Most unit conversion is one multiplication. A metre is 3.28 feet, a kilogram is 2.2 pounds, and the conversion never gets harder than that. But two categories break the pattern badly enough to produce wrong answers in real work — temperature, and digital storage — and both break for reasons worth understanding.
Almost everything converts through one number
A converter does not need a table of every unit paired with every other unit. It needs one base unit per category and a single factor for each unit. To go from inches to centimetres you convert inches to metres, then metres to centimetres. Nine length units need nine numbers, not the eighty-one a pairwise table would.
This works because these units are proportional. Doubling the length doubles the number in every unit, and zero is zero in all of them. Zero metres, zero feet and zero miles are the same nothing.
Temperature has no shared zero
Temperature scales do not have that property. Zero Celsius is the freezing point of water. Zero Fahrenheit is roughly the freezing point of a particular brine solution. Zero Kelvin is absolute zero. These are three different physical conditions that happen to share a label.
So a single multiplier cannot express the relationship. You need a multiplication and a shift — mathematically an affine transformation rather than a linear one:
°F = °C × 9/5 + 32
K = °C + 273.15There is a neat way to check any temperature converter: enter −40. Minus forty is the one point where the Celsius and Fahrenheit scales cross, so a correct converter returns −40. A converter that treats temperature as proportional — multiplying by 9/5 and forgetting the shift — returns −72. It is a single input that separates a correct implementation from a broken one.
The kilobyte problem
Storage units are the other place converters disagree, and here the ambiguity is genuine rather than a bug. Kilo means a thousand everywhere else in measurement, so a kilobyte should be 1000 bytes. But computers address memory in powers of two, and 210 is 1024 — close enough to a thousand that early convention borrowed the prefix.
The standards bodies eventually separated them. A kilobyte (KB) is 1000 bytes; a kibibyte (KiB) is 1024. Megabyte and mebibyte, gigabyte and gibibyte follow the same pattern. The gap compounds: at kilo it is 2.4%, but by tera it is nearly 10%.
This is why a drive sold as 500 GB shows up in Windows as about 465 GB. Nobody is lying. The manufacturer counted 500 × 109 bytes, which is a correct 500 GB, and Windows divided by 1024 three times while still printing the label "GB". Our converter lists both, so you can see which one you are actually being quoted.
Gallons are two different volumes
A US gallon is 3.785 litres. A UK imperial gallon is 4.546 litres. That is a 20% difference hiding behind an identical word, and it propagates down through quarts, pints and fluid ounces. A US pint is 473 ml; a UK pint is 568 ml.
Any converter that offers a single unlabelled "gallon" is guessing on your behalf. Ours labels them, which is less tidy and considerably more useful.
Why the answers carry so many digits
Several imperial units are defined as exact decimal quantities of metric ones — an inch is exactly 25.4 mm, a pound is exactly 0.45359237 kg. Exact in decimal is not exact in binary, which is how computers store these numbers, so the arithmetic accumulates a tiny error. Left alone, converting 100 °C to Fahrenheit produces 211.99999999999994.
Rounding the result to twelve significant digits lands comfortably inside that error and prints 212, while still being precise enough for any practical use. That is the number you see.
FAQ
Why can you not convert Celsius to Fahrenheit by multiplying?
Because the two scales do not share a zero. Multiplying works only when zero means the same thing in both units, as it does for length: zero metres and zero feet are the same nothing. Zero Celsius is the freezing point of water while zero Fahrenheit is a brine freezing point, so converting requires both a multiplication and a shift. Fahrenheit equals Celsius times nine fifths plus thirty-two.
Is a kilobyte 1000 bytes or 1024 bytes?
Both are in use, which is why the answer looks inconsistent. A kilobyte is officially 1000 bytes, following the standard meaning of kilo, and that is what storage manufacturers use. A kibibyte is 1024 bytes. Windows reports file sizes in units of 1024 but labels them KB and MB, which is why a drive sold as 500 GB shows as about 465 GB.
Is a US gallon the same as a UK gallon?
No, and the difference is large enough to ruin a recipe. A US gallon is 3.785 litres while a UK imperial gallon is 4.546 litres, about twenty per cent bigger. US and UK pints and fluid ounces differ too, so any converter worth using lists them separately rather than offering a single gallon.
Why does the converter show 3.28083989501 rather than 3.28?
That is the exact conversion carried to twelve significant digits, because one foot is defined as exactly 0.3048 metres and the reciprocal is not a round number. Twelve digits is enough to be correct for engineering use while short enough to avoid the floating-point noise that would otherwise show up as a long tail of nines. Round it yourself to whatever precision your work needs.
Related guides
- How age calculation really works — the same "units are not uniform" problem, applied to calendars.
- Compressing images in the browser — where kilobytes and mebibytes start to matter in practice.
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