Calculator

Curve radius & layout space helper

Plan before you buy track: see how big a board a simple continuous oval needs at the curve radius you have in mind, how much track goes round it, and how steep a gradient will be. All sizes are in millimetres.

An oval of track

Curve radius (mm, to the track centre line)

Type a radius or pick a sectional-track curve from the list.

Length of each straight (mm)

Space beyond the centre line, each side (mm)

Your choice — room for the sleepers, the overhang of long vehicles and a safety edge. Check your track maker’s figures.

Scale

A gradient (optional)

Rise (mm)

Over a length of (mm)

…or the length needed at a gradient of (%)

Enter a curve radius

The board size, the length of track and the full-size curve appear here.

Sectional track: curve radii

Radii to the track centre line, as the makers publish them. “Spacing” is the distance between parallel curves of neighbouring radii — useful for double track.

Curve radii of common HO sectional track systems
Track system	Curves (radius, mm)	Spacing	Source
Märklin C-track (3-rail AC)30° curves, 12 to a circle. R4 and R5 are parallel to R3 and R4 at 64.3 mm.	R1 360 · R2 437.5 · R3 515 · R4 579.3 · R5 643.6	64.3 mm	Märklin: C-track R1 curve 24130; Märklin: C-track R5 curve 24530
RocoLine (2-rail)30° curves; also R10 888 mm (15°) for turnout curves.	R2 358 · R3 419.6 · R4 481.2 · R5 542.8 · R6 604.4	61.6 mm	Roco: track pages
Roco geoLine (2-rail, with roadbed)30° curves, 12 to a circle, on a 385 × 76.5 mm grid.	R2 358 · R3 434.5 · R4 511.1	76.5 mm	Roco: track pages; Roco geoLine instructions (PDF)
PIKO A-track (2-rail)30° curves; R9 908 mm for turnout curves.	R1 360 · R2 422 · R3 484 · R4 546	61.88 mm	PIKO A-track brochure
Peco Setrack OO/HO (code 100)No.1 from Peco’s own page; No.2–No.4 as listed by retailers.	No.1 371 · No.2 438 · No.3 505 · No.4 572	—	Peco: ST-220 No.1 radius curve; Traintrax: track radius

How tight is too tight?

The NMRA’s recommended practice RP-11 (2018) groups HO equipment into classes by the minimum radius it should run through at restricted speed (20 scale mph). It recommends using a higher class for main-line running and the largest, eased curves you can fit; entry-level models often manage tighter curves, while highly detailed models may need larger ones. Always check the maker’s figure for your own models.

NMRA RP-11 minimum radius classes for HO
Class	E	F	G	H	I	J	K	L	M	N	O	P	Q
Inches	14	16.5	18	20	22	23	24.5	26.5	29.5	32	36	40	44
mm	356	419	457	508	559	584	622	673	749	813	914	1016	1118

Source: NMRA RP-11 (2018), minimum radius. Checked 18 October 2026.

How the sums work

Board length = 2 × radius + straight + 2 × the space you allow; board width = 2 × radius + 2 × the space you allow.
Track round an oval = 2π × radius (the two half-circles) + 2 × the straight.
Full-size radius = model radius × the scale ratio (87 for HO in the NEM standard).
Gradient = rise ÷ length × 100%. “1 in 40” means one unit up for every 40 along — the same as 2.5%.

This is geometry only: it doesn’t know the minimum radius your locos and coaches will run on, or the steepest gradient they can climb. Check the makers’ information — our track & layout guides explain what to look for.

Where the numbers come from

The scale ratios used for the full-size radius come from these standards. Checked 18 October 2026.