Unit Conversion
Metric, imperial, and getting the number right the first time
Most of the world standardized on the metric system decades ago; the US (along with Liberia and Myanmar) still runs primarily on imperial units for everyday life, which is why unit conversion keeps coming up — in recipes, travel, fitness, engineering, and shopping. The two systems aren't just different scales; they're built on different base logic (metric is powered-of-10 and derived from physical constants; imperial units like the foot and pound have historical, often body-based origins), which is part of why mental conversion is error-prone and best left to a tool rather than approximate memory.
Getting a conversion wrong isn't just a minor inconvenience — it's caused real, expensive failures. NASA's Mars Climate Orbiter was lost in 1999 because one engineering team's software used metric units (newton-seconds) while another used imperial (pound-force-seconds), and the mismatch wasn't caught before the spacecraft burned up in the Martian atmosphere. That's an extreme example, but the same category of mistake — silently mixing units — happens constantly at a smaller scale: a recipe using the wrong cup size, a runner misjudging pace, a traveler misreading a weight limit.
Unitto is built around typing a query like "5 km to mi" and getting an instant answer, rather than digging through category menus, and it runs entirely offline — no network calls for a calculation that's pure math and shouldn't need one.
The app
How it compares to other tools
Frequently asked questions
Why does the US still use imperial units?
Historical and economic inertia, mostly. The US Metric Conversion Act of 1975 made metric adoption voluntary rather than mandatory, and by the time serious conversion efforts might have happened, the cost of retooling infrastructure, education, and industry standards around imperial units was seen as prohibitive. Science, medicine, and much of manufacturing in the US already use metric internally — it's everyday life (cooking, road signs, body weight) where imperial persists.
What's a reliable way to estimate metric-imperial conversions in your head?
A few rough anchors cover most everyday cases: a kilometer is about 0.62 miles (roughly 5/8), a kilogram is about 2.2 pounds, a liter is a bit more than a US quart, and Celsius-to-Fahrenheit is roughly (C × 2) + 30 for a quick estimate (the exact formula is C × 9/5 + 32). These mental shortcuts are fine for a ballpark, but anything with real consequences — medication dosing, engineering, cooking chemistry — deserves an exact conversion, not an approximation.
Why did unit-conversion errors cause NASA to lose a spacecraft?
The 1999 Mars Climate Orbiter failure happened because Lockheed Martin's navigation software produced thruster-firing data in imperial units (pound-force seconds) while NASA's Jet Propulsion Laboratory's trajectory software expected metric units (newton-seconds). The mismatch wasn't caught in review, and the resulting navigation error caused the spacecraft to enter the Martian atmosphere far lower than planned, destroying it. It's a widely cited case study in software engineering for exactly this kind of silent, uncaught unit-mismatch bug.
Is it safe to do unit conversions in an offline app versus a web calculator?
Functionally, yes — unit conversion is pure arithmetic and doesn't need a live data source (unlike, say, currency conversion, which needs an exchange-rate feed). An offline converter has no meaningful accuracy disadvantage, and it works without a signal and doesn't send your query to a server.