Revision summary

A measurement needs a clear quantity, a sensible unit and a method suitable for the object or event. Use the links below when you need the reasoning behind a rule.

Base quantities and units

A quantity symbol describes the variable; a unit symbol identifies its unit. In l = 2.0 m, lowercase l means length and m means metre. Kelvin is K, without a degree sign.

Prefixes and conversions

For area, square the length conversion factor; for volume, cube it. Check the order of size before accepting a result: a hair is much thinner than a coin.

Choose an instrument

• Range: can it cover the measurement?
• Resolution: are its divisions or display steps small enough?
• Method: can it be aligned, read and used without distorting the object?

Precision concerns agreement between repeats; accuracy concerns closeness to the true or accepted value. More displayed digits alone guarantee neither.

Read, correct and calculate

Length from a rule

Final end reading - initial end reading. Look perpendicularly at the scale to avoid parallax.

Stable zero offset

Corrected reading = displayed reading - signed zero offset. Repeating alone does not remove the offset.

Period of a repeated motion

Mean elapsed time for N complete oscillations / N. Use the same reference point and direction for each count.

Volume by displacement

Final water volume - initial water volume. Fully submerge a suitable solid; avoid trapped air and lost water. Read the lower water meniscus at eye level.

Use quantity/unit headings in data tables. Record readings consistently with the instrument; preserve the original observations when averaging or correcting. The measurement-method directory links to examples for motion, forces, thermal changes, waves, optics, circuits and radioactivity.

Scalars and vectors

• Scalars have magnitude only: distance, speed, mass, time, temperature and energy.
• Vectors have magnitude and direction: displacement, velocity, acceleration and force, including weight.
• A negative value alone does not make a quantity a vector. State a vector's direction or the chosen positive direction.