Scale Model Calculator - Real Size to Model Size

About the Scale Model Calculator

Model cars, aircraft, ships, railways, dolls' houses, architectural models and wargaming figures are all built to a scale, written as a ratio such as 1:24 or 1:72. The ratio says how many times smaller the model is than the real thing: at 1:24, every centimetre on the model stands for 24 centimetres in real life. Modellers use scale constantly — to scratch-build parts, to check whether a kit will fit a display case, to plan a layout, or to work out the real size of something from a model.

This scale model calculator converts a real size to its model size, or a model size back to full size, at any scale. It shows the result in millimetres, centimetres and inches, or metres and feet, lists the size at common scales from 1:12 to 1:144, and shows how much areas and volumes shrink.

How to Use the Scale Model Calculator

Choose Real size to model or Model size to real.

Enter a length and choose its unit: mm, cm, m, inches or feet. For a real object, such as a car's 4.5 metres, metres or feet are usually easiest; for a model, millimetres or inches.

Enter the scale as the number after "1:" — 24 for 1:24, 87 for HO scale.

The calculator divides a real length by the scale to give the model size, or multiplies a model length by the scale to give the real size.

How Scale Conversion Is Worked Out

  model size  =  real size ÷ N           (scale 1:N)
  real size   =  model size × N
  scale N     =  real size ÷ model size  (same units)
  area ratio  =  N²          volume ratio  =  N³

Step-by-Step Example

A car 4.5 metres long, modelled at 1:24.

  Real length:   4.5 m = 4,500 mm
  Model length:  4,500 ÷ 24 = 187.5 mm
  In cm and in:  18.75 cm, 7.38 in

The model is 187.5 mm long. At other scales the same car would be:

  1:12    375 mm         1:72    62.5 mm
  1:24    187.5 mm       1:87    51.7 mm
  1:43    104.7 mm       1:144   31.3 mm

Working back from a model: a 200 mm long HO scale model at 1:87 represents 200 × 87 = 17,400 mm, which is 17.4 metres or about 57.1 feet — the length of a typical railway coach.

Finding the Scale of a Model

If you know the real size of the subject, divide it by the model's size in the same units. A 4.5 metre car that becomes a 187.5 mm model gives 4,500 ÷ 187.5 = 24, so the model is 1:24. Kits and figures often state their scale, but older models, toys and some ranges are made "to fit the box" and may not match exactly, so measuring is the only way to be sure.

Common Scales

Different hobbies have settled on their own standard scales.

  1:6 to 1:12    large figures, dolls' houses (1:12)
  1:18, 1:24     die-cast and plastic car kits
  1:35           military vehicles and figures
  1:43           collectable die-cast cars, O gauge railways (about 1:43–1:48)
  1:48, 1:72     aircraft kits
  1:76, 1:87     OO (Britain) and HO railways
  1:144, 1:160   airliners; N gauge railways
  1:350, 1:700   ships

Buildings, trees and scenery for a layout are usually bought or built to match the railway scale, so everything on the layout looks in proportion.

Areas, Volumes and Weight

Scale applies to every length, so areas shrink by the scale squared and volumes by the scale cubed. At 1:24, a model has 1/576 of the real surface area and 1/13,824 of the volume. This is why a solid model is so light compared with the original, and why paint, texture and details need to be scaled down too: a real car's paint gloss or a brick's mortar lines, reproduced at full thickness on a small model, look far too heavy.

It also explains why models do not always behave like the real thing. A model aircraft or ship moves through air and water that are not scaled down, so the forces on it are quite different — a problem engineers handle carefully when testing scale models in wind tunnels and towing tanks.

Scaling for Architecture and Design

Architects and designers use the same arithmetic in drawings and presentation models. Common scales are 1:50 and 1:100 for building plans, 1:200 and 1:500 for site layouts, and 1:1,250 for town plans. A 10 metre wall at 1:100 is 100 mm on the drawing; at 1:50 it is 200 mm. Metric plan scales are chosen so the arithmetic stays simple: at 1:100, every metre becomes a centimetre.

Measuring Accurately

Small errors grow when multiplied by the scale. A 1 mm mistake measuring an HO model becomes 87 mm, nearly 9 cm, at full size, so measure models with a steel rule or calipers rather than a tape. When scaling up from a model, measure the longest dimension you can, since the percentage error is smaller on a long measurement than a short one.

Understanding Your Result

The headline is the converted size — the model size in mm, cm and inches, or the real size in metres and feet.

The other units line gives the same result in the remaining units.

The scale line shows the model as a percentage of full size.

The common scales line lists the size at popular scales, handy for choosing a kit or checking a display case.

The area and volume line shows how much surfaces and volumes shrink.

When Should You Use This Calculator?

Use it when scratch-building parts or scenery to match a kit.

Use it to check whether a model will fit a shelf, cabinet or layout before buying.

Use it to find the real size of something shown in a model or on a scaled drawing.

Use it when mixing items from different ranges, to see whether they share a scale.

Common Mistakes

Mixing units. Convert both sizes to the same unit before dividing.

Multiplying instead of dividing. Model size is real size divided by the scale.

Assuming similar scales match. OO at 1:76 and HO at 1:87 look close but differ by about 14 percent.

Scaling only length. Surface detail, weight and volume change far more.

Trusting unlabelled toys. Many are made to fit a package, not to a strict scale.