Lesson 2.2
Metric Scales: The Power of Ten
The metric system used across India, Europe and most of the world. Why it makes scaling almost effortless — and the standard ratios you'll reach for every day.
Start hereAt 1:100, a 4-metre wall is 4 centimetres on paper. At 1:50, it's 8. You did that maths in your head just now — that's the gift of metric.
Because everything is powers of ten, scaling becomes shifting a decimal point. No fractions, no awkward division.
01 — Why metric is fast
Scaling by moving the decimal
Architectural drawings in metric work almost entirely in millimetres — one unit, no switching. A 2400 mm counter, a 900 mm door, a 3600 mm room. Keeping everything in mm means scaling is just division by a round number.
At 1:100, dividing by 100 moves the decimal two places: 3600 mm becomes 36 mm on paper. At 1:50, divide by 50. At 1:10, drop one zero. The whole system is built so the arithmetic stays in your head. So 3600 mm real, drawn at 1:100, is 3600 ÷ 100 = 36 mm on paper.
02 — The standard set
The ratios you'll actually use
Metric practice clusters around a small, recognisable set: 1:1, 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, 1:1000. Learn these and you'll rarely need anything else.
Notice they step by factors of 2 and 5 — 1:5 to 1:10 to 1:20 to 1:50. That keeps every common drawing at a clean, head-calculable ratio. A scale rule (next lesson) carries exactly these.
| Drawing | Usual metric scale |
|---|---|
| Joinery / construction detail | 1:1, 1:2, 1:5 |
| Interior layout, furniture plan | 1:20, 1:50 |
| Floor plan, section, elevation | 1:50, 1:100 |
| Whole building / small site | 1:200 |
| Site / location plan | 1:500, 1:1000 |
03 — A worked example
The same room, two metric scales
Take a 3600 × 4200 mm bedroom — a realistic size in a city apartment anywhere. At 1:100 it draws as 36 × 42 mm; at 1:50 as 72 × 84 mm.
Same room, same real dimensions, two paper sizes. At 1:50 it's four times the area on the page (twice each side) and you can show furniture; at 1:100 it's compact enough to sit alongside the rest of a floor plan. Choosing between them is choosing how much detail the drawing must carry.
Interactive · metric worked examples
A set of worked metric conversions — type a real length, see it divided by 1:50 and 1:100 and laid out side by side. Practise until “move the decimal” is automatic.
Professionals draw in millimetres rather than centimetres or metres for one practical reason: it avoids decimals on the drawing. “2400” is cleaner and less error-prone than “2.4 m” or “240 cm” when scribbled on a busy sheet — a misplaced decimal point in “2.4” can become “24” far too easily, but “2400” is unambiguous. The convention removes a whole category of mistakes. This is standard across metric-using countries, so a drawing dimensioned in mm reads the same in Bengaluru, Berlin or Brisbane.
10 minutes
- Convert these to paper size at 1:50: a 900 mm door, a 1500 mm bed, a 600 mm counter depth.
- Now do the same at 1:100. Notice you just halved every answer.
- A wall is 45 mm on a 1:50 drawing. What's its real length?
- Draw a 2400 × 3000 mm room at 1:50 by hand, using a ruler in mm.
Key terms — added to the Drawing Atlas
- Millimetre convention
- The metric practice of dimensioning drawings in millimetres rather than metres or centimetres, to avoid decimal-point errors.
- Metric scale
- Scale written as a clean ratio (1:50, 1:100) in the metric system; scaling reduces to division by a round number.
- Standard scale set
- The recognised everyday scales. Metric: 1:1, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500. Imperial has its own set.
Check yourself
2 quick questions — pick an answer to see why.
Q1A 3600 mm wall drawn at 1:100 is how long on paper?
Q2Why do architects dimension drawings in millimetres rather than metres?
- Metric drawings work in millimetres — one unit, no switching, fewer decimal errors.
- Scaling is division by a round number; often just moving the decimal point.
- The standard set steps by 2 and 5: 1:1, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500.
- Scale choice = how much detail the drawing must carry.
Metric makes the maths easy. But over a third of the world's drawings use feet and inches — how does scale work when your units aren't tens?