Trackside Prints

Model railway curve radius calculator

Pick a PECO OO Setrack curve or type your own radius. You get how wide the board needs to be for a full circle, how much track the circle uses, and what that curve would be on a real railway.

Work out the space

A full circle needs 976 mm

PECO No.2 radius, 438 mm, makes a circle 876 mm across its centre line. With 50 mm spare on each side, the board has to be 976 mm (0.98 m) wide and deep. One full circle uses 2.75 m of track.

On the real railway, at 1:76.2 (OO (4 mm to the foot)), that curve would be 33.4 m radius, or 1.66 chains.

PECO OO SetrackRadiusCircle acrossBoard with margin
No.1 (ST-220)371 mm742 mm842 mm
No.2 (ST-225)438 mm876 mm976 mm
No.3 (ST-230)505 mm1,010 mm1,110 mm
No.4 (ST-235)571.5 mm1,143 mm1,243 mm
A painting of a steam train and coaches curving through autumn woods above a river gorge, a stone arch bridge behind
A real line bends gently. A model has to bend on a board.

Nothing you type is saved. The radius and margin you pick stay in this page's link, so a bookmark brings them back.

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How to use it

Choose a curve from the list, or pick "My own radius" and type it in millimetres. Then type how much room you want between the track centre line and the edge of the board. That margin is your call: think about overhang on long coaches and how close to the edge you are happy for a train to run.

The answer tells you the board width for a full circle, and a table does the same for all four PECO radii, so you can see what a bigger curve costs you in space.

A large and a small print of the same black steam engine laid on a wooden table beside an open yellow tape measure
Measure the room before you buy the track.

How it is worked out

The radius is measured to the centre line of the track. A full circle is twice the radius across, plus your margin on each side. The track in a full circle is 2 × π × the radius long.

For the real-life figure, the tool multiplies the radius by your scale and turns the metres into chains. A chain is 22 yards, or 20.1168 metres (Weights and Measures Act 1985). A PECO No.2 curve in OO comes out at a little over 33 metres, about one and two thirds of a chain.

The PECO radii

PECO gives its OO Setrack curves as No.1 371 mm, No.2 438 mm, No.3 505 mm and No.4 571.5 mm. For any other make, type the radius from the maker's own box or page.

Sources: PECO Setrack product pages ST-220, ST-225, ST-230 and ST-235; Weights and Measures Act 1985, Schedule 1. Read 30 October 2026.

Written by Craig Fearn, Trackside Prints. Last updated .

Questions, answered

How wide a board do I need for a circle of OO track?
Twice the radius plus your margin on each side. On PECO No.2 radius (438 mm) with 50 mm spare each side, that is 976 mm.
What are the PECO Setrack radii?
In OO: No.1 371 mm, No.2 438 mm, No.3 505 mm and No.4 571.5 mm, as PECO lists them.
Do you recommend a minimum radius?
No. That depends on your locomotives and coaches, so check what their makers say.