I’ve been using unit converter tools almost daily for the past two years. Between cooking recipes in metric, checking material specs in imperial, and converting file sizes at work, these apps get more use than I’d like to admit. Here’s what I found after testing 17 different converters over the past month.
| Tool | Best For | Platform | Categories | Offline | Price |
|---|---|---|---|---|---|
| Google Search | Quick lookups | Web/Mobile | 20+ | No | Free |
| UnitConverters.net | Comprehensive web tool | Web | 70+ | No | Free |
| ConvertAll | Science/engineering | Windows/Linux/Mac | 500+ units | Yes | Free (open source) |
| Unit Converter Ultimate | Mobile all-in-one | Android | 30+ | Yes | Free (ads) |
| Convertio | File + unit conversion | Web | 25+ | No | Free tier |
| RapidTables | Reference tables | Web | 40+ | No | Free |
| Apple Shortcuts | iOS automation | iOS/Mac | 15+ | Yes | Free |
| Measure (by Google) | AR measurements | Android | Length only | Yes | Free |
1. Google Search – The One You Already Have
Type “150 lbs to kg” into Google and you get your answer instantly. No app to install, no bookmarks needed. Google’s built-in converter handles over 20 categories and the interface is clean enough that you rarely need anything else for basic stuff.
What makes Google’s converter work so well is the natural language input. You can type “how many cups in a gallon” or “72 fahrenheit in celsius” and it just figures it out. The converter widget pops up right at the top of search results with a dropdown for switching units.
Currency conversion pulls live exchange rates, though they update every few hours rather than in real-time. For checking hotel prices abroad or estimating shipping costs, that’s more than good enough. For forex trading, you’d want something like XE instead.
The downsides? You need internet access, and the conversion history disappears once you close the tab. There’s also no way to do batch conversions or create custom unit definitions. But for the 90% case where you need one quick answer, nothing beats it.
What I actually use it for
Cooking conversions, mostly. I keep running into European recipes that list everything in grams and milliliters. Typing “200g flour in cups” while my hands are covered in dough is faster than opening any app.
2. UnitConverters.net – When You Need Everything in One Place
This site has been around since 2005 and honestly it shows in the design. But the sheer number of conversion categories makes up for it. Over 70 categories covering everything from length and weight to magnetic flux density and radiation dose equivalent.
Each conversion page includes the formula used, a reference table with common values, and an explanation of both units. If you’re a student trying to understand why there are 4.184 joules in a calorie, this context helps more than just getting a number.
The site loads fast and works well on mobile browsers despite not having a dedicated app. I tested it on a budget Android phone with spotty 3G and conversions still came back in under 2 seconds.
One thing I appreciate is the conversion tables they generate. Need a quick reference card for metric to imperial lengths? The site spits out a printable table. I’ve printed a few of these and taped them to my workshop wall.
Coverage depth
Where this site really pulls ahead is niche categories. Shoe sizes across US, UK, EU, and Japanese standards. Wire gauge conversions. Cooking volume conversions that actually distinguish between US and Imperial cups (yes, they’re different – a US cup is 236.6 ml while an Imperial cup is 284.1 ml). Most converters ignore this distinction and it drives me crazy.
3. ConvertAll – The Power User’s Pick
ConvertAll is a free, open-source desktop application that takes a completely different approach. Instead of picking from dropdown menus, you type unit expressions directly. Want to convert miles per gallon to liters per 100 kilometers? Type “mile/gallon” on one side and “liter/100 km” on the other.
The unit expression parser is where this tool gets interesting. You can combine any units using multiplication, division, and exponents. Need to convert pound-force per square inch to kilopascals? Just type it. The app figures out whether the conversion is dimensionally valid and warns you if it isn’t.
Installation is straightforward on Linux (it’s in most package managers) and Windows. The Mac version requires Python but works fine through Homebrew. The whole application is under 2 MB.
| Feature | ConvertAll | Google Search | UnitConverters.net |
|---|---|---|---|
| Custom unit combos | Yes | No | No |
| Offline mode | Yes | No | No |
| Units supported | 500+ | ~200 | 1000+ |
| Batch conversion | No | No | No |
| Open source | Yes | No | No |
| Learning curve | Medium | None | Low |
Who should use this
Engineers, scientists, and anyone who regularly works with compound units. If you’ve ever needed to convert BTU per hour per square foot per degree Fahrenheit to watts per square meter per kelvin, this is your tool. That specific conversion came up for me when comparing insulation R-values between American and European specs.
