Most dedicated unit converter apps work offline for physical units like length, weight and temperature, because the conversion factors never change and fit inside the app. Currency is the exception: rates move every day, so it always needs a connection. Good offline choices are Convert Units Calculator (iPhone and Android, fully offline, no currency), Converter NOW (Android, offline except currency) and Unit Converter – Convert all or Unit Converter – Pro Units App on iPhone. Google’s search converter doesn’t work at all without a connection.
Why can a unit converter work offline? #
Every physical unit is defined by a fixed number. An inch is exactly 2.54 cm, a pound exactly 0.45359237 kg and a US gallon exactly 3.785411784 liters, and those definitions are published by standards bodies like NIST. A converter only needs a table of these factors and some arithmetic. The whole table is a few kilobytes, so there’s no technical reason for a unit converter to need the internet.
When an app does need a connection for basic units, it’s usually because it’s a web page wrapped in an app, or because it won’t open without loading ads or a login first.
Why does currency always need a connection? #
Exchange rates change constantly, so a currency converter has to download fresh rates. Offline, the best an app can do is reuse the last rates it saved, which may be days or weeks old. For a rough sense of a price that’s fine. For anything that costs real money, check a current rate. Some converters, Convert Units Calculator among them, leave currency out entirely so that nothing in the app can go stale.
Offline unit converter apps compared #
Based on each app’s store or F-Droid listing in September 2026:
| App | Platforms | Units offline | Currency | Ads |
|---|---|---|---|---|
| Convert Units Calculator | iOS, Android | Yes, all conversions | Not included | One banner |
| Converter NOW | Android, web, desktop | Yes | Yes, needs internet | No |
| Unit Converter – Convert all | iPhone, iPad, Mac, Watch | Yes, per its listing | Yes, live rates | Yes |
| Unit Converter – Pro Units App | iPhone | Yes, per its listing | Yes, 148 currencies | No |
| Unit Converter – Scientific | iPhone | Standard conversions | Not listed | No |
| Units – Pro Unit Converter | iPhone, iPad, Mac | Not stated | Daily rate updates | No |
| Google Search | Any browser | No | Yes | Search ads |
Convert Units Calculator: everything on the device #
Convert Units Calculator is our app. Every conversion is computed on your phone from fixed standard factors, so airplane mode changes nothing about the answers. It has no account or sign-in, and your settings and pinned conversions stay on the device. The single banner ad needs a connection to load, so offline you’ll just see no ad. The trade-off is no currency tab, and at 140 units in version 1.0.5 (earlier builds had ten categories) it’s smaller than the big converters. For travel it covers the usual gaps: km and miles, Celsius and Fahrenheit, liters and gallons, kg and lbs, and its Cooking category turns cups into grams.
Converter NOW: offline and open source on Android #
Converter NOW is free, open source (GPL), has no ads and covers 200+ units in 30+ categories. F-Droid’s listing notes that it only needs internet for currency rates, which it updates daily. Everything else works without a connection.
Unit Converter – Convert all and Pro Units App on iPhone #
Both list offline conversion for physical units and live currency when connected. Convert all has the bigger feature set (calculator, clothing sizes, cooking measurements) along with ads and subscription options. Pro Units App has no ads but keeps 12 of its 16 categories behind a one-time $2.99 purchase.
How do you test whether a converter works offline? #
Listings aren’t always precise, so check for yourself before a trip:
- Install the app and open it once while connected, so any first-run setup finishes.
- Turn on airplane mode, and turn off Wi-Fi too.
- Force-quit the app and reopen it. Some apps work offline only while still open from before.
- Convert 1 mile to kilometers. The answer should be 1.609344.
- Convert 100 °F to Celsius. The answer should be 37.78 (37.777778).
- If the app has currency, look for the date of the last rate update. That’s how old your offline rates are.
If the app shows a spinner, a login screen or an error instead of an answer, it isn’t an offline app.
When does offline actually matter? #
- Traveling abroad, where roaming data is expensive or you’re between SIM cards. That’s also when you need conversions most: road signs, weather, fuel, recipes and clothing labels.
- On planes, including in-flight Wi-Fi that’s paid or patchy.
- Job sites, basements and rural areas with no signal.
- Privacy. An app that computes locally doesn’t need to send what you type anywhere. Ads are a separate question: check the App Privacy or Data safety section.
What about the iPhone Calculator and Google? #
Google’s converter is part of Google Search, so it needs a connection every time. It’s excellent when you’re online and useless in airplane mode.
The iPhone’s Calculator app can convert units or currency. Currency needs a connection for fresh rates. Run the test above in airplane mode to see how it behaves on your iOS version before you rely on it. We compare it with a dedicated app in iPhone Calculator unit conversion vs a converter app, and cover other web-free options in Google unit converter alternatives.
For the full field, including apps that need a connection, see our best unit converter app comparison.
Frequently asked questions #
Do unit converter apps need the internet? #
Not for physical units. Length, weight, temperature, volume and similar conversions use fixed factors that can live inside the app. Only currency, and sometimes ads, need a connection.
Does Google’s unit converter work offline? #
No. It’s a Google Search feature, so it needs a connection to load. Save a converter app to your phone if you’ll be without signal.
Can I convert currency offline? #
Only with rates the app saved the last time it was online, and those may be out of date. For purchases, check a current rate when you’re connected.
What is the best offline unit converter for travel? #
One that works fully offline and covers the everyday gaps between countries: distance, speed, temperature, fuel, volume and weight. Convert Units Calculator and Converter NOW both do. If you also need currency, pick an app that caches rates and check how old they are before you use them.
Why does my converter app show no ads offline? #
Ads are downloaded from the internet, so they can’t load without a connection. That’s normal. The conversions keep working as long as the app computes them on the device.