Scale Model Converter
Convert between real-world size and model size for any scale ratio — 1:87 HO, 1:160 N, 1:48 O, 1:35 military, 1:100 architecture and more.
- Model size
- 20 cm
- Scale ratio
- 1:87
Calculator
About this calculator
A scale ratio of 1:N means a model is N times smaller than the real subject it represents. This converter works in both directions: enter a real-world length and a ratio to find the model size, or enter a model measurement to recover the real size. It covers any ratio you type, plus the named hobby and industry scales — 1:87 (HO) and 1:160 (N) model railways, 1:48 (O), 1:35 military armour, 1:24 large scale, 1:100 architecture, and 1:1000 maps — and converts the answer into millimetres, centimetres, metres, inches or feet.
How to read your results
The headline figure is the converted length in your chosen output unit. The result panel also echoes the ratio you used and, when the denominator matches a well-known scale, names it (for example 1:87 → HO). The to-scale comparison bar shows the real length next to the model length so you can see the proportion at a glance. Because the scale factor is a pure division or multiplication, the conversion is exact — only the unit step rounds, and that uses the same exact length factors throughout.
How it's calculated
The conversion has two stages. First the scale factor is applied in the input unit: real-to-model divides the entered length by the ratio N, and model-to-real multiplies it by N. Because dividing or multiplying by a dimensionless ratio does not depend on the unit, this step is unit-agnostic. Second, the scaled length is converted to the requested output unit using the exact international length factors (1 in = 0.0254 m, 1 ft = 0.3048 m, with mm, cm and m as decimal sub-multiples of the metre). No intermediate rounding occurs between the two stages, so the displayed figure is the full-precision result rounded only for presentation. Named scales are matched by looking the ratio denominator up in a small table (87 → HO, 160 → N, 48 → O, 35 → military, 24 → large, 100 → architecture, 1000 → map); an unrecognised ratio simply carries no name.
Worked example
A real locomotive measures 1740 cm long and you are modelling it in HO scale (1:87). Choose "Real → Model", enter 1740, set the input unit to centimetres, the ratio to 87, and the output unit to centimetres.
The model is 1740 ÷ 87 = 20 cm long. The panel labels the ratio as HO scale. Switching the output unit to inches reports the same model as about 7.87 in instead.
Frequently asked questions
What does a scale like 1:87 actually mean?
It means one unit of length on the model equals 87 of the same units on the real subject — the model is 87 times smaller. So a 1740 cm real length becomes 1740 ÷ 87 = 20 cm on the model. The ratio is dimensionless, so it works identically whether you measure in millimetres, inches or feet.
Which named scales does the converter recognise?
It flags the common ones: 1:87 (HO) and 1:160 (N) for model railways, 1:48 (O), 1:35 for military armour kits, 1:24 for large/G scale, 1:100 for architecture, and 1:1000 for maps. Any other ratio still converts correctly — it simply is not given a name. HO is standardised by the NMRA at 1:87.1, which this tool treats as 1:87 for everyday use.
How do I convert a model measurement back to real size?
Pick "Model → Real" and the converter multiplies by the ratio instead of dividing. A 50 mm part at 1:35 represents 50 × 35 = 1750 mm = 1.75 m on the real vehicle. Real-to-model and model-to-real are exact inverses of each other for the same ratio.
Can I mix input and output units, like feet in and inches out?
Yes. The scale factor is applied first, then the length is converted between units using exact factors (1 in = 0.0254 m, 1 ft = 0.3048 m). For example, 1 ft of real prototype at 1:12 becomes exactly 1 in on the model, because 1 ft is 12 inches and 12 ÷ 12 = 1.
Is HO really 1:87 or 1:87.1?
The NMRA standard defines HO precisely as 1:87.1. The difference from 1:87 is about one tenth of one percent, far below the tolerance of most measuring and modelling work, so this converter uses the round 1:87 that hobbyists quote. If you need the exact standard figure, enter 87.1 as the ratio.
Sources
Reviewed by the YouCalc Team · Last reviewed
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