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.
Before standard units existed, length was measured against the human body: the ancient Egyptian cubit (roughly the length of a forearm) was used to build the pyramids, and the English foot is traditionally said to derive from the length of a king's actual foot. This caused chaos in trade, since a merchant's cubit rarely matched a customer's. The modern metre was defined during the French Revolution in 1793 as one ten-millionth of the distance from the North Pole to the Equator, measured along a meridian through Paris – an attempt to create a single, universal unit that belonged to no king and no country. Today the metre is defined even more precisely, using the distance light travels in a tiny fraction of a second, but the goal is the same one the French revolutionaries had: a length that anyone, anywhere, can reproduce exactly.
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
4.7 × 1,000 = 4,700 m.
230 ÷ 100 = 2.3 m.
5 ft = 5 × 30.48 = 152.4 cm. 9 in = 9 × 2.54 = 22.86 cm. Total = 175.26 cm.
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.