How unit conversion works
Every unit conversion involves three steps: express the input value in a base unit, then express the base unit value in the output unit. The converter uses SI base units (metres, kilograms, seconds, etc.) as the intermediate representation.
For example, converting 5 miles to kilometres:
- 5 miles → metres: 5 × 1609.344 = 8,046.72 m
- 8,046.72 m → kilometres: 8,046.72 ÷ 1,000 = 8.04672 km
For most unit categories, this is just multiplication. Temperature is the exception — it requires a full formula because Celsius, Fahrenheit, and Kelvin have different zero points (offsets), not just different scales.
Length: the inch and the metre
The metre is the SI base unit of length, defined since 1983 as the distance light travels in a vacuum in 1/299,792,458 of a second.
The inch was redefined in 1959 to be exactly 2.54 centimetres — a precise, internationally agreed definition that makes the entire imperial/metric length system fully interoperable. From this single definition:
- 1 foot = 12 inches = 30.48 cm
- 1 yard = 36 inches = 91.44 cm
- 1 mile = 5,280 feet = 1,609.344 m
These are exact by definition — not approximations.
Weight vs mass
Technically, the kilogram is a unit of mass (matter content), not weight (gravitational force). Weight is measured in Newtons. However, in everyday usage and in this converter, "weight" and "mass" are treated as interchangeable — we're converting between units of mass, which is what bathroom scales and kitchen scales actually measure.
The kilogram became the SI base unit of mass in 1889. It was defined by the International Prototype Kilogram (a platinum-iridium cylinder in Paris) until 2019, when it was redefined in terms of the Planck constant.
Temperature: why there's no simple multiplier
The Celsius, Fahrenheit, and Kelvin scales agree on the size of a degree (Kelvin and Celsius have the same degree size; Fahrenheit degrees are 5/9 the size), but they start counting from different zero points:
- Kelvin zero (0 K = −273.15 °C): absolute zero — the coldest possible temperature, where all molecular motion stops
- Celsius zero (0 °C): the freezing point of water
- Fahrenheit zero (0 °F): approximately the coldest temperature Daniel Fahrenheit could achieve with a salt-ice mixture
Because of these offsets, you cannot simply multiply to convert — you must apply the full formula:
- °C to °F: (°C × 9/5) + 32
- °F to °C: (°F − 32) × 5/9
- °C to K: °C + 273.15
Data: binary (IEC) vs decimal (SI) prefixes
This is the source of the classic confusion: "Why does my 1 TB hard drive show up as 931 GB in Windows?"
- Manufacturers use decimal SI prefixes: 1 TB = 1,000,000,000,000 bytes
- Operating systems (Windows, macOS, Linux) report sizes in binary IEC prefixes: 1 TiB = 1,099,511,627,776 bytes
The converter uses binary prefixes (1 KB = 1,024 bytes), matching what your OS shows. The 1 TB drive contains exactly 1,000,000,000,000 bytes, which the OS reports as ~931 GiB.
The IEC standardized this in 1998 with new prefixes: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB). Most people still say "gigabyte" when they mean "gibibyte" — the converter handles both interpretations under the "binary (IEC)" assumption.
Speed: m/s, km/h, mph, and knots
Speed conversions come up in travel, aviation, and physics. The base unit is metres per second (m/s). Key relationships:
- 1 km/h = 1/3.6 m/s (approximately 0.2778 m/s)
- 1 mph = 0.44704 m/s exactly (derived from the 1959 inch definition)
- 1 knot = 1 nautical mile per hour = 1.852 km/h exactly
- 1 Mach at sea level standard atmosphere = 340.29 m/s (varies with altitude and temperature)
Pressure: Pa, bar, psi, and atm
Pressure is force per unit area. The SI unit is the pascal (Pa = N/m²). Common reference points:
- 1 standard atmosphere (atm) = 101,325 Pa exactly
- 1 bar = 100,000 Pa (close to, but not exactly, 1 atm)
- 1 psi (pound-force per square inch) = 6,894.757 Pa
- Standard tyre pressure: ~2.2 bar (32 psi)
- Weather maps use hectopascals (hPa), where 1 hPa = 1 millibar = 100 Pa
Energy: joules, calories, kWh, and BTU
Energy is the capacity to do work. The SI unit is the joule (J). Different fields use different units:
- 1 kcal (food calorie) = 4,184 J. Nutrition labels list kilocalories as "calories" (capital C).
- 1 kWh (electricity billing) = 3,600,000 J = 3,600 kJ
- 1 BTU = 1,055.06 J. Still common in HVAC and US energy ratings.
- 1 eV (electron-volt) = 1.60218 × 10⁻¹⁹ J. Used in particle physics and semiconductor work.
The most common conversions
| From | To | Result | |------|-----|--------| | 1 km | miles | 0.621371 mi | | 1 mile | km | 1.609344 km | | 1 kg | lb | 2.20462 lb | | 1 lb | kg | 0.453592 kg | | 1 litre | gallons (US) | 0.264172 gal | | 1 inch | cm | 2.54 cm (exact) | | 0 °C | °F | 32 °F | | 100 °C | °F | 212 °F | | 100 km/h | mph | 62.137 mph | | 1 bar | psi | 14.5038 psi | | 1 kcal | kJ | 4.184 kJ |