigo, the fertile window with a sky blue outline, and ovulation with a dot, with a text legend so nothing depends on color alone.
Your inputs are saved in your browser’s local storage so they are still there next time you open the page. Nothing leaves your device, and there is a “clear my data” button that wipes everything the tool saved whenever you want.
How it compares to Flo
The full Flo app does far more than predict dates. It logs symptoms, moods, discharge, and dozens of other signals, then refines its predictions from what you actually record rather than from a fixed average. It also offers health insights reviewed by medical experts, an anonymous community, and a dedicated pregnancy mode that follows you week by week. If you want a daily companion that learns your body over time, a calculator does not replace that.
Our tool predicts from averages only. That works reasonably well if your cycle is fairly regular, and it gets less accurate the more your cycle varies from month to month. Flo handles irregularity better because it adjusts to each logged period, while a calculator keeps projecting the same interval forward. We think that trade is worth being upfront about: you give up adaptive accuracy and symptom tracking, and in return nothing about your cycle ever leaves your device.
There is also no account here, no email, and no subscription. Flo’s deeper features sit behind a paid tier, while the basics are free.
Get the full app
If you want symptom logging, smarter predictions, and pregnancy tracking, the full app is the way to go. Flo is free to download, with a premium subscription for the advanced features.
FAQ
How accurate are period calculators?
They are estimates, not guarantees. A calculator projects your average cycle length forward, so it is most reliable if your cycles are regular. Stress, illness, travel, and hormonal changes all shift real cycles, and irregular cycles reduce accuracy noticeably. Treat the dates as a planning guide, not a certainty.
Is my data private?
Yes, by design. The calculator stores your inputs only in your browser’s local storage, on your device. There is no server, no account, no analytics, and the tool makes zero network requests, so nothing you enter can be uploaded anywhere. The “clear my data” button deletes everything it saved.
Can I use this for contraception?
No. Predicted fertile windows are estimates based on averages, and ovulation can shift from cycle to cycle. This tool is not medical advice and must not be used as a contraception method. Talk to a doctor or healthcare provider about reliable options.
Does it work offline?
Once the page has loaded, yes. The calculator is a single self-contained file with no external requests, so it keeps working without a connection, and your saved inputs are still there when you come back.
Is this the official Flo app?
No. Cycle Compass is an original calculator we built, and it is not affiliated with, endorsed by, or connected to Flo Health in any way. Flo is a full tracking app with symptom logging and expert-reviewed insights; our tool covers the prediction basics with a strict on-device privacy model.
