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Model Train Scale Converter

Convert a length between the named model railway scales — Z, N, TT, HO, OO, S, O and G — or work out the real-world size it represents. Press Convert.

Written and reviewed by SM Arijul Islam · Last reviewed 29 October 2026 · Editorial standards · Report an error

About the model train scale converter

Railway modelling scales are written as ratios, but each one also has a letter name. HO is 1:87.1, N is 1:160 and Z is 1:220, so a 100 mm coach in HO represents 8.71 metres of real rolling stock. Choose 1:1 real size as the target to see the full-size dimension.

Handy check: to move from a larger scale to a smaller one, multiply by the old ratio and divide by the new one. A 100 mm HO wagon becomes 100 × 87.1 ÷ 160 = 54.4 mm in N.

Scale and track gauge reference

Model railway scale ratios with standard track gauge
Name	Ratio	Track gauge	Typical height of an adult figure
Z	1:220	6.5 mm	8 mm
N	1:160 (1:150 in Japan)	9 mm	10.5 mm
TT	1:120	12 mm	15 mm
OO	1:76	16.5 mm	24 mm
HO	1:87.1	16.5 mm	20 mm
S	1:64	22.5 mm	28 mm
O	1:48 (US), 1:43.5 (EU)	32 mm	38 mm
G	1:22.5 to 1:29	45 mm	80 mm

North American O uses the same 32 mm track as European O but a different scale ratio, which is why O models from different makers do not always sit comfortably side by side.

Worked examples

A 26.4 m real coach in HO (1:87.1) is 26400 ÷ 87.1 = 303 mm long.
100 mm of N scale track represents 100 × 160 = 16,000 mm, or 16 m.
Converting a 250 mm HO building to N: 250 × 87.1 ÷ 160 = 136 mm.
A 1.8 m person in O scale (1:48) stands 37.5 mm tall.

Gauge, scale and clearance are three different things

Model railway scale is a ratio, but the track is a fixed width, and the two do not always agree. HO and OO both run on 16.5 mm track, yet HO is 1:87 and OO is 1:76 — so OO models are slightly oversized for the gauge. British manufacturers made that compromise a century ago and never abandoned it, which is why an OO locomotive looks subtly wider than it should beside a true 1:87 scale figure.

Common model railway scales and their track gauges
Scale	Ratio	Track gauge
Z	1:220	6.5 mm
N	1:160	9 mm
TT	1:120	12 mm
HO / OO	1:87 / 1:76	16.5 mm
S	1:64	22.5 mm
O	1:48 / 1:43	32 mm
G	1:22.5	45 mm

Loading gauge is the third number, and it is the one that causes trouble on a layout. A model can be perfectly correct to scale and still foul a platform edge, a tunnel portal or an overhead catwalk if the scenery was built to a different clearance. Real railways leave generous space above and beside the track for exactly this reason. Work out the real clearance in millimetres at your scale before building the tunnel, not after.

Narrow-gauge models add one more layer: a metre-gauge prototype modelled at 1:87 runs on 12 mm track (HOm), not 16.5 mm, because the track is scaled to the narrower real gauge. If you mix stock from different manufacturers, check the gauge as well as the ratio.

Frequently asked questions

Why is HO written as 1:87.1 and not 1:87?

HO evolved from OO and the 16.5 mm gauge. Working back from that gauge gives a ratio of roughly 1:87.1, and the decimal has stuck. For layout maths, 1:87 is close enough.

Are OO and HO the same size?

No. Both run on 16.5 mm track, but OO bodies are built to 1:76 while HO bodies are 1:87.1, so OO models are noticeably larger.

What is narrow gauge, such as HOn3?

Narrow gauge describes the real railway's gauge rather than the model's scale. HOn3 is HO scale (1:87.1) running on 10.5 mm track to represent a 3 foot prototype gauge.

Which scale should a beginner choose?

HO if space is available, because the choice of stock, track and buildings is the widest. N fits a believable layout into a small room but is fiddlier to handle and detail.

Common mistakes to avoid

Assuming gauge and scale are the same thing. Gauge is the track width; scale is the ratio of the model to the real object.
Buying O scale figures from the wrong continent — 1:48 and 1:43.5 are about 10% apart.
Scaling scenery by eye. A doorway is around 2 m and a carriage about 26 m; check the math on those two and everything else falls into line.
Mixing G scale brands freely. The letter covers a range from 1:22.5 to 1:29.

Sources and standards

Model railway scales are ratios, not gauges: HO is 1:87.1 and N is 1:160, while the track gauge stays close to 16.5 mm and 9 mm respectively. The National Model Railroad Association publishes the scale and gauge standards this page follows, including the difference between a scale ratio and a track gauge.