The two systems, and why both survive
The metric system — formally the SI — is decimal throughout, so every conversion is a shift of the decimal point. It is used officially by every country except the United States, Liberia and Myanmar.
The imperial and US customary systems descend from older English units and are not decimal: 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile. The awkwardness is historical, and the units survive because they are embedded in culture, infrastructure and habit.
When unit confusion goes wrong
The Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds. The spacecraft entered the atmosphere at the wrong altitude and broke up — a $327 million unit conversion error.
Temperature is different
Every other category here converts by multiplying a factor, because the scales share a zero point. Temperature scales do not, so conversion needs an offset as well.
°C = (°F − 32) × 5/9
K = °C + 273.15
| °C | °F | K | |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Scales coincide | −40 | −40 | 233.15 |
| Room temperature | 20 | 68 | 293.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
The −40 coincidence is a useful check: it is the one temperature where Celsius and Fahrenheit read the same. Kelvin has no degree symbol and no negative values, because it starts at absolute zero.
Why your hard drive looks smaller than advertised
Digital storage has two competing definitions, and the mismatch is why a "1 TB" drive shows as roughly 931 GB in Windows.
| Unit | Decimal (SI) | Binary (IEC) | Difference |
|---|---|---|---|
| Kilo | 1,000 | 1,024 (kibi) | 2.4% |
| Mega | 1,000² | 1,024² (mebi) | 4.9% |
| Giga | 1,000³ | 1,024³ (gibi) | 7.4% |
| Tera | 1,000⁴ | 1,024⁴ (tebi) | 10.0% |
| Peta | 1,000⁵ | 1,024⁵ (pebi) | 12.6% |
Drive manufacturers use decimal — 1 TB means exactly 1,000,000,000,000 bytes. Windows reports in binary but labels it "GB", so the same drive reads as 931 GiB. macOS and most Linux distributions switched to decimal reporting years ago, which is why the same drive shows a larger number there.
This converter offers both. The IEC binary units — KiB, MiB, GiB — are unambiguous and worth using in technical writing.
Frequently asked questions
What is the difference between a US gallon and an imperial gallon?
A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, about 20% larger. The difference carries through to quarts, pints and fluid ounces.
A US fluid ounce is 29.57 ml while an imperial fluid ounce is 28.41 ml — so a US pint is 16 fl oz and an imperial pint is 20.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32. So 20°C is 20 × 1.8 + 32 = 68°F.
A quick mental approximation is to double and add 30 — that gives 70 for 20°C, close enough for weather.
Why does my 1 TB drive show as 931 GB?
The manufacturer uses decimal units where 1 TB is 1,000,000,000,000 bytes. Windows divides by 1,024 three times but still labels the result "GB", giving 931.
No storage is missing — it is a labelling mismatch. The correct term for the Windows figure is 931 GiB.
How many kilometres is a mile?
One mile is exactly 1.609344 kilometres, and one kilometre is 0.621371 miles.
For mental conversion, multiply miles by 1.6, or use the Fibonacci trick: consecutive Fibonacci numbers approximate the ratio, so 5 miles ≈ 8 km and 8 miles ≈ 13 km.
Which countries do not use the metric system?
Only the United States, Liberia and Myanmar have not adopted it officially. The UK is formally metric but retains imperial units for road distances, draught beer and personal height and weight.
Even in the US, science, medicine and the military all run on metric.