4. Unit Converter Ultimate (Android)
Look, most unit converter apps on the Play Store are identical wrappers around the same conversion formulas with different color schemes. Unit Converter Ultimate stands out because it actually includes useful extras beyond basic conversion.
The app covers 30+ categories with a clean material design interface. Conversions happen instantly as you type. But the real selling point is the built-in calculator that lets you do math before converting. Need to add up three measurements in inches and then convert the total to centimeters? You can do that in one flow.
Currency conversion uses rates from the European Central Bank, updating daily. The app caches these rates so you can still get recent conversions when offline, though it warns you the rates may be stale.
The free version has banner ads at the bottom. They’re not intrusive – no full-screen popups or video ads. The pro version costs $2.99 and removes ads, which I think is fair for an app I open 4-5 times a week.
Battery usage is minimal. I checked my stats after a month of regular use and it didn’t even register in the battery usage breakdown. The app is 8 MB installed.
5. Convertio – Not Just for Files
Most people know Convertio as a file converter, but it has a surprisingly capable unit conversion section tucked away in the navigation menu. The same clean interface that makes file conversion pleasant works well for units too.
The 25+ conversion categories cover standard needs. Nothing exotic like ConvertAll offers, but the interface is so much friendlier that I reach for it first when I’m already in a browser. Conversions include a brief explanation and the mathematical formula, which is handy when you need to show your work.
Where Convertio falls short is the free tier limit. While unit conversions are unlimited, if you’re using the same account for file conversions, you share the daily limit of 10 operations. This only matters if you’re a heavy file converter user too.
6. RapidTables – Reference Tables That Actually Help
RapidTables takes the reference approach to conversions. Instead of just giving you a single converted value, every conversion page includes comprehensive tables showing dozens of pre-calculated values. Converting feet to meters? You’ll see a table going from 1 foot all the way up to 100 feet in neat increments.
The site also includes electrical calculators, math tools, and color code converters. If you work in electronics, the resistor color code calculator and Ohm’s law calculators save time. The LED resistor calculator alone has saved me from blowing up a handful of LEDs.
Load times are excellent. The site uses minimal JavaScript and no heavy frameworks, so it renders almost instantly even on slow connections. There’s no account system, no cookies consent banner taking up half the screen, and no newsletter popup. Just conversions.
The conversion tables advantage
Here’s the thing about conversion tables – sometimes you don’t need a precise answer, you need a rough sense of scale. When I’m reading a European weather forecast in Celsius, I don’t want to convert each temperature individually. I want to glance at a table and know that 18C is roughly 64F and 25C is roughly 77F. RapidTables makes this kind of quick reference easy.
7. Apple Shortcuts – The Hidden Converter on iOS
If you have an iPhone or Mac, you already have a surprisingly powerful unit converter built into the Shortcuts app. The “Convert Measurement” action handles 15+ unit types including length, mass, temperature, volume, speed, pressure, energy, and more.
The real power comes from automation. You can build a shortcut that asks for a value, presents a list of unit categories, and gives you the result – all triggered by a single tap or Siri command. I made one that converts between US and metric cooking measurements and put it on my home screen. “Hey Siri, convert 350 degrees” gives me 177 Celsius instantly.
You can also chain conversions into other workflows. One shortcut I use takes a distance in miles from my clipboard, converts it to kilometers, and adds it to a note. Takes about 5 minutes to set up and works forever after that.
The limitation is that you can’t do anything Apple hasn’t already defined as a measurement type. No shoe sizes, no wire gauges, no cooking-specific volumes. For those, you’ll still need a dedicated converter.
8. Google Measure – When You Need to Measure First
Google Measure (and similar AR measurement apps on newer Android phones) solves a different problem. Instead of converting a number you already have, it measures real-world objects using your phone’s camera and then gives you the result in whatever unit you want.
Accuracy depends heavily on your phone’s sensors. On a Pixel 8, I measured a table at 152 cm, and the actual measurement with a tape measure was 153.4 cm. That’s within 1% and good enough for checking if furniture will fit. On an older phone, the error was closer to 5%, which is too much for anything precise.
