Most unit conversion mistakes are not arithmetic errors. They happen because two units share a name, because a shortcut factor was rounded too early, or because the quantity is not what it seems: a temperature difference instead of a temperature, bits instead of bytes, a US gallon instead of an imperial one. This article goes through the mix-ups that cause real confusion, each with the exact figure. Every number here comes from the unit converter, which stores each unit as an exact fraction of its base unit and rounds only once, at the end.
kB and KiB: two kilobytes
Computers count memory in powers of two, so for decades "kilobyte" was used loosely for 1,024 bytes. The international standard (IEC 80000-13) settled the ambiguity: the decimal prefixes kilo, mega, giga and tera mean 1,000, 1,000,000 and so on, exactly as in kilometre, and the binary multiples got their own names: kibibyte (KiB, 1,024 bytes), mebibyte (MiB, 1,024²), gibibyte (GiB, 1,024³) and tebibyte (TiB, 1,024⁴).
The two systems drift further apart with every prefix:
| Decimal unit | Bytes | Same amount in binary units |
|---|---|---|
| 1 kB | 1,000 | 0.9765625 KiB |
| 1 MB | 1,000,000 | 0.9536743164 MiB |
| 1 GB | 1,000,000,000 | 0.9313225746 GiB |
| 1 TB | 1,000,000,000,000 | 0.9094947018 TiB |
At the kilobyte level the gap is 2.3%; at the terabyte level it is over 9%. That is the whole explanation for the most common storage complaint.
Why a 1 TB drive shows 931 GB
Drive makers label capacity in decimal units: a 1 TB drive holds 10¹² bytes. Windows divides by 1,024³ but still writes "GB", so it reports 931 GB. Nothing is missing and nothing was used by the formatting in that figure; the same number of bytes is being described in a different unit. A "500 GB" drive shows as 465.66 GiB, which Windows prints as 465 GB. macOS and iOS switched to decimal units years ago, so the same drive shows as 500 GB on a Mac and 465 GB on a PC.
Memory is the opposite case: a "16 GB" RAM module really is 16 GiB (17,179,869,184 bytes), because memory chips are built in powers of two.
Bits and bytes
A byte is eight bits. Storage is measured in bytes (capital B), network speed in bits per second (lower-case b). A 100 Mbit/s connection therefore moves at most 12.5 MB of data per second, before any protocol overhead. Downloading a 4 GB file over that line takes at least 320 seconds (five minutes and twenty seconds), not 40 seconds.
When you read a speed, look at the case of the b. "MB/s" in a download manager and "Mbps" in an internet plan differ by a factor of eight.
Temperatures and temperature differences
Converting a temperature needs an offset: °F = °C × 9/5 + 32. So 20 °C is 68 °F, 100 °C is 212 °F, and −40 is the one point where both scales agree.
A temperature difference has no offset. If the forecast says it will be 20 °C warmer than yesterday, that is 36 °F warmer, not 68. The same trap appears in recipes ("lower the oven by 20 degrees") and in engineering tolerances. The unit converter converts temperatures; for a difference, multiply by 9/5 (Celsius to Fahrenheit) or 5/9 (back) and do not add or subtract 32. Kelvin and Celsius differences are equal: a change of 1 K is a change of 1 °C.
The converter also refuses anything below absolute zero (0 K, −273.15 °C, −459.67 °F), because a lower temperature cannot exist. If a calculation produces one, a difference was probably converted as if it were a temperature.
Rounding a factor too early
Shortcuts like "a foot is about 0.3 m" or "a kilo is 2.2 pounds" are fine for a rough idea and bad for anything multiplied. A mile is 5,280 feet: with 0.3 m per foot you get 1,584 m instead of the exact 1,609.344 m, 25 m short. 100 kg is 220.46 lb, not 220 lb.
Many common factors are exact by definition, so there is no reason to round them:
- 1 inch = 0.0254 m exactly, so 1 foot = 0.3048 m and 1 mile = 1,609.344 m.
- 1 pound = 0.45359237 kg exactly.
- 1 US gallon = 231 cubic inches = 3.785411784 litres exactly.
- 1 imperial gallon = 4.54609 litres exactly.
- 1 nautical mile = 1,852 m exactly.
The converter keeps these as fractions and rounds the final result to the number of significant digits you choose, and it says whether the shown value is exact. 1 metre is 3.280839895 feet at ten digits, and that one is rounded, because 1 ÷ 0.3048 never ends.
US and imperial volumes
The United States and the United Kingdom use the same names for different volumes. The US gallon is 3.785 litres, the imperial gallon 4.546 litres, about 20% more. US fluid ounces are slightly larger than imperial ones, but there are 16 of them in a US pint and 20 in an imperial pint, so the imperial pint is bigger. A US cup is 236.5882365 mL.
This matters most in recipes and fuel economy: a British car magazine and an American one will print different mpg figures for the same car.
Fuel economy runs backwards
Litres per 100 km measures consumption: lower is better. Miles per gallon measures distance per fuel: higher is better. One is the inverse of the other, so they cannot be converted by multiplying by a factor. The exact relation for US gallons is mpg = 235.2145833 ÷ (L/100 km).
| L/100 km | mpg (US) | mpg (UK) |
|---|---|---|
| 5 | 47.04 | 56.50 |
| 7.84 | 30.00 | 36.03 |
| 10 | 23.52 | 28.25 |
The inverse relation also explains a famous puzzle. Going from 10 to 5 L/100 km saves 5 litres every 100 km, while going from 30 to 50 mpg saves less fuel than going from 10 to 15 mpg. Equal steps in mpg are not equal savings; equal steps in L/100 km are.
Pressure units in tyres and weather
A tyre at 2.2 bar is at 31.9 psi, and 1 bar is 14.50377377 psi. A standard atmosphere is 101,325 Pa, which is 1,013.25 hPa (the weather unit, equal to millibars) and 14.69594878 psi. The millimetre of mercury and the torr look identical but are not quite: 1 atm is exactly 760 Torr and 759.9998917 mmHg. The difference only matters in a laboratory, but it is why both exist.
Common mistakes, in short
- Reading a decimal GB figure as binary, or the other way round. Check the unit: GB versus GiB.
- Comparing a speed in Mbit/s with a size in MB without dividing by eight.
- Adding 32 when converting a temperature difference.
- Rounding the factor (0.3 m, 2.2 lb) before multiplying a large number.
- Mixing US and imperial gallons, pints or fluid ounces.
- Treating L/100 km and mpg as proportional.
How to check a conversion yourself
Choose the quantity in the unit converter, type the value and look at three things: the result, the "1 X = Y" line that shows the factor used, and the label that says whether the result is exact or rounded. The table below the result lists the same value in every unit, which is the quickest way to spot that you meant KiB rather than kB. For money, use the currency converter, which is a different problem: there the rate changes every day.