Prime Number Checker — Is n Prime?
Test whether a non-negative integer is prime and list helpful factors when it is composite.
How it works
- 1Enter your values in the fields above
- 2Click "Calculate" — all math runs in your browser
- 3View your results instantly
What to do next
About Prime Number Checker
Prime Number Checker handles a focused step in the modern calculation workflow. Test whether a non-negative integer is prime and list helpful factors when it is composite. The page loads with the upload area, controls and result panel all visible at once, so the path from "I have a file" to "I have the result" is one screen long.
Prime Number Checker is shaped around the recurring needs of two audiences: engineers sanity-checking conversions, who use it as a quick utility between bigger tools, and professionals validating quick estimates, who use it as their primary way of getting the job done. Both groups get the same defaults and the same speed.
Reach for Prime Number Checker when you need a predictable result on a single file. The page works on the first visit, the controls are visible without a menu, and the output is delivered the moment the engine finishes.
Under the hood, Prime Number Checker uses standard browser APIs to do the actual work. Input runs through the same engine, with a per-file ceiling of 0 MB so memory usage stays predictable on lower-end laptops and tablets. The engine ships as part of the page bundle, so once the page is loaded the tool keeps working even if your network connection drops.
The browser sandbox isolates the page's JavaScript from the rest of the system, the same way it isolates every other tab you have open. Prime Number Checker works inside that sandbox: it reads the file you give it, processes it with standard browser APIs, and writes the result back. Nothing leaves the page's memory unless you choose to download or copy it.
For multi-step jobs, Prime Number Checker sits next to Prime Factorization Calculator, GCD Calculator, and Random Number Generator. None of them depend on each other — you can use Prime Number Checker on its own — but together they cover the common variations of the task this page exists to handle.
The hard constraints are easy to remember. Maximum input: 0 MB. Multiple files per run: no — one input at a time, by design, to keep results predictable. The same controls apply on every run.
Prime Number Checker keeps the control set focused. Every option on the page is there because a real workflow needs it, and the defaults aim at the most common case so a first-time user can get the right output without changing any settings.
Output handling is intentionally boring: Prime Number Checker produces a single output file and triggers your browser's standard "save" behaviour. If you have a default download folder configured, that is where it will land. There is no Favtoo-side history of jobs you have run.
From a product perspective, Prime Number Checker is one of the simplest possible expressions of "do one thing well." The catalog contains dozens of related tools that each handle a slightly different calculation task, and every one is a separate page rather than a tab inside a larger app. That separation keeps each tool fast to load and easy to bookmark.
Prime Number Checker runs as a regular web page, so there is no install step or permission grant before the first run. The page can be audited by viewing the source or by watching the developer-tools Network tab while a job runs.
If you want to get the most out of Prime Number Checker, three small habits help. Drag-and-drop is faster than the file picker once you get used to it. The keyboard shortcut for downloading the result is whatever your browser uses for "save link as," because the result is a normal download. And if you are working on a sensitive file, processing in an Incognito or Private window is a good extra layer — it leaves no trace in browser history when the tab closes.
If the result is not what you expected, the most common causes are easy to check. Confirm the input is under the 0 MB ceiling — files just above the cap fail silently because the engine refuses to allocate the buffer. Confirm the input is one of the supported formats. And if the page itself feels slow, try closing other heavy tabs to free up memory; the engine runs in your browser, so it competes for the same resources as everything else open.
If Prime Number Checker solved your problem, sharing the page link with someone who has the same problem is the most useful thing you can do. The catalog grows mostly through word of mouth; visitors arriving through a recommendation tend to be the ones the tool serves best.
How it works
- 1Land on the Prime Number Checker page. The tool is ready to use the moment the page renders.
- 2Add your calculator input by dropping it onto the page or browsing for it.
- 3Tweak the controls if the defaults are not quite right for your input. The options are kept short and labelled in plain language.
- 4Trigger processing. standard browser APIs reads your input, applies the transformation, and writes the result back into the page.
- 5Download the result. The file is generated in your browser and saved through your normal download flow.
- 6Repeat the process for additional inputs whenever you need to. The page stays loaded, so subsequent runs are quick.
Common use cases
- Plan a project budget on a phone in a meeting using Prime Number Checker.
- Estimate a finance schedule before approaching a bank.
- Compare two scenarios side by side without spinning up a spreadsheet.
- Work out a percentage change between two figures.
- Split a restaurant bill cleanly between a group.
- Estimate how much paint or material a room will need.
- Sanity-check a quote before sending it to a customer.
- Forecast a fitness target without a paid app.
- Convert a foreign currency amount into your local one.
FAQ
What algorithm is used?
Trial division by all integers up to the square root, with an even-number shortcut after checking two.
Is zero or one prime?
Neither is considered prime; the tool reports them as not prime by the usual definition.
What factors are listed for composites?
Non-trivial divisors strictly between 1 and n when available from the factor listing.
How large can n be?
JavaScript integers are safe up to about 9e15; very large inputs may be slow or hit precision limits.
Is my number private?
Yes — testing runs only in your browser.
Should I use prime factorization instead?
Use factorization when you need the full multiset of prime factors; this tool answers primality first.
Does Prime Number Checker work with screen readers?
Prime Number Checker uses native HTML controls wherever possible, which means keyboard navigation, focus rings, and screen-reader labels work the way the platform expects. The drop zone accepts files via the keyboard-accessible file picker as well as drag-and-drop, and result downloads use standard browser download flows. If you spot an accessibility gap, Favtoo treats it as a bug worth fixing.
Can I self-host Prime Number Checker for my team?
Prime Number Checker is a static page running an open-source engine in your browser, so a typical corporate firewall does not get in the way as long as it allows JavaScript to load from Favtoo. For teams that need to host it themselves on an internal network, the underlying engine (standard browser APIs) is open-source and can be packaged into a private build with the same behaviour. Reach out via the Contact page if that is something you are exploring.
How do I know I am using the latest version of Prime Number Checker?
Prime Number Checker is updated whenever the underlying engine releases an improvement or a bug fix. Because the tool is delivered as a static page, every visit fetches the latest version automatically — there is no "version" to manage on your end. If a particular release ever changes default behaviour, the change is documented on Favtoo's changelog so you can confirm what shifted.
Which file formats does Prime Number Checker accept?
The accepted formats are listed in the upload area on the tool itself. If your input is in a format that is not directly supported, convert it first using one of Favtoo's converter tools — every Favtoo converter outputs a file that is a clean input to the next tool in the chain.
Where does my file actually go when I use Prime Number Checker?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.
Is Prime Number Checker really free?
Prime Number Checker is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
Can I use Prime Number Checker on iOS or Android?
Prime Number Checker runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 0 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
Is there a desktop version of Prime Number Checker?
No installation is needed. Prime Number Checker runs as a normal web page, with no browser extension, no native helper, and no separate desktop client to download. That is partly a privacy choice — extensions can request broad permissions, while a regular page is sandboxed by default — and partly a convenience one: you can use Prime Number Checker on any computer you have temporary access to without leaving anything installed on it.
How do I run Prime Number Checker over a folder of files?
Prime Number Checker processes one input at a time by design — it keeps memory usage predictable on lower-end devices and makes results easier to verify. To handle a folder, run the tool once per file; the page stays loaded between runs and remembers your last-used settings, so the second run is essentially instant.