The app measures length, height, and area. It won’t weigh things or measure temperature. But for the specific use case of “will this shelf fit in that space” or “how tall is that ceiling,” it beats carrying a tape measure around.
How I Picked These Tools
I started with 17 unit converter apps and websites that showed up in search results and app store rankings. Then I tested each one against the same set of conversions:
- 10 basic conversions (length, weight, temperature)
- 5 compound unit conversions (speed, pressure, fuel economy)
- 3 obscure conversions (nautical miles, troy ounces, astronomical units)
- Currency conversion accuracy vs XE.com rates
- Load time on a 25 Mbps connection
I cut tools that got any basic conversion wrong (two apps failed on Fahrenheit to Celsius, which is embarrassing), those with aggressive ads that covered the conversion interface, and those that required a paid account for basic functionality.
I also used each surviving tool daily for two weeks to see which ones I naturally reached for and which ones gathered dust. The eight tools above are the ones that earned a permanent spot in my workflow.
Quick Tips for Better Unit Conversions
After thousands of conversions, here are some things I’ve learned the hard way:
Bookmark your most common conversions. If you convert Celsius to Fahrenheit ten times a day, save a bookmark with the Google converter pre-filled. Saves you from typing the same query repeatedly.
Watch out for US vs Imperial measurements. A US gallon is 3.785 liters. An Imperial gallon is 4.546 liters. That’s a 20% difference. Most converters default to US units but don’t always make this clear. If you’re reading a British recipe or manual, double-check which system they’re using.
Don’t trust currency conversions for large transactions. Free converters typically lag 4-12 hours behind real exchange rates. For amounts over a few hundred dollars, check a real-time source like your bank or XE.com.
Temperature conversion is the one formula worth memorizing. Multiply Celsius by 1.8 and add 32 to get Fahrenheit. Going the other way, subtract 32 and divide by 1.8. Once you’ve done it a few dozen times, you stop needing an app for common temperatures.
What About Built-in OS Converters?
Windows Calculator has a converter mode that’s honestly pretty decent. Click the hamburger menu in the top left, and you’ll find converters for volume, length, weight, temperature, energy, area, speed, time, power, data, pressure, and angle. It works offline, launches fast, and the interface is clean. If you’re on Windows and just need basic conversions, you might not need anything else.
macOS doesn’t have a built-in converter in Calculator, which is a weird gap. Spotlight can do some conversions if you type them in natural language, but it’s inconsistent about which queries it recognizes. The Shortcuts approach I described above is more reliable.
Android has a converter in the Google app (not the Calculator app, confusingly). Long-press the home button, type your conversion, and you get the same Google Search converter in a compact format. Samsung phones include a converter in the Samsung Calculator app under the “tools” tab.
Frequently Asked Questions
What is the most accurate free unit converter app?
For general conversions, Google’s built-in converter is the most accurate since it pulls from constantly updated databases. For scientific and engineering work, ConvertAll and GNU Units offer the highest precision with support for obscure and derived units.
Do unit converter apps work offline?
Most desktop unit converters (ConvertAll, Unit Converter Ultimate, Convertio desktop) work fully offline. Currency conversion always needs an internet connection for live exchange rates. Mobile apps like Unit Converter by Digit Grove cache conversion factors locally so basic conversions work without internet.
Can I convert currencies with a unit converter app?
Some unit converter apps include currency conversion with live exchange rates. Google Search, UnitConverters.net, and Unit Converter Ultimate all support currency. Dedicated currency converters like XE give more accurate rates since they update every 60 seconds versus every few hours for general-purpose tools.
Are web-based unit converters as good as desktop apps?
For quick everyday conversions, web-based tools like Google Search and UnitConverters.net are faster and more convenient. Desktop apps like ConvertAll work better when you need custom unit expressions or work in environments without internet access. The accuracy is identical for standard conversions.
What unit categories do free converters typically support?
Most free unit converters cover 15-25 categories including length, weight, temperature, volume, area, speed, time, pressure, energy, power, data storage, fuel economy, and cooking measurements. Specialized tools may add engineering units like torque, viscosity, and luminous intensity.
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