Length - How We Measure Distance
Length is one of the most fundamental measurements in everyday life. Whether you are cutting a piece of string, planning a room layout, or describing the distance between two cities, you are measuring length – the straight-line distance from one point to another.
Metric Units of Length
The metric system uses the metre (m) as its base unit for length. All other units are powers of ten away from the metre.
| Unit | Symbol | Relation to 1 metre | Typical use |
|---|---|---|---|
| Millimetre | mm | 1 m = 1,000 mm | Thickness of a coin |
| Centimetre | cm | 1 m = 100 cm | Height of a person |
| Metre | m | Base unit | Length of a room |
| Kilometre | km | 1 km = 1,000 m | Distance between towns |
Converting Metric Units of Length
| Conversion | Operation | Example |
|---|---|---|
| km → m | × 1,000 | 3.5 km = 3,500 m |
| m → km | ÷ 1,000 | 750 m = 0.75 km |
| m → cm | × 100 | 2.4 m = 240 cm |
| cm → m | ÷ 100 | 85 cm = 0.85 m |
| cm → mm | × 10 | 6.2 cm = 62 mm |
| mm → cm | ÷ 10 | 45 mm = 4.5 cm |
Imperial Units of Length
| Unit | Symbol | Equivalence | Metric approx. |
|---|---|---|---|
| Inch | in | 12 inches = 1 foot | 1 in = 2.54 cm |
| Foot | ft | 3 feet = 1 yard | 1 ft = 30.48 cm |
| Yard | yd | 1,760 yards = 1 mile | 1 yd ≈ 0.914 m |
| Mile | mi | — | 1 mi ≈ 1.609 km |
Measuring Tools
- Ruler – measures up to 30 cm; ideal for straight lines on paper.
- Tape measure – flexible; used for furniture and rooms.
- Trundle wheel – rolls along a surface counting each metre outdoors.
- Laser distance meter – bounces a laser beam for instant digital readings.
Worked Examples
Convert 4.7 km to metres.
4.7 × 1,000 = 4,700 m.
Convert 230 cm to metres.
230 ÷ 100 = 2.3 m.
A bookshelf is 5 ft 9 in tall. Convert to centimetres.
5 ft = 5 × 30.48 = 152.4 cm. 9 in = 9 × 2.54 = 22.86 cm. Total = 175.26 cm.
Convert 3 miles to kilometres.
3 × 1.609 = 4.827 km.
Key Takeaways
- The base metric unit of length is the metre.
- Multiply to convert to a smaller unit; divide to convert to a larger unit.
- 1 km = 1,000 m | 1 m = 100 cm | 1 cm = 10 mm.
- 1 inch = 2.54 cm | 1 mile ≈ 1.609 km.
Practice Questions
- Convert 6.3 km to metres.
- Convert 850 mm to centimetres and then to metres.
- A swimming pool is 25 m long. How many lengths equal 1 km?
- Convert 5 ft 4 in to centimetres.
- A road sign shows 12 miles to the next town. How far is that in kilometres?