Article
Why your converter app works in airplane mode (and should)
Unit conversion is arithmetic with published constants — a server adds nothing but latency and tracking. Here’s which conversions genuinely need the internet, and which never did.
Put your phone in airplane mode and open a converter app. The length, weight, and temperature conversions still work. The currency tab probably shows a spinner — or worse, a cached rate with no date on it.
That split tells you everything about which conversions need the internet and which never did.
Units are math, not data
A mile is 1.60934 kilometers. That number was true before servers existed and will be true after. Every unit conversion — length, weight, volume, temperature, speed, data storage — is multiplication by a published constant. An app that needs a server for this is either badly built or built for a different purpose (the analytics SDK phones home either way).
Cooking conversions are the same story with slightly fuzzier constants: a cup of flour is roughly 120 grams, a tablespoon is 14.8 milliliters. Fuzzy, but fixed — no API call required.
Currency is the exception — and it's a timestamp problem
Exchange rates change. A USD→EUR conversion genuinely needs a number from the real world, and that number goes stale. The honest design is a cached rate with a visible timestamp: "1 USD = 0.92 EUR, updated 3 hours ago." You can decide whether that's fresh enough for a menu or whether you need a live quote for a wire transfer.
The dishonest design is a cached rate with no date, or a spinner that refuses to work offline at all. Cached rates in airplane mode aren't a limitation — they're the feature. What good is a currency converter that dies the moment you land in a foreign country with no roaming?
What to look for
A converter worth keeping does three things: every unit category fully offline, currency rates cached with visible timestamps, and no ads between you and the answer. Currency apps are among the most ad-dense utilities on the store — banners over the keypad, video ads between conversions — for a function that is, again, multiplication.
The airplane-mode test is the whole review. If it converts with the radios off, the app is a tool. If it won't, the app is a billboard that occasionally does math.
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