The art of scale modeling is recreating life-like objects in a smaller scale, or size. As such most modeling efforts, including railroad modeling are based on a ’scale’ model such as 1:10 in which every 10 inches equals 1 foot for example. In additional a ‘gauge’ or distance between tracks needs to be specified, and normally is reflected by the scale name but not always. In railroad modeling the most common standards that are used include:
HO - 1:87.1
N - 1:148 to 1:160
O - 1:43.5
Z - 1:122
The ‘names’ such as HO (which literally stands for Half O scale) or N that are given to a particular scale is being built also give an indication for the track gauge being used. The actual scale associated with any given name may vary depending on the common gauge being modeled although in ‘narrow’ gauge the name usually also reflects the gauge. Standard gauge is 1435 mm or 4′ 8.5” in real life and is scaled down appropriately and rounded down or up depending on the scale model definitions and prototyping being used.
COMMON GAUGES
The following is a partial list of gauges for railways in various parts of the world:
| Standard | Metric | Country of Origin | |
| Broad | 7′ 0-1/4" | 2140mm | Britain |
| 5′ 6" | 1676mm | Spain, Portugal, India, China | |
| 5′ 3" | 1600mm | Ireland, Australia, Brazil | |
| 5′ 0" | 1524mm | Russia, Finland | |
| Standard | 4′ 8-1/2" | 1435mm | Australia, Austria, Belgium, Bolivia, Bulgaria, China, Czechoslovakia, Canada, Denmark, Egypt, France, Germany, Britain, Holland, Hungary, Italy, Japan, Yugoslavia, Mongolia, Norway, Poland, Peru, Rumania, Sweden, Switzerland, Turkey, USA |
| BNarrow | 3′ 6" | 1067mm | Australia, New Zealand, South Africa, Japan, Spain, East Africa, Canada, Ecuador, Java |
| 3′ 3-3/8" | 1000mm | Germany, Switzerland, Spain, India, Burma, Kenya, Malaysia, Thailand, Uganda, Chile | |
| 3′ 1-1/2" | 950mm | Italy | |
| 3′ 0" | 914mm | Isle of Man, Ireland, USA, Canada (WP&Y) | |
| 2′ 7-1/2" | 800mm | Wales, Switzerland | |
| 2′ 6" | 760mm | Australia, Wales, Austria, India | |
| 2′ 5-1/2" | 750mm | Germany, Egypt, Indonesia | |
| 2′ 3" | 686mm | Wales | |
| 2′ 0" | 610mm | Australia, Fiji | |
| 1′ 11-5/8" | 600mm | Wales, Greece | |
| 1′ 3" | 381mm | United Kingdom, New Zealand |
STANDARD GAUGES
The following are scales that are often used by modelers:
| Scale Name | Ratio | Gauge |
| 4 | 1:15 |
3 in. |
| 3 | 1:18 | 2-1/2 in. |
| 2 | 1:24 | 2 in. |
| 1 | 1:32 | 45mm / 1-3/4 in. |
| O | 1:43.5 | 32mm |
| Scale 7 | 1:43.5 | 33mm |
| O | 1:45 | 32mm |
| O17 | 1:45 | 1-1/4 in. |
| O | 1:48 | 1-1/4 in. |
| Q | 1:48 | 1-3/16 in. |
| Proto 48 | 1:48 | 29.89mm |
| S or H1 | 1:64 | 7/8 in. |
| OO | 1:76.2 | 19mm |
| P4 / S4 | 1:76.2 | 18.83mm |
| EEM | 1:76.2 | 18.8mm |
| EMF | 1:76.2 | 18.2mm |
| EM | 1:76.2 | 18mm |
| HO | 1:87.1 | 16.5mm |
| QO | 1:96 | 3/5 in. |
| TT | 1:100 | 13 mm |
| Scale 3 | 1:102 | 14.12mm |
| TT | 1:120 | 12mm |
| HOO | 1:145 | 10mm |
| OOO | 1:152 | 9.5mm |
| N | 1:160 | 9mm |
| K | 1:174 | 8mm |
| QOO | 1:174 | 7.62mm |
| Z | 1:220 | 6mm |
| X | 1:304.8 | 4.5mm |
NARROW GAUGES
The following are the narrow gauge scales that are often used by modelers:
| Scale Name | Ratio | Gauge |
| SM45 | 1:19 | 45mm |
| SM32 | 1:19 | 32mm |
| 2N3 | 1:24 | 45mm |
| 2M | 1:22.5 | 45mm |
| G | 1:22 or 1:29 | 45mm |
| GN15 | 1:22 | 16.5mm |
| OM | 1:48 | 22.5mm |
| ON3 | 1:48 | 19mm |
| ON 2-1/2 | 1:48 | 16.5mm |
| OE | 1:48 | 16.5mm |
| O16.5 | 1:43.5 | 16.5mm |
| On2 | 1:48 | 14mm |
| Sn3-1/2 | 1:64 | 16.5mm |
| Sn3 | 1:64 | 14mm |
| Sn2 | 1:64 | 9mm |
| OOn3 or OO12 | 1:76.2 | 12mm |
| HOM | 1:87.1 | 12mm |
| HON3 | 1:87.1 | 10.5mm |
| HoE | 1:87.1 | 9mm |
| HON2-1/2 or HO30 | 1:87.1 | 9mm |
| HON2 | 1:87.1 | 6mm |
| TTN3 | 1:101.1 | 9mm |
| NM or NN3 | 1:160 | 6mm |
The size of models varies from around 700 mm (or 28 inches) tall for the largest ‘usable’ size models of a 1:8 size, down to matchbox size in Z -scale. Typical HO engines are about 50 mm (2") tall, by 100 mm to 300 mm (4" to 12") in length.
The five most popular scales are: G, O, HO (also known in Britain as OO), TT and N. The Z scale is also growing in popularity and the Nn3 scale and Hon3 Scale are popular ‘narrow’ gauge scales.
