Intro
Unit conversion means expressing the same quantity in a different measurement system. The arithmetic may be multiplication, division, or, for temperature, a scale change with an offset.
Knowing the unit definition matters as much as knowing the number.
Metric prefixes scale by powers of ten
Kilo means 1,000, centi means one hundredth, and milli means one thousandth. This regular structure is why metric conversions are easy to chain.
Customary units use fixed factors
One inch is exactly 2.54 centimetres, one foot is 12 inches, and one pound is 16 ounces. These relationships are fixed, but they do not follow powers of ten.
MB is not MiB
Decimal SI prefixes use powers of 1,000: 1 MB is 1,000,000 bytes. Binary prefixes use powers of 1,024: 1 MiB is 1,048,576 bytes. Using the correct label avoids storage surprises.
Units are labels, not decoration
A conversion is meaningful only when the unit and quantity are known. Length, mass, area, volume, speed, energy, and temperature each have different relationships. Squared and cubed units require the factor to be squared or cubed too: converting a length scale does not use the same numerical factor as converting an area.
Metric prefixes are powers of ten, but customary and domain-specific units use fixed definitions. Digital storage is a frequent trap: decimal kB, MB, and GB are different from binary KiB, MiB, and GiB. A specification, operating system, and manufacturer may use different conventions, so preserve the original unit in a report.
Temperature and compound quantities
Celsius, Fahrenheit, and Kelvin are affine scales, so temperature conversion includes an offset as well as a scale. A temperature difference of one degree Celsius equals one kelvin, but a temperature reading does not convert by multiplying alone. Similar care is needed for rates such as km/h and mph, where the time unit is part of the quantity.
State the source unit, target unit, precision, and rounding policy. Do not round intermediate values when a later calculation depends on them, and do not imply scientific accuracy greater than the input supports. The unit converter is useful for common values; confirm specialist engineering, medical, laboratory, and legal measurements against the governing standard.
Common conversions and formulas
Useful reference relationships include 1 inch = 2.54 centimetres exactly, 1 kilogram = 2.2046226218 pounds approximately, and 1 mile = 1.609344 kilometres exactly. For temperature, °F = (°C × 9/5) + 32, °C = (°F − 32) × 5/9, and K = °C + 273.15. State whether a value is a measurement, a rounded display, or an exact defined relationship.
Digital units need an explicit convention. Decimal prefixes use powers of 1,000: 1 MB = 1,000,000 bytes. Binary prefixes use powers of 1,024: 1 MiB = 1,048,576 bytes. A product label, operating system, and technical specification may use different terms, so do not silently replace MB with MiB or vice versa.
Significant figures and compound units
Do not report more precision than the input supports. Keep extra precision during intermediate calculations, then round once at the presentation boundary. For an area conversion, square the length factor; for a volume conversion, cube it. For speed, convert numerator and denominator consistently rather than converting only the distance.
Imperial and US customary units also differ for some measures, especially gallons and fluid ounces. If a recipe, engineering drawing, medical instruction, or legal document is involved, confirm the governing definition before converting. A general-purpose tool cannot infer which regional convention the source intended.
Practical takeaway
Before converting, write down the quantity, source unit, target unit, precision, and rounding policy. Pay special attention to squared or cubed units, temperature offsets, and decimal versus binary prefixes. A converter removes arithmetic friction, but only the person doing the measurement can confirm that the chosen units describe the real thing.
FAQ
How many inches are in a centimetre?
One centimetre is exactly 1 divided by 2.54 inches, approximately 0.3937008 inches.
What is the difference between kg and lb?
A kilogram is a metric mass unit; a pound is a customary mass unit. One kilogram is approximately 2.2046226 pounds.
Why are temperature conversions different?
Celsius, Fahrenheit, and Kelvin use different zero points as well as different step sizes, so conversion needs multiplication and an offset.