Proportion Calculator — a/b = c/d
Solve a/b = c/d for whichever variable is unknown using cross-multiplication.
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 Proportion Calculator
Proportion Calculator is shaped around how people actually use calculation utilities online: open the page, drop in a file, get the result. Solve a/b = c/d for whichever variable is unknown using cross-multiplication. The interface stays out of the way once the work begins so the engine can use the available CPU and memory for the actual transformation.
Internally the tool runs on standard browser APIs — the same processing stack used by professional desktop pipelines, just compiled for the browser. 0 MB is the practical ceiling, set so the tool stays responsive on phones and older laptops.
Proportion Calculator fits naturally into the workflow of professionals validating quick estimates and students checking homework answers, both of whom typically need a fast result inside the browser. There is no learning curve to budget for: anyone who has used a typical web upload form can complete a run on the first try.
The architecture is local-first by design. Once the page is loaded, you can disconnect from the network and the tool still completes the job. The processing stack — standard browser APIs and the small UI shell wrapping it — ships with the page itself, so the tool keeps working in offline conditions, on a captive-portal Wi-Fi, or behind a corporate proxy that limits what the tab can reach.
Proportion Calculator works well as a bookmarked utility you reach for when you need it. The first visit shows you what the tool does; the second is when you realise it is a low-friction option for the task and worth keeping in your tab list.
Output handling is intentionally boring: Proportion Calculator 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.
The only practical limit is the 0 MB per-file ceiling, which keeps the tool responsive across a wide range of devices. Run the tool ten times in a row, run it ten thousand times — it behaves the same way and produces the same quality of result.
Proportion Calculator is intentionally narrow in what it does, which makes it easy to slot into a longer workflow. Take its output, hand it to whichever next tool fits the job, and Proportion Calculator stays out of your way until the next time you need it.
Some notes on the design of Proportion Calculator. The page is intentionally narrow: one input, the controls relevant to the task, and one output. Adding unrelated features would make the common case slower for the majority of users, so the surface is held to what people actually use.
From a product perspective, Proportion Calculator 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.
Proportion Calculator fits the gap where opening a desktop app feels heavy and writing a script feels overkill. The page handles the common calculation task with sensible defaults so a single visit usually completes the job; for highly specialised work, a dedicated desktop application can offer more knobs to turn.
If you want to get the most out of Proportion Calculator, 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 Proportion Calculator appears to hang, the engine is almost certainly still working — large inputs simply take longer to process inside a browser than they would on a server with multi-core scheduling. For inputs near the 0 MB cap, give it up to a minute on a typical laptop before assuming something is stuck.
That is essentially everything Proportion Calculator does and how it does it. Open the tool above, drop in your input, and the work happens in the page. If you find yourself reaching for it often, bookmark the page — it loads quickly on subsequent visits, and your most-recent settings are remembered for the rest of the session.
How it works
- 1Open Proportion Calculator in your browser. The page loads quickly and the tool is ready to use the moment it becomes interactive.
- 2Drop a calculator file onto the upload area, or click to pick one from your device.
- 3Adjust the options to match what you need. Sensible defaults cover the most common case, so you can usually skip this step.
- 4Click to start the job. The engine (standard browser APIs) processes the input in the page; you can watch the progress indicator until it completes.
- 5Save the output when it is ready.
- 6Run additional jobs as needed. The same controls and defaults apply on every run.
Common use cases
- Convert a measurement on the fly while shopping using Proportion Calculator.
- Sanity-check a quote before sending it to a customer.
- Work out a percentage change between two figures.
- Estimate how much paint or material a room will need.
- Forecast a fitness target without a paid app.
- Estimate a finance schedule before approaching a bank.
- Split a restaurant bill cleanly between a group.
- Confirm a unit conversion before quoting it in a report.
FAQ
Which fields must I fill in?
Enter the three letters that are not the unknown; the unknown’s box can be left blank or ignored.
How is the unknown computed?
Cross-multiplication rearranges ad = bc to isolate the missing value, watching for division by zero.
What does the check row show?
It recomputes a/b and c/d with the solved value to confirm they match within floating tolerance.
Is it private?
Yes — all algebra is local.
Can denominators be zero?
If a chosen arrangement implies division by zero, the tool reports an error.
Difference from ratio calculator?
This tool supports any of a, b, c, d unknown in the standard proportion equality.
Why did Proportion Calculator reject my input?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is in a supported format and that it is below 0 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.
Are jobs run with Proportion Calculator stored anywhere?
Favtoo keeps no copy of your file because Favtoo never receives your file. Proportion Calculator runs entirely in your browser, the input is held only in your tab's memory, and closing the tab discards it. There is no opt-in cloud history, no "recent jobs" panel synced to an account, and no server-side retention to configure — the architecture simply has nowhere for your file to be stored.
How accessible is the Proportion Calculator interface?
Proportion Calculator 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.
Does Proportion Calculator work on a phone or tablet?
Proportion Calculator 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 Proportion Calculator?
No installation is needed. Proportion Calculator 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 Proportion Calculator on any computer you have temporary access to without leaving anything installed on it.
What is the maximum file size for Proportion Calculator?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run Proportion Calculator as often as you need; every run produces a full-quality result.
Is Proportion Calculator really free?
Proportion Calculator 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.