# SI units

The standard units scientists share worldwide: metre (m) for length, kilogram (kg) for mass and second (s) for time.

Physics (school) · Part 01: Motion and measurement · Card 1 of 8 · Last updated 2026-10-19

## Analogy

Like everyone agreeing that a cup of tea means the same amount, so recipes work anywhere.

## Example

**Converting units**

2.5 km = 2.5 x 1,000 = 2,500 m
450 g = 450 / 1,000 = 0.45 kg
90 min = 90 x 60 = 5,400 s

## In practice

Always convert to SI units before using a formula, or the answer comes out wrong.

## Practice

### Flashcards

- **What are the SI units for length, mass and time?** Metre (m), kilogram (kg) and second (s).
- **Convert 36 km/h to m/s.** 36 / 3.6 = 10 m/s.

## Questions people ask

### Why do we use SI units in physics?

Physics formulas only work when every quantity uses matching units. SI units are the agreed international set, so a result in Japan means the same as one in Brazil. Mixing, say, grams with kilograms inside one formula gives an answer that is wrong by a factor of a thousand.

### What is the difference between a base unit and a derived unit?

A base unit stands alone, such as the metre, kilogram or second. A derived unit is built by combining base units. Speed is measured in metres per second (m/s), and force in newtons (N), where one newton equals one kilogram metre per second squared.

## Related concepts

- [Speed](https://studyvisually.com/learn/science/physics-school/physics-school-part-1/physics-school-part-1-concept-3)
- [Scalar and vector](https://studyvisually.com/learn/science/physics-school/physics-school-part-1/physics-school-part-1-concept-2)

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