Last reviewed 16 Sept 2026 · 8 min read
Scales
Objects are often too large (buildings, bridges) or too small (watch parts) to be drawn full size. A scale is the ratio of the size of the drawing to the actual size of the object.
FormulaRepresentative fraction
Length of scale maximum length to be measured
| Type of scale | RF | Examples |
|---|---|---|
| Full-size scale | = 1 | 1 : 1 |
| Reducing scale | < 1 | 1 : 2, 1 : 100 |
| Enlarging scale | > 1 | 2 : 1, 10 : 1 |
Recommended scales (SP 46)
- Enlargement: 50 : 1, 20 : 1, 10 : 1, 5 : 1, 2 : 1
- Full size: 1 : 1
- Reduction: 1 : 2, 1 : 5, 1 : 10, 1 : 20, 1 : 50, 1 : 100, 1 : 200, 1 : 500, 1 : 1000, 1 : 2000, 1 : 5000, 1 : 10 000
Typical uses in civil engineering: building plans 1 : 100 (or 1 : 50), details 1 : 20, 1 : 10, 1 : 5, site plans 1 : 500 or 1 : 1000, layout/location plans 1 : 2000 or smaller.
The scale must always be stated in the title block; if a drawing may be enlarged or reduced during reproduction, a graphic (bar) scale is drawn on the sheet.
Types of scales
| Scale | Features |
|---|---|
| Plain scale | Divided into main units and first sub-units (e.g. metres and decimetres) — reads two units |
| Diagonal scale | Uses diagonals to read three units (e.g. metres, decimetres and centimetres) — based on similar triangles; accurate small subdivisions |
| Vernier scale | A main scale and a sliding/auxiliary vernier scale; reads small fractions of the smallest main scale division — forward (direct) or backward (retrograde) vernier |
| Comparative scales | Two scales with the same RF but different units (e.g. km and miles) |
| Scale of chords | Marks angles by chord lengths — used when a protractor is not available |
| Map scales | Expressed as RF (1 : 50 000), statement ("1 cm = 0.5 km") or graphic bar scale |
Construction of a plain scale (steps)
- Compute RF and length of scale.
- Draw a line of that length; divide it into the required main units.
- Divide the first main division (at the left) into sub-units.
- Mark zero at the end of the first main division; number main units to the right and sub-units to the left.
- Draw the scale as a thin rectangle (commonly about 10 mm wide), write the RF and units below.
Construction of a diagonal scale (principle)
- If a short length is to be divided into parts, draw a perpendicular of any length, divide it into equal parts, join to , and draw lines parallel to through the division points. The -th parallel line cut by the diagonal has length .
- Thus, sub-units are subdivided further by 10 equal vertical divisions and diagonals.
Vernier scale
FormulaLeast count
(forward vernier: vernier divisions = main scale divisions)
Backward vernier: vernier divisions = main scale divisions; LC = 1 VSD − 1 MSD.