Universal Model Railroad Scale Converter
Convert real-world measurements to any model railroad scale — HO, N, O, Z, G, S, TT, or enter a custom ratio. Handles any direction, any unit.
This universal converter handles all major model railroad scales and any custom ratio. Select a scale from the dropdown, enter your measurement, and instantly see the model equivalent — or reverse it to find prototype size from a model measurement. For architectural and diorama scales, enter a custom ratio like 1:200 or 1:35.
Enter Real-World Measurement
HO Scale (1:87.1) Equivalent
Scale Model Size
139.98mm
| In millimeters | 139.977 mm |
| In centimeters | 13.9977 cm |
| In inches | 5.5109" |
| Input converted to mm | 12192.00 mm |
| Scale factor applied | 1:87.1 |
Model Railroad Scale Reference Table
Quick comparison of all major scales — ratio, track gauge, and how 1 prototype foot converts. All values from verified NMRA/NEM standards.
| Scale | Ratio | Track Gauge | 1 Foot = |
|---|---|---|---|
| Z | 1:220 | 6.5mm | 1.39mm / 0.055" |
| N | 1:160 | 9mm | 1.91mm / 0.075" |
| TT | 1:120 | 12mm | 2.54mm / 0.100" |
| HO | 1:87.1 | 16.5mm | 3.50mm / 0.138" |
| OO | 1:76.2 | 16.5mm | 4.00mm / 0.157" |
| S | 1:64 | 22.43mm | 4.76mm / 0.188" |
| O | 1:48 | 31.75mm | 6.35mm / 0.250" |
| G / LGB | 1:22.5 | 45mm | 13.55mm / 0.533" |
How Model Railroad Scale Conversion Works
Scale conversion is simple division and multiplication. Given a ratio of 1:X:
The Two Formulas
Real World → Scale Model: Scale size = Real size ÷ X
Scale Model → Real World: Real size = Model size × X
For example, at HO scale (1:87.1): a 40-foot boxcar's over-coupler length of 528 inches becomes 528 ÷ 87.1 = 6.062 inches = 153.9mm. To verify: 153.9mm × 87.1 = 13,404mm = 44 feet.
Converting Between Scales
To convert a model measurement from Scale A to Scale B, use two steps:
- Prototype size = Model size (Scale A) × Scale A ratio
- Model size (Scale B) = Prototype size ÷ Scale B ratio
Example: Converting a 140mm HO building to N scale: 140mm × 87.1 = 12,194mm prototype → 12,194mm ÷ 160 = 76.2mm N scale.
Scale vs. Gauge: Understanding the Difference
Scale is the size ratio — how large the model is compared to the prototype. Gauge is the track measurement — the distance between the rails. These are related but not the same thing, and the confusion between them is the source of most scale mix-ups in the hobby.
The clearest example: HO scale (1:87.1) and OO scale (1:76.2) both use 16.5mm gauge track. HO is the correct ratio for 4'8.5" prototype gauge at 1:87.1. OO is technically running on under-gauge track — the correct OO gauge would be about 18.8mm, but manufacturers standardized on 16.5mm for compatibility.
Similarly, "O gauge" (Lionel three-rail) and "O scale" (1:48 two-rail) are not the same. Lionel O gauge runs at roughly 1:43.5 to 1:48 depending on the product. Both use 31.75mm (1.25") NMRA-standard track, while Proto:48 fine-scale uses 29.9mm (true 1:48 gauge).
For this calculator, the scale ratio controls the size math. Track gauge is a separate specification — check NMRA standards or NEM standards for your specific scale's track requirements.
Frequently Asked Questions
What is a model railroad scale ratio?
A scale ratio describes how much smaller a model is compared to the real thing. A ratio of 1:87.1 (HO scale) means that 1 unit on the model represents 87.1 of the same units in real life. Divide any real-world measurement by the scale ratio to get the model size. Multiply any model measurement by the ratio to get the real-world size.
How do I convert between two different scales?
To convert a model measurement from one scale to another, first convert it to real-world size (multiply by the original scale ratio), then convert to the target scale (divide by the new ratio). For example, a 153.9mm HO scale boxcar (40-ft PS-1 at 1:87.1): 153.9mm × 87.1 = 13,404mm prototype, then 13,404mm ÷ 160 = 83.8mm in N scale. This calculator handles all of this automatically.
Which model railroad scale should I choose?
The right scale depends on your space, budget, and modeling goals. HO (1:87.1) offers the widest selection of equipment and accessories and fits most average rooms. N scale (1:160) is best for limited space or ambitious geography. O scale (1:48) rewards modelers who want large, highly detailed equipment. Z scale (1:220) is for apartment dwellers or truly minimal footprints. G scale (1:22.5) thrives in garden railways.
What is the difference between scale and gauge?
Scale refers to the size ratio (how large the model is relative to prototype). Gauge refers to the track measurement — the distance between the rails. Confusingly, different scales sometimes share track gauges: HO (1:87.1) and OO (1:76.2) both use 16.5mm track. Similarly, "O gauge" (Lionel three-rail) runs at a different ratio than true "O scale" (1:48 two-rail).
What is a 1:200 scale converter?
A 1:200 scale is used mainly in architectural models. Enter 200 as your custom scale ratio in this calculator to convert any real-world dimension. At 1:200, 1 foot becomes 1.524mm. This scale is common for presenting building designs where the model needs to show a city block or larger context at tabletop size.
How do I use a custom scale ratio?
Select "Custom scale..." from the scale dropdown, then enter any ratio number. For example, enter 87.1 for HO (NMRA standard), 32 for 1:32, or 200 for architectural 1:200 scale. The calculator divides your real-world measurement by this number to get the model size.
What scale is best for large layouts with long trains?
N scale (1:160) allows the longest trains and broadest geographic scope in a given room. A 100-car coal drag that would need a 55-foot mainline run in HO fits in about 30 feet in N scale. For those who want operational realism with scale-length trains, N scale is the practical choice in anything smaller than a dedicated train room.
What is TT scale?
TT scale (1:120) sits between HO and N. "TT" stands for Table Top. It was popular in the 1950s-60s in the US and remains significant in Eastern Europe and Russia. American TT scale uses 12mm track gauge. It is a niche scale in North America with limited equipment availability, but European manufacturers like PIKO and Tillig offer extensive TT product lines.
Dedicated Scale Calculators
Each scale-specific page includes a full reference table, FAQ, and layout planning data:
- HO Scale Calculator (1:87.1) — Most popular scale worldwide
- N Scale Calculator (1:160) — Best for space-limited layouts
- O Scale Calculator (1:48) — Classic tinplate and fine-scale O
- Scale Detection Calculator — Identify unknown scales from measurements
- Speed Scale Calculator — Find out how fast your model is running at scale speed
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Shop Now →Scale ratios per NMRA S-1.1 (HO 1:87.1) and NMRA S-1.2. Track gauge data per NMRA S-4.2 and NEM 010. OO scale per BRMSB standards. G/LGB scale per NEM 010. All values from verified scaleConstants.js.
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Try Our Other Calculators
HO Scale Calculator (1:87)
Dedicated HO scale converter with full reference tables.
N Scale Calculator (1:160)
Dedicated N scale converter with layout planning data.
O Scale Calculator (1:48)
Dedicated O scale converter for Lionel and two-rail modelers.
Scale Detection Calculator
Identify your model's scale from measurements.