The one idea behind almost every unit conversion
Nearly every category on this page works the same way under the hood: each unit has a fixed factor relative to one "base" unit in its category — metres for length, kilograms for weight, litres for volume, seconds for time. To convert anything, you convert it to the base unit first, then convert from the base unit to whatever you actually want.
Result = Value × (factor of source unit ÷ factor of target unit)
Worked example: converting 5 miles to kilometres. A mile's factor is 1,609.344 (metres), a kilometre's factor is 1,000. So 5 × 1,609.344 ÷ 1,000 = 8.05 km. The calculator does exactly this arithmetic — it just hides the base-unit step and shows you the direct factor.
Once you see it this way, the reason two seemingly random units convert cleanly stops feeling like magic — they're both just multiples of the same underlying quantity.
Why temperature breaks the pattern
Temperature is the one category here that can't use a simple multiplication, because Celsius, Fahrenheit, and Kelvin don't all start counting from the same zero. Water freezes at 0°C, 32°F, and 273.15 K — three different numbers for the exact same physical temperature — so converting between them needs an offset, not just a ratio:
| Conversion | Formula |
|---|---|
| °C → °F | (°C × 9/5) + 32 |
| °F → °C | (°F − 32) × 5/9 |
| °C → K | °C + 273.15 |
| K → °C | K − 273.15 |
What each category covers
Ten categories are built into this converter, each anchored to its own base unit:
| Category | Base unit | Example units |
|---|---|---|
| Length | Meter | mile, foot, inch, light-year |
| Weight | Kilogram | pound, ounce, stone, carat |
| Temperature | — | Celsius, Fahrenheit, Kelvin, Rankine |
| Volume | Liter | gallon, cup, tablespoon, cubic foot |
| Area | Square meter | acre, hectare, square mile |
| Speed | Meter/second | mph, knot, Mach |
| Time | Second | week, month, decade |
| Digital storage | Byte | kilobyte, gibibyte, megabit |
| Pressure | Pascal | bar, PSI, atmosphere |
| Energy | Joule | calorie, kWh, BTU |
Metric vs. imperial: why both still exist
The metric system builds everything from base-10 multiples of a handful of base units — a kilometre is exactly 1,000 metres, a kilogram is exactly 1,000 grams — which makes the arithmetic behind it clean and predictable. Imperial and US customary units (inches, feet, pounds, gallons) evolved from older, more local standards, so their ratios are less tidy: 12 inches to a foot, 3 feet to a yard, 5,280 feet to a mile.
Almost every country has adopted metric as its official system; the United States remains the largest holdout for everyday imperial use, which is exactly why a converter that speaks both fluently earns its keep.
Kilobytes vs. kibibytes: the storage size mismatch
This is the one everybody's bumped into without knowing the cause: you buy a "1 TB" drive, plug it in, and your computer reports something closer to 931 GB. Nothing is missing — two different counting conventions are colliding.
Storage manufacturers use the decimal (SI) definition, where 1 KB = 1,000 bytes and 1 TB = 1,000,000,000,000 bytes. Most operating systems report file and drive sizes using the binary convention instead, where 1 KiB = 1,024 bytes and the units compound from there (1 MiB = 1,024 KiB, and so on). Do the math on a 1 TB drive under the binary convention and you land at roughly 931 GiB — the same physical storage, reported on a different scale.
This calculator includes both — kilobyte/megabyte/gigabyte for the decimal convention, and kibibyte/mebibyte/gibibyte for the binary one — so you can convert between whichever pair actually matches what you're comparing.