Track planning
Turnout Number Chart
Quick answer
A number 6 turnout diverges at 9.53 degrees and needs about 10.74 in of parallel track spacing in HO. Number 4 is sharper at 14.25 degrees; number 10 is nearly straight at 5.72 degrees. Higher turnout numbers are gentler, longer, and reach a parallel track farther down the ladder.
A turnout number describes how many units a diverging route travels forward for every one unit it moves sideways. A number 6 turnout moves 6 units forward for every 1 unit sideways, which is a gentler angle than a number 4 and a sharper one than a number 8. The number is a ratio, not a length, so the same turnout number produces a different physical footprint in every scale.
Two things fall directly out of the turnout number: the frog angle, in degrees, and how far down a parallel siding a train travels before the two tracks reach standard track-center spacing. Both are worked below using the turnout geometry calculator's own formulas.
What frog angle does each turnout number produce?
Frog angle is calculated as 2 x arctan(1 / (2 x turnout number)), in degrees. It is the actual angle the diverging rail cuts away from the straight route at the frog.
A number 4 turnout diverges at 14.25 degrees, roughly two and a half times sharper than a number 10 at 5.72 degrees.
| Turnout number | Frog angle |
|---|---|
| Number 4 | 14.25 degrees |
| Number 5 | 11.42 degrees |
| Number 6 | 9.53 degrees |
| Number 8 | 7.15 degrees |
| Number 10 | 5.72 degrees |
How far down a ladder does a turnout reach a parallel track?
Ladder reach is turnout number multiplied by the scale's track center spacing (the standard gap between two parallel tracks, itself scaled from a 13 foot prototype separation). Track centers used here are 0.98 in for N, 1.79 in for HO, and 3.25 in for O.
A number 6 turnout reaches its parallel track after 10.74 in in HO, 5.88 in in N, and 19.50 in in O.
| Turnout number | N ladder reach | HO ladder reach | O ladder reach |
|---|---|---|---|
| Number 4 | 3.92 in | 7.16 in | 13.00 in |
| Number 5 | 4.90 in | 8.95 in | 16.25 in |
| Number 6 | 5.88 in | 10.74 in | 19.50 in |
| Number 8 | 7.84 in | 14.32 in | 26.00 in |
| Number 10 | 9.80 in | 17.90 in | 32.50 in |
Ladder reach = turnout number x track centers. HO figures match the worked examples on the turnout geometry calculator; N and O are the same formula applied to their own track-center spacing.
How do I pick a turnout number for a yard ladder versus a mainline?
Lower numbers save space and cost sharper diverging speed; a number 4 turnout is standard in yards and industrial spurs where trains move slowly and space is tight, like the sets sold as Atlas HO Code 83 Number 6 Left-Hand Super Switch in code 83 or Atlas Code 100 Nickel Silver Mark 4 Number 8 Turnout Left Hand HO Scale at the higher number in code 100. Higher numbers, number 8 and up, belong on a visible mainline crossover where a train should not visibly slow down through the diverging route. A double crossover between two mainlines, such as Kato N Scale 310 mm Double Crossover in N scale, is usually built at a higher number than a hidden staging ladder for exactly that reason.
Why do track centers matter for the ladder math?
Track centers are the minimum safe spacing between two parallel tracks, scaled down from a 13 foot real-world separation. That single number, 1.79 in in HO, sets how far apart parallel tracks sit everywhere on the layout, not just at a turnout, which is why it also drives clearance planning on the track centers and clearance chart. A turnout number multiplies against that fixed spacing, so a wider scale (larger track centers) always produces a longer ladder for the same turnout number than a narrower scale does.
Sources
- Turnout frog angle geometry, 2 x arctan(1 / (2 x turnout number))
Frequently asked questions
What does a higher turnout number actually mean on the layout?
A higher number means a gentler, longer turnout: less sideways angle for the same forward distance, a shallower frog angle, and a train that can take the diverging route at a higher speed without a visible lurch. The tradeoff is length; a number 10 turnout is noticeably longer on the benchwork than a number 4.
Is a number 6 turnout good enough for a mainline crossover?
Number 6 is a common minimum for mainline work and is the most widely stocked size across brands, including Peco HO Scale Code 83 Electrofrog Number 6 Right-Hand Turnout and Peco HO Scale Code 100 Electrofrog Number 6 Right-Hand Turnout. Number 8 or higher looks and runs better at realistic mainline speeds, but number 6 is an accepted, functional standard for a visible crossover.
Why does the same turnout number take up more room in O than in N?
Turnout number is a ratio between forward travel and sideways travel, and it produces a physically longer turnout in a larger scale because everything in that scale is proportionally bigger. A number 6 turnout is a specific length in N, a longer specific length in HO, and longer again in O, even though the ratio and the frog angle are identical in all three.
What turnout number is standard for a yard ladder?
Number 4 is the traditional yard and industrial-spur standard because yard moves happen at low speed and space is usually the limiting factor. A number 4 like Atlas Code 83 Nickel Silver Wye Turnout HO Scale style geometry lets a yard fit more tracks into less benchwork than the same ladder built with number 6 or 8 turnouts.
Does turnout number affect how reliably a train runs through it?
Turnout number sets the geometry, not the reliability directly; reliability comes more from frog type (electrofrog versus insulfrog), wheel gauge, and how well the points are aligned and powered. A well-maintained number 4 will run more reliably than a poorly wired number 10, though the higher number will always look smoother at speed.
Can I mix turnout numbers on the same layout?
Yes, and most real layouts do. Lower numbers in yards and spurs where space matters more than speed, higher numbers on the visible mainline where appearance and running speed matter more, is standard practice and does not create any electrical or mechanical compatibility problem between the two.
Researched, not professional advice. This page is compiled from published manufacturer specifications, instruction manuals, published NMRA standards and recommended practices, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are hobby convention rather than a standard, and they are labelled that way wherever they appear. Always check a radius, a clearance or a grade against your own equipment before you cut wood or lay rail, because manufacturers vary and so does what your specific models will tolerate. Any permanent mains wiring in a layout room is work for a licensed electrician to your local code.