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Unit Converter Online Free

Length, weight, temperature, digital storage, time, speed. Pick a category, type a value, see the result. No conversion button, no ads.

In your browser—your files never leave your device.

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About this tool

Google does single-shot unit conversions just fine. Where Google falls down is when you need to switch between five different unit types in five minutes (homework, kitchen, travel planning, cabling), and each query needs you to retype "convert X to Y". This tool keeps all the categories visible and switching takes one click. Pick a category, type the value, choose source and target. Conversions update live as you type, no submit button. Swap button flips source and target if you typed them backwards. The conversion factors are the standard ones (1609.344 metres per mile, 0.45359237 kilograms per pound, the usual base-2 binary prefixes for digital storage), so it's fine for homework, engineering estimates, and the kind of "wait is that a lot" sanity checks that come up daily.

How to unit converter online free

  1. Pick a category

    Twelve tabs at the top: Length, Weight, Temperature, Data, Time, Speed, Area, Volume, Pressure, Energy, Fuel economy, Angle. The tab selection swaps in the units for that category, so picking Length shows meters, kilometers, miles, yards, feet, inches as the unit options; picking Weight shows kilograms, pounds, ounces, etc. Switching tabs resets the unit selection to the first two options for that category.

  2. Type a value

    Number goes in the From input on the left. The result on the right updates as you type; no submit button to click. Decimals work (type 3.5 for three-and-a-half). Negative numbers work for things like Celsius temperatures or differences. Scientific notation also works (1e6 for one million).

  3. Pick source and target units

    Two dropdowns, one under each input, set what you're converting from (left) and what you're converting to (right). The dropdowns are populated with the units for the active category. If you set them backwards, click the swap arrows button in the middle (or below the inputs on mobile) and they flip.

  4. Copy the result if you need it

    Copy button next to the result drops the converted number onto your clipboard. Useful when the result has six decimal places and you'd rather not retype them by hand into a spreadsheet, report, or homework assignment. Paste it wherever; the clipboard format is plain text.

Why use this tool

You're cooking from a US recipe and need 3/4 cup in millilitres. You're shopping for a 50-inch TV and want to know if it'll fit a 130 cm-wide cabinet. You're reading a server spec sheet and want to convert 128 GB to TB to fit it into a capacity report. You're packing for a trip and the airline says 23 kg max but your bathroom scale reads in pounds. These are the actual use cases. You don't need a scientific calculator with thirty unit categories; you need fast switching between the eight or so categories that come up in normal life. That's what this tool is. Length, weight, temperature, data, time, speed are visible all the time as tabs. Switching is one click. The live conversion means you can hold the result steady in your head while you adjust the input. It works offline once the page has loaded, so airplane mode is fine. Currency is intentionally not included because exchange rates need an API and a daily refresh, and any tool offering "free offline currency conversion" is giving you cached rates from some unknown date which is worse than just asking your bank.

Features

Twelve categories, fast switching

Length (meters, kilometers, centimeters, millimeters, miles, yards, feet, inches, nautical miles), weight (kilograms, grams, milligrams, pounds, ounces, metric tons, stones), temperature (Celsius, Fahrenheit, Kelvin), digital storage (bytes through petabytes plus bits), time (milliseconds through years), speed (km/h, mph, m/s, knots, ft/s), area (m², km², hectares, acres, square miles/yards/feet/inches), volume (litres, US and Imperial gallons/quarts/pints/cups/fl oz), pressure (Pa, kPa, bars, atm, PSI, Torr, mmHg, inHg), energy (joules, calories, kWh, BTU, foot-pounds, electronvolts), fuel economy (L/100 km, mpg US/UK, km/L), and angle (degrees, radians, gradians, turns, arc-min/sec). Switching categories is one click and the unit dropdowns refresh to the right options for that category. The categories cover the conversions that come up in cooking, travel, homework, engineering, and IT capacity planning without dragging in obscure units nobody uses outside specific industries.

Live conversion as you type

No "convert" button to click. The result updates on every keystroke. Sounds minor but matters when you're trying to find a value that fits a constraint ("how many kilograms lands me under the 50 lb baggage limit?"). Hold one number in your head, slide the input up or down, watch the output land at the value you wanted. Same trick works for "what speed in km/h is 30 mph" or "what value in GB matches 500 MB". The instant feedback lets you home in on the answer faster than retyping queries.

Precision when it matters

Conversion factors are the SI/IEEE standard exact values (1 mile equals 1609.344 meters exactly, 1 pound equals 0.45359237 kilograms exactly, 1 GB equals 1073741824 bytes in binary mode). Output is shown with up to 6 decimal places of precision. For very small or very large results, the tool automatically switches to scientific notation, so you get "3.4e-6" instead of "0.0000034" or "1.23e12" instead of an unreadable string of digits. Suitable for engineering estimates, scientific calculations, and undergraduate homework where exact factors matter.

