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Free model railroad scale calculator for linear prototype-to-layout and layout-to-prototype conversions. Pick Z, N, TT, HO, S, O (US or UK), or G presets—or define your own 1:N—then enter length in feet, inches, meters, centimeters, or millimeters for benchwork, scratch structures, and consist planning.
Last updated: April 17, 2026
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HO — 1:87 · Model size
0.140138 m
140.1379 mm · 5.5172 in · 0.45977 ft
Track gauge, wheel standards, and era-specific O scales differ by region. Use manufacturer drawings for turnout geometry; this tool is linear length only.
Tip:
For a 40 ft boxcar in HO (1:87), choose prototype → model, enter 40 ft, scale HO—output is the scaled length along the car body axis only (not track centerline offset).
Ratios
1:220 … 1:22.5
Museums & narrow gauge
Flexible N
Input
One dimension at a time
Model → prototype
Ruler audits
Scope
Length along one axis
CNC & laser
Sub-mm output
40 ft prototype freight car in HO (1:87), prototype → model:
5.5172 in
≈ 140.1379 mm · true scale length along the body
Model railroading scales lengths by a fixed ratio between the real world and the layout. If the ratio is 1:N, every N units of prototype length equals one unit on the model. The calculator converts your input length to millimeters internally, divides or multiplies by N depending on direction, then shows millimeters, inches, feet, and meters so you can copy the number into CAD, a chop saw fence, or a caliper without hand-converting units.
model_length = prototype_length ÷ Nprototype_length = model_length × NLengths are converted to millimeters first so ft, in, m, cm, and mm inputs stay consistent.
Also explore our RC car gear ratio calculator for surface RC drivetrain math in another hobby aisle.
Get Custom Calculator for Your PlatformShare it with your modular group or scenery build night thread.
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