Model railroad minimum radius calculator.

How the radius is calculated

This calculator reads the NMRA's Recommended Practice RP-11, Curvature and Rolling Stock, revised March 2018 (read 26 October 2026). RP-11 sorts equipment into classes E to Q. Each class has a degree of curvature, a prototype radius from 100 feet (class E) to 320 feet (class Q), a model radius for each scale, and limits on the equipment that class can run: the rigid wheelbase of a locomotive, the total length of a diesel or electric on swivel trucks, the longest passenger car, and the longest freight car with body-mounted or truck-mounted couplers.

For each item you enter, the calculator picks the first class, in order from E, whose limit covers it. The class that governs the layout is the highest of those. The model radius is then read from RP-11's own table for your scale, so the figures are the NMRA's published numbers, not a recalculation. For HO those are 14 inches (356 mm) for class E up to 44 inches (1118 mm) for class Q.

RP-11 Note 2 raises an articulated locomotive four classes above the class set by its longer set of drivers. Note 7 lets a locomotive with blind centre drivers and a rigid wheelbase of 15 feet or more drop one class. Both are built in, and both only apply to the rigid-frame choice. RP-11 is a recommended practice, not a rule: manufacturers publish their own minimums, and entry-level models often run on tighter curves than the class says.

Worked examples with real numbers

• HO six-axle diesel and long passenger cars. A 70-foot locomotive on six-wheel trucks is class N (the limit for that column is 70 feet). An 85-foot passenger car is class O. The layout needs class O: 36 inches (914 mm) in HO.
• HO rigid-frame steam locomotive, 15-foot rigid wheelbase. A 15-foot wheelbase is class I, 22 inches (559 mm) in HO. With a blind centre driver, Note 7 allows class H, 20 inches (508 mm).
• An articulated steam locomotive. RP-11 gives the example of a Pacific with 69-inch drivers as class H and a Challenger with the same drivers as class L. For a rigid set of drivers on an 11-foot wheelbase (class F), the articulated rule gives class J, 23 inches (584 mm) in HO.
• The same classes in N. Class O is 19.625 inches (498 mm) and class E is 7.5 inches (191 mm).

HO radius by RP-11 class

What this calculator leaves out

It uses the locomotive-hauled table only. RP-11 has a separate table for streetcars and interurbans, which this tool does not cover. It does not model turnouts: RP-11 lists a minimum frog number for each class, and the page for your turnouts is the manufacturer's. It does not take the cushion-underframe column into account. RP-11 lists 60 feet for the longest passenger car at class J and 65 feet at class I; this tool takes the first class, in order, whose limit covers the length, so a 65-foot car reads as class I.