I built Calculum for Mac, 281 calculators that work offline

I built Calculum for Mac, a free and open source app with the 281 calculators from calculum.dev, the library I launched in July. It’s the same calculators in a native window. It works without a connection, keeps the calculations you save, and comes with a command for Terminal.

Here’s the app in a minute:

The sidebar lists the 20 categories, each in its own color, plus Favorites, Recent and Saved. ⌘F searches all 281 calculators at once.

Pick one and type your numbers. The result updates as you type, with every supporting figure under it. Below that there’s a chart of what changes the result, three realistic examples, the formula, the assumptions, and a short guide with the edge cases worth checking. Loans, savings and investments also get a chart of how the numbers develop over time.

Pick your country at the bottom of the sidebar to get its currency, number format, and metric or US customary units. There are 21 to choose from.

Saving and comparing calculations

Press ⌘S and Calculum saves the calculation: the inputs as you typed them, and the result with all its details. It’s a record of the answer, so it doesn’t change later, even if you switch country.

Saved calculations get their own page. You can rename one, copy it as text, or open it in the calculator again to try something else.

Save two calculations from the same calculator and you can compare them. ⌘-click both, and Calculum lines up every input and every figure of the result, with the change next to each one.

In this comparison, the 25-year mortgage at 5.75% costs $195.97 more per month than the 30-year one at 6.25%. It also saves $89,336.62 in total interest, on a more expensive home.

A command for Terminal

Calculum comes with a calculum command. Choose Calculum → Install Command Line Tool… to link it into /usr/local/bin, and every calculator works from the terminal:

calculum mortgage-payment price=450000 rate=5.5

You pass the inputs you want to change as name=value, and the others keep their example values. It prints the inputs and the full result. calculum show mortgage-payment lists the input names, and calculum search paint finds a calculator.

-q prints only the answer, which is handy in a script:

calculum tip-and-bill-split bill=186 people=4 -q

That prints $58.90. --json prints every input and every figure of the result, and --country IT switches to euros and metres.

How it works

The calculators are JavaScript, the same code that runs on calculum.dev. Instead of rewriting 281 formulas in Swift, the app bundles them into one file and runs it in JavaScriptCore, the JavaScript engine built into macOS. The app and the calculum command share that engine, so the Mac app, the terminal and the website give the same answers.

Everything else is SwiftUI, with no dependencies. Your favorites, saved calculations and numbers stay in one JSON file on your Mac. There are no accounts and no analytics. The only time Calculum goes online is the daily check for a new version on GitHub.

Get it

The code is on GitHub at flaviocopes/calculum, MIT licensed.

Download the zip from the latest release, unzip it, and drag Calculum to your Applications folder. It runs on macOS 15 Sequoia or later, on Apple silicon and Intel Macs.

The app isn’t signed with an Apple Developer ID or notarized. The first time you open it, macOS says it “could not verify Calculum is free of malware”. Click Done, go to System Settings → Privacy & Security, scroll down to Calculum, and click Open Anyway.

You can also remove the quarantine flag in Terminal, then open the app again:

xattr -dr com.apple.quarantine /Applications/Calculum.app

The README explains what to do on a work laptop, and covers every feature and every command option.

After that, Calculum updates itself. It checks GitHub once a day, and Install and Relaunch puts the new version in place.

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