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Physical quantity
Beginner
Anything measurable, expressed as a number times a unit.
A physical quantity is a property you can measure, written as a number together with a unit. The number alone is meaningless: "5" could be seconds, metres or newtons. Quantities are either base (chosen as independent, such as length and time) or derived (built by combining base ones, such as speed).
A car travels 90 km in 1.5 h. The quantity is a distance of 90 km, a time of 1.5 h, and the derived quantity speed is 60 km/h.
Never drop units in intermediate steps. Carrying them through is the cheapest error check you have.
In more detail
A physical quantity is anything you can measure, such as length, mass or time. Every quantity is a number multiplied by a unit, and the unit tells you what the number means.
Real-world uses
- Recipes and medicine doses rely on consistent quantities and units to avoid dangerous mistakes.
- Scientists compare results from different labs only when they report quantities in agreed units.
- A doctor records a patient's temperature as 37.0 °C, and a pharmacist records a dose as 5 mg. Both are physical quantities with a number and a unit.
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Frequently asked questions
Is temperature a physical quantity?
Yes. It is measured in kelvin in physics, although everyday life often uses degrees Celsius or Fahrenheit.
Can a quantity have no unit?
Yes. Ratios such as refractive index or relative density are dimensionless, so their value is a pure number.
Is a unit always needed for a physical quantity?
Yes. A number without a unit does not say what was measured. Only pure ratios and counts, such as the number of turns in a coil, are dimensionless.
Related topics in Measurement & Math Tools
What is physics? · SI base units · Derived units · Prefixes (milli, kilo, mega)