Standard Units of Length

The International System of Units (SI)

To solve the confusion caused by local units (like inches, feet, or handspans), countries across the world adopted a standard set of units known as the International System of Units (SI Units).

Sub-units of the Metre

Ideally, we use different units depending on the size of the object we are measuring.

1. Millimetre (mm): Used for very small lengths (e.g., thickness of a coin, tip of a pencil).

   • $1 \text{ cm} = 10 \text{ mm}$
   • $1 \text{ mm} = 0.1 \text{ cm}$

2. Centimetre (cm): Used for medium-small lengths (e.g., length of a pencil, notebook, mobile phone).

   • $1 \text{ m} = 100 \text{ cm}$

3. Kilometre (km): Used for long distances (e.g., distance between two cities).

   • $1 \text{ km} = 1000 \text{ m}$

Conversion Visualization

Rules for Writing Units

1. Lowercase: Units are usually written in lowercase (e.g., metre, not Metre), unless at the start of a sentence.
2. Symbols: Symbols are lowercase (km, m, cm).
3. No Plurals: Never add an ‘s’ to the symbol.
   • Correct: 50 kg, 10 cm.
   • Incorrect: 50 kgs, 10 cms.
4. No Full Stop: Do not put a dot after the symbol unless it is the end of a sentence.
5. Spacing: Always leave a space between the number and the unit (e.g., $15 \text{ cm}$).