Unit Conversion Reference: Length, Weight, Temperature

Unit conversion is one of those skills that quietly underpins a huge amount of practical work — from reading scientific papers to following recipes, sizing rooms to planning international shipments. Most people learn one or two conversions by heart (Celsius to Fahrenheit, miles to kilometers) and look up the rest when needed. This reference puts the most useful factors in one place, with the mental shortcuts that approximate them well enough for daily use and the exact figures you should use when accuracy matters.

One important warning up front: not all imperial units are the same. A US gallon is not a UK gallon, a US ton is not a metric tonne, and a US pint is not a British pint. Where this matters, the tables below call out the difference explicitly.

Length: Metric and Imperial

Length is the easiest set of conversions because the factors are stable across regions. The base SI unit is the meter, and the conversion to imperial units is fixed by the international yard (defined as exactly 0.9144 m since 1959).

The 5/8 and 8/5 shortcuts are accurate to within 0.6% — perfectly fine for travel and conversation. For navigation, scientific work, or anything regulated, use the exact figure or rely on an automated converter to avoid compounding rounding errors.

Weight (Mass): Pounds, Kilograms, Tons

The international pound has been defined as exactly 0.45359237 kg since 1959. The complication is "ton," which depends on country.

Two traps to remember: an ounce of gold is a different ounce than an ounce of flour (troy vs. avoirdupois). And the "ton" in a news article about cargo could be any of three sizes — short ton (US), long ton (UK), or tonne (metric). When precision matters, always specify which.

Temperature: The One With Formulas

Temperature is the only conversion in this article that uses a real formula instead of a multiplier, because the Celsius and Fahrenheit scales have different zero points.

Useful anchor points to memorize: −40° is the same temperature in both scales. 0°C = 32°F (water freezes). 100°C = 212°F (water boils at sea level). Body temperature is 37°C = 98.6°F. Room temperature is around 20–22°C = 68–72°F. Once you have these in your head, you can interpolate the rest. For a precise answer, run the numbers through a Celsius to Fahrenheit converter.

Volume: Where US and UK Diverge Most

Volume is the messiest category because the US and UK invented their gallons separately and they differ by about 17%. A US gallon is 3.785 liters; a UK ("imperial") gallon is 4.546 liters. A car that gets 30 US MPG actually gets 36 UK MPG, even though nothing about the car changed.

For recipes that use "cups," the unit varies: US cups (236 mL), metric cups (250 mL), and the small Japanese cup (200 mL) are all in active use. Cooking outcomes generally tolerate the difference, but baking does not. When following an international recipe, weigh ingredients in grams whenever possible.

Avoiding Conversion Bugs in Software

Unit conversions in software are responsible for some of the most expensive bugs in history — the loss of the Mars Climate Orbiter in 1999 traced to one team using pound-seconds and another using newton-seconds for the same parameter. The defensive practice is to make units explicit in your data model. Store values in a single canonical unit (SI is the conventional choice — meters, kilograms, seconds, Celsius) and convert only at the boundary where the value is presented to a human. Never let a function accept a bare number for a physical quantity without documenting the expected unit in the signature.

Stronger languages let you encode units in the type system: a Length type that internally stores meters and offers .toFeet() on the way out makes a unit mistake a compile error rather than a runtime catastrophe. For dynamic languages, the equivalent is wrapper classes or naming conventions like distance_m rather than distance. This is the same discipline that JSON benefits from — be explicit about what the number means and resist the urge to "just multiply by 3.28."

Frequently Asked Questions

Why does the US not use the metric system?

The US officially adopted the metric system in 1975 (Metric Conversion Act) but never mandated it for daily use. Science, medicine, and the military use metric internally; consumer goods, road signs, and weather forecasts still use customary units. The dual system is unlikely to change soon.

How precise should I be with conversion factors?

For shopping, cooking, and travel, three significant figures are plenty. For engineering, manufacturing, or anything where rounding compounds across many operations, use the full precision factor and round only at the final result. The conversion factor between inches and centimeters (2.54) is exact by definition; many others, like miles to kilometers, are exact to the displayed digits.

What about digital units — KB, MB, GiB?

Digital units have two systems: SI (KB = 1000 bytes) and binary (KiB = 1024 bytes). Storage manufacturers advertise in SI; operating systems often report in binary, which is why a "1 TB" drive shows as 931 GB. The IEC binary prefixes (KiB, MiB, GiB) exist to remove this ambiguity, and you should use them whenever the distinction matters.