HO Scale Track Radius Guide: 18-Inch vs 22-Inch Curves and Beyond

An 18-inch HO curve makes a 36-inch circle at the track centerline. A 22-inch curve makes a 44-inch centerline circle. Neither number is the finished table width: the track and roadbed extend beyond that line, and the train needs clearance as it moves around the curve.

Start with the locomotive and cars you want to run, then check their stated curve requirements against the space available. If the equipment requires a broader curve than the room allows, changing the train or the track plan is a useful decision to make before buying the track.

Measure from the centerline, then plan the outside edge

Radius runs from the center of the circle to the track centerline, midway between the rails. Diameter is twice the radius. Moving from 18 to 22 inches therefore adds eight inches to the centerline diameter, even though the radius changes by only four.

On a hypothetical 48-inch-wide board, a centered 22-inch-radius circle leaves two inches from the centerline to each board edge. Part of that space is occupied by the track and its base. It is not a two-inch clear strip outside the roadbed, and it does not establish adequate space for the train or scenery.

Mark the proposed centerline on the actual surface or use a full-size track plan. Then account for the outer edge of the track system and the movement of the equipment. A quoted circle diameter is useful geometry, but it is not a complete layout footprint.

Four curved sections do not always make the same fraction of a circle

Radius describes how broad the curve is. The angle of each section determines how much of the circle a pack supplies. Bachmann's two HO nickel silver E-Z Track packs make the distinction easy to see.

These counts follow from the official 44501 specification and official 44503 specification. They describe the curved geometry only. A complete powered layout still needs a suitable electrical connection and controller, and the parts list changes if the plan uses a terminal rerailer or other section in the curve.

Check the whole train's curve requirements

A locomotive's published minimum radius does not establish the limit for every car coupled behind it. Check the exact releases you intend to use, including passenger cars, long freight cars and any special coupling or drawbar arrangement. A broad category such as “steam” or “diesel” cannot replace those specifications.

If one item calls for more than a 22-inch radius, neither of these two packs settles that requirement. Choose a suitable broader geometry or reconsider the equipment. If every item is specified for 18 inches, a 22-inch curve may still be worth trying for the way the train looks and moves, but it is not a promise that every track plan will operate well.

For an example of why exact models matter more than category labels, see our steam versus diesel locomotive guide. It compares specific current releases with different published curve guidance.

Leave room inside the curve as well as outside it

On curves, car ends swing outward while the middle of the body reaches inward. The NMRA's track-center and obstacle-clearance guidance explains why curves need additional attention around adjacent tracks and fixed objects.

Check tunnel openings, platform edges, bridge sides and nearby structures against the equipment's full movement. Looking only at the rails can miss a body corner that reaches beyond them. For parallel curves, also check two trains passing one another, rather than testing each track with the other one empty.

Bachmann describes the 18-inch curve as an inner partner for its 22-inch curve. That establishes a planning arrangement within this track family; check the actual trains and nearby scenery before treating the arrangement as clearance approval for every vehicle.

Look at the joins between curves

A curve is part of a route. Examine the places where it meets a straight, a turnout or a curve in the opposite direction. A single broad bend does not establish that the complete route, including those transitions, will work with the intended train.

Assemble the proposed route before fastening it permanently. Try the locomotive and the longest intended cars together, at low speed in the directions you plan to operate. Inspect coupled cars as well as individual vehicles. Test the junctions and return curves with scenery still removable so a clearance problem does not become a major rebuilding job.

If a turnout is part of that route, use our HO #4 versus #6 turnout guide for its geometry and footprint. A turnout number and a curve radius describe different features of the plan.

For the track system's materials and connections, read the Bachmann E-Z Track guide. Browse our model train collection with the scale, control system and route requirements already noted.