Why some conversions are "exact" and others aren't
It's easy to assume unit conversion is just "multiply by some ratio," and for most physical quantities that's true. An inch was formally redefined in 1959 as exactly 2.54 centimeters — not a rounded approximation, an exact international agreement. The same goes for the pound (0.45359237 kg exactly). That's why a converter can show 10 decimal places of precision without ever introducing rounding error: the underlying constants are exact, even if the everyday numbers you deal with (your height, your weight) aren't.
Where it gets messier is when a "unit" isn't standardized the same way everywhere. A gallon in the US is not the same as a gallon in the UK. A ton in US shipping documents is not the same as a metric tonne. Getting these mixed up is a common, very real source of billing and shipping errors — more on that below.
Length and weight: the everyday US ↔ metric pairs
Say, hypothetically, you're comparing a US clothing size chart (which uses inches) to a European one (which uses centimeters) — a chest measurement listed as "40 in" converts to about 101.6 cm. Or a hypothetical example: a package listed as weighing "22 lb" on a US shipping label works out to roughly 10 kg, which matters if a foreign customs form asks for kilograms specifically. These are illustrative examples only, not fixed real-world figures — the actual conversion depends on the exact number you're starting with.
| From | Multiply by | Example |
|---|---|---|
| Inches → cm | 2.54 | 12 in ≈ 30.48 cm |
| Feet → meters | 0.3048 | 6 ft ≈ 1.83 m |
| Miles → km | 1.609344 | 26.2 mi (marathon) ≈ 42.2 km |
| Pounds → kg | 0.453592 | 150 lb ≈ 68 kg |
| Ounces → grams | 28.3495 | 8 oz ≈ 226.8 g |
Temperature: why multiplication doesn't work
This trips people up more than any other conversion on this list. If temperature scaled like length or weight, 0°F would equal 0°C — it doesn't (0°F is about −17.8°C). That offset is why the formula subtracts 32 before scaling. One genuinely useful fact: −40° is the one point where Fahrenheit and Celsius agree exactly, which is a decent way to sanity-check whether you've applied the formula correctly.
A hypothetical example: if a recipe (written for a European oven) calls for "180°C," that converts to 356°F — worth double-checking against your oven's actual dial before you commit a dish to it, since ovens vary and this is an illustrative conversion, not a cooking guarantee.
Area, volume, and the "which gallon?" trap
Real estate listings are the most common place area conversion actually matters day-to-day — a lot described as "2 acres" is roughly 8,094 m², useful context if you're comparing it to a property listed in a country that uses metric land measurements. Volume has its own trap: because "gallon" means different things in different countries, a recipe or fuel-economy figure quoted in gallons needs a quick check of which gallon is meant before converting — get it wrong and you're off by about 20%.
Bonus: why your 1TB drive shows up as 931GB
This one isn't really a conversion mistake so much as two industries using the same word for two different quantities. It's worth knowing the difference exists so a "missing" 7% of storage doesn't send you down a troubleshooting rabbit hole that was never a hardware problem to begin with.