Clue built its reputation on one number: the day you are most likely to conceive. Millions of people open the app specifically to check their fertile window, and that single prediction is the feature most reviews and forum threads point to first. Getting it usually means an account, an onboarding flow, and a copy of your cycle history sitting on someone else’s server. We built a simpler path: a free ovulation calculator that runs entirely in your browser, needs no signup, and keeps every date on your own device.
Find your fertile window now
Set the first day of your last period and your average cycle length, and the timeline redraws as you type.
How to use it
Two inputs drive everything: the first day of your last period, and a slider for your average cycle length, defaulted to 28 days and adjustable from 21 to 35. There is no calculate button. Move the slider or change the date and the timeline, the headline countdown, and the key dates below it all update immediately.
The visual timeline marks four things on a single bar: when your period started, your estimated fertile window, your estimated ovulation day, and where today sits relative to all of it. A short list underneath repeats the same information as plain text, so nothing depends on reading the chart alone.
Everything you enter is saved in your browser’s local storage so it is still there next time you open the page, and nothing you type is ever uploaded anywhere. There is no account, no analytics, and no network request the tool makes on its own. A “clear my data” button wipes what it saved whenever you want.
How it compares to Clue
Clue’s core prediction and ours run on the same basic idea: count backward roughly 14 days from your next expected period to estimate ovulation, then mark the several days before it as fertile. Our calculator does that math and draws it on a timeline, which covers what most people actually open a fertility app to check.
What it does not do is everything else Clue is built around. Clue logs symptoms, mood, discharge, and dozens of other signals, then adjusts its predictions based on what you actually record instead of a fixed average. It also tracks PMS patterns, offers a research-backed content library, and can sync with a partner. None of that lives in our tool, and we would rather say so plainly than let a simple calculator pretend to be a daily companion.
The trade runs the other way too. Clue’s smarter, adaptive predictions live behind data you hand over, including an account and, for some features, a subscription. Our calculator only ever sees the two numbers you type into it, and it stays free with no tier to unlock. If you want a tool that learns your body over months, get the real app. If you just want today’s estimate without creating an account, this covers it.
Get the full app
Clue is free to download, with an optional subscription that unlocks personalized insights and additional tracking categories. It works on Android and iOS and supports cycle, symptom, and medication tracking well beyond ovulation prediction.
If you are comparing period trackers more broadly, we also built a private period cycle calculator inspired by Flo that predicts your next periods the same no-signup way.
FAQ
Can I calculate ovulation online without an app?
Yes. Our calculator runs entirely in the browser tab you are reading this in, so there is nothing to install to get a fertile window and ovulation estimate. Enter the first day of your last period and your average cycle length, and the timeline updates immediately.
Is this the official Clue app?
No. Cycle Bloom is an original calculator we built, and it is not affiliated with, endorsed by, or connected to BioWink or the Clue app in any way. Clue is a full tracking app with symptom logging, PMS patterns, and expert-reviewed content; our tool covers the ovulation and fertile window basics with a strict on-device privacy model.
How accurate is an ovulation calculator?
It is an estimate, not a medical diagnosis. The calculator projects ovulation and your fertile window from the average cycle length you enter, so accuracy depends on how regular your cycles actually are. Stress, illness, travel, and hormonal shifts can all move real ovulation earlier or later than the projection.
Is my data private?
Yes, by design. The calculator stores your inputs only in your browser’s local storage, on your own device. There is no server, no account, and no analytics, and the tool makes zero network requests once the page has loaded. The “clear my data” button removes everything it saved.
Can I use this for contraception?
No. Predicted fertile windows are estimates based on averages, and ovulation timing shifts from cycle to cycle for most people. This tool is not medical advice and should not be used as a method of contraception. 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.
