Scale and gauge
Model Railroad Scale and Gauge Chart
Quick answer
HO scale is 1:87.1 with a 16.5 mm (0.650 in) gauge, sitting between N (1:160, 9 mm) and O (1:48, 31.75 mm). Scale is the size ratio to the real train; gauge is the distance between the rails. G scale breaks the pattern entirely, running ratios from 1:32 to 1:20.3 on the same 45 mm track.
Scale and gauge get used interchangeably in casual conversation and they are not the same measurement. Scale is a ratio, how much smaller the model is than the real locomotive it represents. Gauge is a distance, the spacing between the two rails the model rolls on. Two models can share a gauge and run different scales, which is exactly what happens in G scale, and the confusion that causes is common enough to be worth a chart of its own.
The figures below come from NMRA Standard S-1.2, the document that defines scale and gauge for the hobby in the United States. Every calculator on this site that converts a prototype length into a model length, including the scale converter, uses these exact ratios.
On this page
What is the difference between scale and gauge?
Scale describes size. HO scale is 1:87.1, meaning a real 87.1 foot object becomes a 1 foot model. Gauge describes spacing. HO gauge is 16.5 mm between the railheads, which is the correct spacing for standard-gauge prototype track (4 feet 8.5 inches) scaled down at 1:87.1. The two numbers move together for every scale except one. A caliper or a track gauge tool measures gauge directly off the rails; scale has to be worked out from a model's length against the real equipment it copies.
What are the exact ratio and gauge for every NMRA scale?
These are the seven scales the NMRA defines a standard for, ordered smallest to largest. Ratio and gauge in millimeters are the NMRA reference figures; gauge in inches is the same number converted for a US toolbox.
HO (1:87.1, 16.5 mm) and N (1:160, 9 mm) are the two most common scale-and-gauge pairs sold in the hobby today.
| Scale | Ratio | Gauge (mm) | Gauge (in) | Notes |
|---|---|---|---|---|
| Z | 1:220 | 6.5 mm | 0.256 in | Smallest common commercial scale |
| N | 1:160 | 9 mm | 0.354 in | Most compact scale with wide model selection |
| TT | 1:120 | 12 mm | 0.472 in | Between N and HO, small US catalog |
| HO | 1:87.1 | 16.5 mm | 0.650 in | The most common scale in North America |
| S | 1:64 | 22.42 mm | 0.883 in | American Flyer heritage scale |
| O | 1:48 | 31.75 mm | 1.250 in | Toy-train heritage, also serious scale modeling |
| G | 1:22.5 (varies 1:32 to 1:20.3) | 45 mm | 1.772 in | One gauge, several different ratios |
Ratios and gauges are the NMRA S-1.2 reference values. HO is frequently rounded to "1:87" in casual use; 1:87.1 is the figure the standard actually specifies.
Why does G scale break the scale-and-gauge pattern?
Every other scale on this chart pairs one ratio with one gauge. G scale does not. "G" covers models built at anywhere from 1:32 down to 1:20.3, and all of them run on the same 45 mm track because that track represents a range of real narrow-gauge and standard-gauge prototypes at different sizes. A 1:32 G scale model represents standard-gauge equipment; a 1:20.3 G scale model represents narrow-gauge equipment, and the two are visibly different sizes sitting on the identical rail spacing. This is normal for G and not a manufacturing inconsistency, but it means two G scale trains from different product lines will rarely look proportional coupled together.
What radius does a starter set oval actually ship with?
The oval packed into a beginner train set is chosen for the box it has to fit in, not for what the locomotive needs. These are the classic set-track minimum radii for the three most common starter scales.
A beginner set ships with roughly 9.75 in radius in N, 18 in in HO, and 31 in in O, and none of those numbers come from an NMRA standard.
| Scale | Classic set-track minimum radius |
|---|---|
| N | 9.75 in |
| HO | 18 in |
| O | 31 in |
This is manufacturer set-track convention, not an NMRA figure. NMRA RP-11 covers recommended practice for curvature and easements; a manufacturer's stated minimum for a specific locomotive always overrides a rule of thumb. See the full breakdown on the minimum radius chart.
How do I convert a real locomotive length into a model length?
The formula is the same for every scale: model length equals the prototype length in feet, multiplied by 12 to get inches, divided by the scale ratio. An 85 foot passenger car in HO (1:87.1) works out to 85 x 12 / 87.1, which is 11.71 in. The same car in N (1:160) is 85 x 12 / 160, or 6.38 in. Run that math for the whole roster on the scale converter calculator instead of doing it by hand car by car, then check the result against the minimum radius chart before laying benchwork, since model length is the number that decides how sharp a curve a car can take.
Sources
- NMRA Standard S-1.2, Standards for scale and gauge
Frequently asked questions
Is HO scale exactly 1:87 or 1:87.1?
NMRA S-1.2 specifies HO at 1:87.1. "1:87" is a common shorthand in conversation and in older hobby writing, and the difference is small enough that it never affects whether a locomotive fits a curve or a car couples correctly. Specification sheets and precision work should use 1:87.1.
What gauge is N scale track?
N scale gauge is 9 mm, or 0.354 in, between the railheads. That number is fixed regardless of manufacturer; Kato Unitrack and Atlas N flex track both hold 9 mm gauge because both are built to the same NMRA S-1.2 standard, even though the rail profile and tie spacing look different between brands.
Can narrow-gauge O scale equipment run on standard O gauge track?
No. Standard O gauge track is 31.75 mm (1.25 in) wide, built for 1:48 standard-gauge prototypes. Narrow-gauge O variants exist in the hobby and run narrower rail spacing at the same 1:48 body scale, so they need their own track and cannot share an oval with standard O equipment.
Why do two HO locomotives from different brands look slightly different sizes?
If both are built to 1:87.1 against the correct real-world prototype dimensions, they should match closely. Small visible differences usually come from selective compression, a common modeling shortcut where a manufacturer shortens a long prototype slightly so the finished model fits typical curves and shelf depths, not from an actual scale error.
Does a larger scale ratio number mean a bigger train?
No, it is the opposite. The ratio is a division: 1:220 (Z) divides a real train by 220, producing the smallest model on this chart. 1:22.5 (G) divides by roughly 22.5, producing the largest. A bigger second number in the ratio always means a smaller model.
Is G scale the same thing as G gauge?
They get used to mean the same product line, but strictly, 45 mm is the gauge and "G" is a loose label covering several different scale ratios that all run on that gauge. Two trains both correctly called "G scale" are not guaranteed to share a body scale, only a rail spacing.
Researched, not professional advice. This page is compiled from published manufacturer specifications, instruction manuals, published NMRA standards and recommended practices, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are hobby convention rather than a standard, and they are labelled that way wherever they appear. Always check a radius, a clearance or a grade against your own equipment before you cut wood or lay rail, because manufacturers vary and so does what your specific models will tolerate. Any permanent mains wiring in a layout room is work for a licensed electrician to your local code.