A unit converter that doesn’t need the internet.
Length, mass, temperature, volume, area, speed, data and time — 57 units in 8 categories, one keystroke from anywhere. No page to load, no ads, no cookie banner: the factors ship inside the app, so it answers at the same speed on a plane.
Category, number, done.
Pick a category from the chips, type the number, choose the two units. The result updates as you type and a click copies it — and when you inevitably typed it the wrong way round, the swap button flips both units without retyping anything.
It opens with the global shortcut wherever you are, which is the actual difference: the conversion itself takes a second, but going to find a converter costs you a tab, a search, a page load and a cookie banner first. Here it is the same second every time, and the app is already open.
- Live result — no “convert” button to press
- Swap the two units without retyping
- Works with the popover pinned, or in its own window
- Lives in the Math tab, next to the Calculator
Eight categories, 57 units.
Enough for the conversions that actually interrupt you — a recipe in cups, a package in pounds, a drive in gibibytes — and deliberately not a units encyclopedia nobody scrolls.
Length
mm, cm, m, km · in, ft, yd, mi · nautical miles
Mass
mg, g, kg, tonnes · oz, lb · stones
Temperature
°C, °F, K — with the real formula, not a factor
Volume
ml, l, m³ · tsp, tbsp, cup, fl oz, pt, qt, gal
Area
cm², m², km² · in², ft² · hectares, acres, mi²
Speed
m/s, km/h, mph, ft/s, knots
Data
bit, B · KB→TB (1000s) · KiB→GiB (1024s)
Time
ms, s, min, h, days, weeks
The conversion was never the slow part.
Typing “12 inches in cm” into a search box works. What it costs is a tab, a page full of ads, a consent banner and a result you then have to select by hand — for arithmetic your machine could do in a microsecond, offline, without telling anyone what you were measuring.
This one is a keystroke away, answers with no network at all, and copies with a click. The honest limit is scope: it converts units, not currencies, and it is not trying to be a page that ranks for every conversion pair in existence.
A foot is 0.3048 metres. A dollar is not.
Every number in this widget is a constant, which is why it can promise an answer with no network. An exchange rate is a live quote that has to be fetched, cached and dated — a different tool with different failure modes. Toolbox has one: Currency Converter, with several quotes at once and the gap between them.
It comes with Toolbox. There is nothing to install.
The Converter is built into Webstarted Toolbox, the app that lives in your menu bar on macOS and your system tray on Windows — so it is already in the Math tab the moment you install it. These are a few of its neighbors:
Questions, answered.
Does it work without internet?
Completely. Every conversion is a factor that ships inside the app, so there is nothing to fetch — it works on a plane, on a locked-down network, and at the same speed either way. That is the whole point: a unit conversion is arithmetic, and arithmetic does not need a server.
Which units does it cover?
Eight categories and 57 units: length (mm to nautical miles), mass (mg to stones), temperature (°C, °F, K), volume (ml, litres, cups, pints, quarts, gallons, teaspoons and tablespoons), area (cm² to square miles, plus hectares and acres), speed (m/s, km/h, mph, ft/s, knots), data (bits and bytes, decimal and binary) and time (milliseconds to weeks).
Is a KB 1000 bytes or 1024?
Both are there, as separate units, because they are separate units. KB, MB, GB and TB are decimal — 1000 each step, the way drive manufacturers count. KiB, MiB and GiB are binary — 1024 each step, the way your operating system usually counts. Picking one and calling it “KB” is exactly how you end up off by 7% and unable to explain why.
Is the temperature conversion just a multiplication?
No, and it cannot be. Length and mass are ratios, so a single factor works; temperature scales have different zero points, so °C to °F is a multiplication plus an offset, and Kelvin shifts by 273.15. The widget runs the real formula for each pair instead of squeezing temperature into the factor table.
Can it convert currencies?
Not here, and on purpose. Every conversion in this widget is a constant — a foot is 0.3048 metres today and next year. An exchange rate is a live quote that has to be fetched, cached and dated, which is a different kind of tool. Toolbox has a Currency Converter widget for exactly that, with multiple quotes and the gap between them.
How do I copy the result?
One click on the result copies it. There is also a swap button for when you typed it the right way round but wanted the other one, which is most of the time.
Can Claude or another AI agent convert for me?
If you allow it, per widget. The widget exposes a conversion action through the local MCP server, and it works out the category from the units themselves — you say “12 in to cm”, not “length, 12, in, cm”. Asking for something impossible like metres to kilograms fails with the list of units that are actually valid for that category, instead of quietly returning a number that means nothing.
Where does it live in the app?
In the Math tab, next to the Calculator and the spreadsheet-style grid — the place you already go when the answer is a number.
Is it free?
Yes. Webstarted Toolbox is a free beta, needs no account, and the Converter comes built into the app — there is nothing to install and nothing to enable.
Get Webstarted Toolbox
Free to use. The Converter is built in — download the app and it is already in the Math tab. macOS 12+ and Windows 10+.
One icon, many small tools
This is just one of the widgets in Webstarted Toolbox. Here are the rest: