Look at this map of a local park. Down in the corner there is a little scale bar. A scale bar is a small ruler-like marker on a map that shows how much real distance a short paper length stands for. Two trees are drawn close together on the paper, but in real life they might be far apart. How could that tiny scale bar tell us the real distance between the two trees without ever walking it?

Look at this classroom plan. It shows paper lengths and the scale above it. The same rule turns each paper length into a real distance.

The same rule works on any scale drawing. On a treasure map where one cm stands for 10 m, four cm on the page is 40 m of ground. On an Ordnance Survey map drawn at 1:50,000, one cm stands for half a kilometre, so two cm is a full kilometre. The numbers get much bigger, but you still multiply the paper length by the scale number.

Today we work through this together: turn a real classroom, 6 m long and 5 m wide, into a plan at 1:100. We take each real length, change it to centimetres, divide by the scale number, and write down the paper length. Then we sketch the plan on the board as a class.

Key Takeaways

• A scale ratio such as 1:100 links a paper length to a real length: one paper unit stands for that many real units.
• A verbal scale like '1 cm = 10 m' is the same idea as a colon ratio (1:1,000) once both lengths are in the same unit.
• Real to paper: match the units, then divide by the scale number.
• Paper to real: multiply the paper length by the scale number.
• Every line on a scale drawing should carry both its paper length and its real length.