What this calculator does
A scale calculator converts between a real-world size and its scaled-down (or occasionally scaled-up) equivalent, given a scale ratio such as 1:24 for a model car, 1:87 for HO model railways, or 1:100 for an architectural drawing. The ratio says how many real-world units are represented by one unit on the drawing or model.
This calculator works in both directions: enter a real-world measurement and it returns the scale size, or enter a scale measurement and it returns the real-world size. Both values need to be entered in the same unit, whether that is millimetres, centimetres, inches or feet, since the ratio itself carries no unit.
The formula
A scale of 1 : ratio means one unit on the scale drawing or model corresponds to "ratio" units in real life. To go from real-world to scale, divide the real-world value by the ratio. To go the other way, multiply the scale value by the ratio.
| Term | Meaning |
|---|---|
| Scale ratio | The number after the colon in a scale such as 1:24, meaning the real world is 24 times larger than the drawing or model. |
| Real-world value | The full-size measurement of the object, building or distance being represented. |
| Scale value | The measurement as it appears on the drawing, plan or model, after shrinking (or occasionally enlarging) by the scale ratio. |
The inputs explained
| Field | What to enter |
|---|---|
| Scale ratio (1 : ___) | The number after the colon in the scale, e.g. enter 24 for a scale of 1:24, or 87 for 1:87. |
| Value to convert | The value to convert, in whichever unit is being used (mm, cm, inches, feet). Enter both values in the same page in the same unit. |
| Convert | Choose whether to convert a real-world measurement down to scale, or a scale measurement up to real life. |
When to use it
Building a scale model from real dimensions
A model car at 1:24 scale, or a model railway locomotive at 1:87 (HO scale), needs each real-world dimension divided by the ratio to find the size to build or print at.
Reading a measurement off an architectural or engineering drawing
A plan drawn at 1:100 shows a wall as 4 cm long on paper; multiplying that by 100 gives the real 4 m wall it represents.
Working out how to calculate scale for a photo or map enlargement
A map or diagram scaled at 1:25,000 converts a measured distance on the page directly to the real-world distance it stands for, without needing a separate ruler-based estimate.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Scale calculator: real-world to scale, across common model and drawing scales
A fixed 6-unit real-world measurement, converted to scale size across several common ratios used for models and drawings.
| Scale ratio | Scale value | As a percentage |
|---|---|---|
| 1:12 | 0.5000 | 8.33% |
| 1:24 | 0.2500 | 4.17% |
| 1:43 | 0.1395 | 2.33% |
| 1:87 | 0.0690 | 1.15% |
| 1:100 | 0.0600 | 1.00% |
| 1:160 | 0.0375 | 0.625% |
Scale calculator: scale to real-world, for a 1:100 architectural drawing
A range of measurements taken directly off a 1:100 scale architectural drawing, converted to their real-world size.
| Scale value (drawing units) | Real-world value | As a percentage |
|---|---|---|
| 1 | 100.000 | 1.00% |
| 2 | 200.000 | 1.00% |
| 4 | 400.000 | 1.00% |
| 6 | 600.000 | 1.00% |
| 10 | 1,000.00 | 1.00% |
| 20 | 2,000.00 | 1.00% |
Questions
How do I calculate scale from real to model size?
Divide the real-world measurement by the scale ratio. For a 1:24 scale and a 6 m real length, the model size is 6 ÷ 24 = 0.25 m, or 25 cm.
How do I convert a scale measurement back to real life?
Multiply the scale measurement by the ratio. A 4 cm measurement on a 1:100 drawing is 4 × 100 = 400 cm, or 4 m, in real life.
What common scale ratios are used for models?
1:24 and 1:18 are common for model cars, 1:87 (HO) and 1:160 (N) for model railways, and 1:72 or 1:48 for aircraft and military models. Architectural drawings commonly use 1:50 or 1:100.
Does the unit matter for this scale converter?
The ratio itself has no unit, so any unit works as long as both the real-world and scale values use the same one, whether that is millimetres, centimetres, inches or feet.
For converting a length between metric and imperial units directly, see the length converter. For a photo or image resized by a percentage or target dimension, see the aspect ratio calculator.