Lesson 2.4
Reading & Setting Scale: The Rule and the Cursor
The scale rule turns ratios into a tool you read directly — no arithmetic. And in CAD, scale works the opposite way round. Here's both, side by side.
Start hereA scale rule looks like an ordinary ruler with too many numbers. Those extra edges are the trick: each one is pre-divided so you read real-world lengths straight off the drawing.
And in CAD, you don't scale as you draw at all — you draw everything full size, then scale only when you print. Two opposite habits, one concept.
01 — By hand: the scale rule
Read lengths, don't calculate them
A scale rule (often a triangular one with six edges) carries several scales at once — say 1:50 and 1:100 on metric, or ¼" and ⅛" on imperial. You pick the edge matching your drawing, lay it along a line, and read the real length directly. The rule has done the division for you.
Drag to lay the marker along the 1:50 edge below. The rule reads real metres directly — no “× 50” in your head. An imperial architect's scale works identically, just reading feet off a ¼"=1' edge instead.
Reads off the rule
3 metres real · 60 mm on paper at 1:50
Drag the marker along the 1:50 edge. The rule reads real metres directly — no '× 50' in your head.
02 — On screen: scale at output
Draw full size, scale when you plot
CAD flips the whole habit. You draw at 1:1 — actual size — a 3600 mm wall is modelled as 3600 units. Scale isn't applied while you draw; it's applied when you place the drawing on a sheet and print. You tell the layout “show this at 1:50” and the software shrinks it to fit.
This is a genuine advantage: because the model is always true size, you can output the same plan at 1:50 for one sheet and 1:100 for another without redrawing anything. The scale becomes a property of the view, not the geometry.
| By hand | On screen |
|---|---|
| Choose a scale before you start | Draw at full size (1:1) always |
| Draw everything reduced by that ratio | Apply scale only at the layout/print stage |
| Read lengths off the scale rule | Software computes the reduction |
| Changing scale = redraw | Changing scale = change one setting |
03 — The shared discipline
Always state the scale
Hand or digital, metric or imperial, one rule never changes: every drawing must declare its scale, in the title block and often under each view. A drawing without a stated scale is unreadable — the reader has no way to know whether a 40 mm line means 2 metres or 4. The scale statement is part of the drawing's honesty, the same promise you met in 2.1.
The reason CAD can output any scale from one model is that it stores real-world units, not paper sizes. You set the drawing's units once (millimetres, or feet-and-inches) and every length lives at true size internally. A “scale” is then just a transform applied to a viewport. This is also why getting units right at the start matters enormously — a model built in the wrong units prints at the wrong size everywhere. Same discipline as the hand rule “every dimension true at its scale”, enforced by the software instead of your eye.
15 minutes, both hands
- If you have a scale rule, lay its 1:50 edge along a 60 mm line and read the real length. (No rule? Calculate: 60 × 50.)
- In CAD, draw a 3000 mm line at full size. Set a layout to 1:50 and confirm it prints 60 mm.
- Change that layout to 1:100 without redrawing. Confirm the line now prints 30 mm.
- Add a scale note (“1:50”) under your drawing, as you would in a title block.
Key terms — added to the Drawing Atlas
- Scale rule
- A ruler (often triangular, six edges) carrying several pre-divided scales, so real lengths read directly off a drawing with no calculation.
- Full size (1:1)
- Drawing at actual size, with no reduction. In CAD, all geometry is drawn at 1:1 and scale is applied only at output.
- Output scale
- In CAD, the scale applied to a view when it is placed on a sheet and printed — a property of the view, not the geometry.
- Stated scale
- The scale a drawing declares in its title block or under each view. A drawing without one is unreadable.
Check yourself
1 quick questions — pick an answer to see why.
Q1In CAD, when is scale typically applied?
- A scale rule reads real lengths directly off the drawing — no arithmetic. Pick the edge matching your scale.
- CAD reverses the habit: draw at full size, apply scale only when you plot.
- Because the model is true-size, one drawing outputs at any scale by changing a setting.
- Every drawing must state its scale — the same honesty rule from 2.1.
You can work in metric or imperial, by hand or on screen. The last skill is moving safely between them — and never trusting a scaled measurement blindly. Why not?