How to choose a minimum curve radius for HO
Tight curves save space but limit what you can run. Use the makers' radius tables and the NMRA's recommended practice to choose the smallest curve your trains will accept.
- Time
- 30 minutes
- Difficulty
- Easy
- Cost
- Free
- Steps
- 7
- Checked
What you’ll need
- Tape measure
- Calculator
Step by step
-
Step 1: Know the set-track radii
Common first-radius curves in HO:
- Märklin C-track: R1 360 mm, R2 437.5 mm, R3 515 mm, R4 579.3 mm, R5 643.6 mm.
- Piko A-track: R1 360 mm, R2 422 mm, R3 484 mm, R4 546 mm.
- RocoLine: R2 358 mm, R3 419.6 mm, R4 481.2 mm, R5 542.8 mm, R6 604.4 mm.
- Roco geoLine: R2 358 mm, R3 434.5 mm, R4 511.1 mm.
- Peco Setrack OO/HO: No.1 371 mm (No.2 438 mm, No.3 505 mm, No.4 about 572 mm).
-
Step 2: Check the NMRA classes
NMRA RP-11 gives minimum radii by equipment class for running at a restricted 20 scale mph. In HO they range from 14 in (356 mm) for class E to 44 in (1,118 mm) for class Q — for example 18 in (457 mm) for class G, 22 in (559 mm) for class I and 24.5 in (622 mm) for class K. For main-line speeds the NMRA says use a higher class.
Next: step 3 -
Step 3: Read your models' instructions
Manufacturers usually state a minimum radius for each loco and coach. Entry-level models often manage tighter curves; highly detailed brass models may need much larger ones. Long coaches and big six-axle locos are usually the fussiest.
Next: step 4 -
Step 4: Pick your minimum and stick to it
Choose the smallest radius your longest, fussiest model accepts, then use bigger curves wherever space allows. Keep curves inside tunnels and on gradients generous.
Next: step 5 -
Step 5: Leave room between tracks on curves
Long vehicles overhang on curves, so double track needs wider spacing on bends. Set-track systems build this in: Märklin's R3–R4–R5 are 64.3 mm apart, Piko's spacing is 61.88 mm, Roco geoLine's 76.5 mm.
Next: step 6 -
Step 6: Ease into curves
Where you lay flexible track, ease into the curve gradually rather than going from straight to full curve in one step; the NMRA recommends eased curves for best running and looks.
Next: step 7 -
Step 7: Test before you fix
Lay the curve loose and push your longest train through it, forwards and backwards, coupled together.
Tips
- Curved points: NMRA RP-11 suggests going one frog number larger if the inner curve is at your minimum radius.
Questions people ask
What's the smallest curve I can use?
Many set-track systems start at about 360 mm. Whether your models run on it depends on the model — check the instructions.
Why do my long coaches derail on curves?
Usually the radius is too tight, the curve starts too abruptly, or the couplers can't swing enough.
Sources
Checked 18 October 2026. We wrote this guide in our own words, using:
- Märklin: curved C-track R1 = 360 mm (24130) (maerklin.de)
- Märklin: curved C-track R2 = 437.5 mm (24230) (maerklin.de)
- Märklin: curved C-track R4 = 579.3 mm (24430) (maerklin.de)
- Märklin: curved C-track R5 = 643.6 mm (24530) (maerklin.de)
- PIKO A-track brochure (piko-shop.de)
- Roco: H0 track and Z21 (roco.cc) (roco.cc)
- Roco geoLine instructions (German, via Conrad) (asset.conrad.com)
- Peco: ST-220 Setrack No.1 radius curve (371 mm) (peco-uk.com)
- Wikipedia: OO gauge (en.wikipedia.org)
- NMRA RP-11: curvature and rolling stock (2018) (nmra.com)
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Printed from locoho.co.uk/guides/how-to-choose-a-minimum-curve-radius/ · Checked 18 October 2026 · General information only. Always follow the manufacturer’s instructions for your trains, track, controllers and decoders.