Swap button between From and To

Small arrows button sits between the From and To inputs and flips source and target units in one click. Useful when you set up the conversion backwards and don't want to manually re-pick both dropdowns. On mobile the swap button moves below the inputs (where it stays reachable with a thumb instead of getting squeezed off the side). Combined with the live update, this means flipping units is essentially free; you just see the result switch sides.

Works offline after first load

After the page loads once, everything is local JavaScript. Disconnect Wi-Fi, switch to airplane mode, hide in a tunnel; the conversions still work. The conversion factors are baked into the page bundle, no API call needed. The only thing you can't do offline is the initial page load. Useful when you're cooking from a foreign recipe in a basement kitchen with no signal, or doing capacity-planning on a flight without paying for the in-flight Wi-Fi.

Privacy & security

This tool runs entirely in your browser. Your files are never uploaded to a server — every step of the process (reading, transforming, downloading) happens on your device using JavaScript and the Web APIs. You can verify this in your browser's network tab: clicking the tool's main action triggers zero requests to our servers. The page itself is served over HTTPS, but once it loads, your data stays put. No accounts, no tracking of file contents, no scanning your inputs.

Frequently asked questions

Is it free?
Yes. No signup, no ads.
How is privacy handled?
All conversion runs in your browser. The numbers you type never reach a server. No analytics on the values, no logging of the unit pairs you converted, nothing.
Which categories are supported?
Twelve: length, weight, temperature, digital storage, time, speed, area, volume, pressure, energy, fuel economy, and angle. Length covers metres through nautical miles; weight covers grams through stones; volume covers both US and Imperial gallons/quarts/pints so US-vs-UK recipe conversions work cleanly; pressure spans Pa to PSI to mmHg; energy covers joules through electronvolts; and fuel economy lets you flip between L/100 km, mpg US, mpg UK, and km/L which is the conversion every car-spec-reader needs.
Why isn't currency included?
Currency conversion needs current exchange rates pulled from an API, and rates change every few seconds during market hours. A currency converter that's offline-capable is by definition working from stale rates, which is worse than no conversion at all if you're making a buying decision. Use your bank's app or xe.com for currency; they refresh from real feeds.
Are these conversions accurate for homework?
Yes. The factors are the SI/IEEE standard ones (1 mile = 1609.344 m exactly, 1 lb = 0.45359237 kg exactly, etc.) with 6 decimal places of output. For an undergraduate physics or chemistry assignment, that's plenty. For ultra-high-precision metrology (atomic clocks, gravimetry, anything beyond 6 decimal places), use a domain-specific tool.
Why does my GB-to-MB conversion give a different number than Windows?
Two definitions exist. The IEC binary prefix (used by Windows file sizes and most operating systems) treats 1 GB as 1024 MB, which is 2^30 bytes. The SI decimal prefix (used by hard drive marketing and network speeds) treats 1 GB as 1000 MB, which is 10^9 bytes. This tool uses the binary 2^30 definition, which matches what your file manager shows. That's why a "1 TB drive" from the store reads as 931 GB in Windows.
How does it compare to Google's built-in unit converter?
Google handles one query at a time and you have to type each conversion fresh ("convert 5 miles to km"). This tool keeps the category panel visible, so once you're in Length mode, you can flip between unit pairs without retyping the value. For one-off conversions, Google is fine. For sustained back-and-forth (cooking, packing, capacity planning), this is faster.
Mobile?
Yes. The numeric keyboard pops up automatically on phones because the input is type="number". The swap button moves below the inputs on narrow screens so it stays reachable with a thumb.
Why is Kelvin in there?
Because chemistry homework and physics labs use Kelvin all the time, and converting between Celsius and Kelvin in your head means remembering to subtract 273.15, which most people get wrong under exam pressure. The Kelvin scale starts at absolute zero (negative 273.15 Celsius), so there are no negative Kelvin temperatures. The tool will happily convert negative Celsius to positive Kelvin without complaining.
What about feet and inches together (like 5'10")?
Not supported directly; you have to convert each piece. 5 ft = 152.4 cm and 10 in = 25.4 cm, total 177.8 cm. For height conversion specifically, a dedicated height calculator is faster. The mismatch between "feet-and-inches as one value" versus single-unit inputs is annoying but a known limitation.
Does it handle the difference between US gallons and Imperial gallons?
Yes. The volume category lists US gallons (3.785 litres) and Imperial gallons (4.546 litres) as separate units, plus quarts, pints, cups and fl oz in both systems, so there's no silent ambiguity for US-vs-UK recipe converters or fuel economy comparisons. The two systems differ by about 20%, which matters: a 1-gallon recipe is 79 vs 95 percent of a 4-litre jug depending on the system, and that's a real cooking failure if you guess wrong.
What's the difference between weight and mass?
In physics, mass is the amount of matter (measured in kg) and weight is the force gravity exerts on that mass (measured in newtons). On Earth, the two scale linearly, so we sloppily use "weight" to mean "mass" in everyday speech and on bathroom scales. This tool follows the everyday convention: the "weight" category converts between kg, lb, oz, etc., which are all technically mass units. If you need actual force units (newtons, pound-force), you need a physics calculator.