Model Train Track Radius and Arcs

So you’re working on your model train layout and want to make a full circle, or even a part of a circle to make ovals or smooth turns in your track, but don’t know how much track you’ll need?

Read on!

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The Difference Between Radius and Arc

Okay, first things first, lets get some definitions out of the way.
Radius refers to a straight line from the center of your circle, to the edge of your circle. So if you had a 10″ radius, your circle is 20″ from side to side, and so on.

The reason we use radius is so you have a central location to work from, so you can do 90′ or 180′ circles instead with ease.

So what’s an arc then?

Well, an arc refers to just a segment of the circle. A full circle has an arc of 360 degrees. so to make that circle, you need track pieces with an arc that equal 360.

The most common types of track pieces are usually 15, 22.5, 30, and 45 degree arcs.

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How to Work Out What Model Train Track Radius you can Use

This is totally dependant on your scale of the train, and what you want to achieve with your turning circle.

Each train scale has a minimum turning circle. This is the absolute minimum radius you can use to make a perfect circle for your train.

You need to remember that the larger the scale such as O or even G, is going to be far bigger than N or Z. So your ability to turn is going to depend on the track you have and the size of your model train layout.

If for some reason you don’t have a large enough space for a 180 degree turn to allow the train to come back, you might need to consider something like a switching station instead; these allow the train to run forwards and backwards, instead of turning.

So now let’s look at what kind of radius you’ll need for your scale!

SCALE	RADIUS		DIAMETER
	INCHES	MILLIMETERS	INCHES	MILLIMETERS
S	20	508	40	1016
O	15.5	393.7	31	787.4
O27	13.5	342.9	27	685.8
OO	14.6	371	29.2	742
HO	15	381	30	762
TT	10.5	267	21	762
N	8.5	216	17	432
Z	5.7	145	11.4	290

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O scale track radius

The diameter for an O scale circle is 31 inches. So to do a full 180 turn, you’ll need a minimum of those 31 inches available on your layout.

O-42 gauge track requires 12 pieces of track to complete a full circle.

Anything about that, from O-45 through to O-99, will require at least 16 pieces of track.

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HO Scale Track Radius

The diameter for HO scale circle of track is 30 inches, so pretty similar to an O scale, regardless of the size difference.

HO code 100 and 83 need somewhere between 12 and 16 pieces of the track also.

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N Scale Track Radius

Luckily, N scale is dramatically smaller, so you’ll need a lot less layout space to complete your turning circle.

N code 55 can complete a full circle with a diameter of 17 inches, and you’ll need about 16 pieces to get it done. N code 80 only requires 12 pieces, however.

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Z Scale Track Radius

Atlas Z gauge track use the same code 55 track as N scale above, so a 17″ turning circle requiring 16 pieces.

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As above, you’re going to need somewhere between 10-20 pieces of curved track to make a full circle. From there, you can half the amount for a 180 degree turn, or just use a quarter to make a smooth 90 degree turn for your train.

If you’re looking for something that’s less stringent, you should consider using something like Flex-E-Trak instead, which will let you place the track however you want.