Track geometry
Turnout Geometry Calculator
Quick answer
A turnout number is the rate of divergence: a number 6 moves one unit sideways for every six forward, giving a 9.53 degree frog. In HO at 1.79 inch track centres it needs 10.74 inches to reach a parallel track, and a crossover takes about 21.5 inches.
A turnout's number is the only thing you really need to understand about turnouts, and it is simpler than the jargon suggests. The number is the rate at which the diverging route pulls away from the straight one, measured at the frog: a number 6 moves one unit sideways for every six units forward. Everything else follows from that by geometry.
Two consequences drive every decision. A higher number means a shallower angle and a gentler divergence, so long equipment takes it better. A higher number also means a longer turnout, so it costs more space. Choosing a turnout number is choosing a point on that trade, and the place it bites hardest is a yard ladder, where every track you add costs the full reach again.
The figures below are exact geometry rather than manufacturer specifications. A real turnout also includes straight rail before the points and after the frog, so the packaged product is always longer than the geometry alone implies.
Turnout geometry calculator
Frog angle
Turnouts of this number
Top matches update when you change your numbers.
Atlas HO Code 83 Number 6 Left-Hand Super Switch
$27.40A number 6 in code 83, which is the general purpose turnout for main line and yard throat work and the one most track plans assume.
Best for: main line and passing siding work in HO
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Atlas Code 100 Nickel Silver Mark 4 Number 8 Turnout Left Hand HO Scale
$30.72A number 8 is long and shallow, which is what long passenger equipment needs to take the diverging route without lurching. It costs a lot of length to install.
Best for: main line crossovers where long cars diverge
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Peco HO Scale Code 83 Electrofrog Number 6 Right-Hand Turnout
$44.59An Electrofrog turnout powers the frog rather than leaving it dead, which is what stops a short wheelbase locomotive stalling on it. It needs its wiring done properly to avoid a short.
Best for: reliable slow running over the frog, with correct wiring
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Atlas Code 83 Nickel Silver Manual Snap Switch Left HO Scale
$26.98A snap switch is geometrically a sharp turnout designed to fit sectional track spacing. Convenient, and sharper than a number 6, so keep long equipment off the diverging route.
Best for: sidings on a sectional track layout
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Kato N Gauge Electric Point Number 6 Left
$19.80The number 6 Unitrack turnout, noticeably longer and shallower than the number 4 and the better choice on a main line.
Best for: N scale Unitrack main line junctions
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Kato N Scale 310 mm Double Crossover
$60.96A complete double crossover in one section. Building this from four separate turnouts is a difficult piece of trackwork, and this removes the problem entirely.
Best for: crossing between two N scale main lines
Check price on AmazonEvery turnout number in HO scale
Frog angle, the run needed to reach a parallel track at standard HO centres, and the length of a crossover between two parallel tracks.
In HO scale a number 6 turnout has a 9.53 degree frog and needs 10.74 inches to reach a parallel track, while a number 8 halves the angle and needs 14.32 inches.
| Number | Frog angle | Reach to parallel track | Crossover length | What it suits |
|---|---|---|---|---|
| Number 4 | 14.25 degrees | 7.16 in | 14.3 in | Tight industrial spurs and cramped yards, where the space matters more than the look and nothing long has to take the diverging route. |
| Number 5 | 11.42 degrees | 8.95 in | 17.9 in | A workable compromise for branch line and secondary track where a number 6 will not fit. |
| Number 6 | 9.53 degrees | 10.74 in | 21.5 in | The general purpose turnout. Yard throats, passing sidings and most main line work. |
| Number 8 | 7.15 degrees | 14.32 in | 28.6 in | Main line crossovers and anywhere long passenger equipment takes the diverging route at speed. |
| Number 10 | 5.72 degrees | 17.9 in | 35.8 in | High speed main line and display trackwork. Long, expensive, and unmistakably correct. |
Track centre spacing is scaled from 13 prototype feet. Curves need more, and NMRA Standard S-8 is the standards document on track centres and clearances.
What a ladder costs in each scale
A yard ladder is a series of turnouts, each peeling off one track, and each one needs its full reach before the next can start. This is the number that decides whether a yard fits.
Each track in an HO yard ladder costs 7.16 inches with number 4 turnouts and 14.32 inches with number 8s, so a five track yard spends between 28 and 57 inches on the ladder alone.
| Scale | Track centres | Number 4 | Number 6 | Number 8 |
|---|---|---|---|---|
| N | 0.98 in | 3.92 in | 5.88 in | 7.84 in |
| HO | 1.79 in | 7.16 in | 10.74 in | 14.32 in |
| O | 3.25 in | 13 in | 19.5 in | 26 in |
Where to spend the space and where not to
The guidance that follows from the arithmetic is unusually clear. Use the highest number you can afford on the main line, where trains run fast and long equipment takes the diverging route at speed. Accept a lower number in yards and on industrial spurs, where everything moves at walking pace and nothing long goes through the diverging route.
A number 6 on the main and a number 4 in the yard is a very common and entirely defensible combination. It is not a compromise made out of ignorance: it is spending length where length buys something.
The one place a sharp turnout genuinely hurts even at low speed is where a long car has to take the diverging route regularly. An 85 foot passenger car through a number 4 will lurch visibly and may pull its couplers apart under load. If that movement is part of how the layout operates, pay the length.
Frog type is a separate decision from the number
The number tells you the geometry and says nothing about whether the frog carries current.
An insulated or dead frog has a plastic frog casting, so there is a short gap where no power reaches the wheels. It is simple to wire, it cannot short, and a short wheelbase locomotive can stall on it.
A powered or live frog carries current and is switched to match the route, which eliminates the stall. It must be wired correctly or it shorts the instant a wheel bridges the rails.
The practical rule: a modern locomotive with all wheel pickup and a flywheel usually copes with a dead frog. Small switchers and short wheelbase steam often do not. A keep alive circuit in the decoder solves the same problem from the other end, and is worth considering if you would rather not rewire turnouts.
Frequently asked questions
What does the number on a turnout mean?
It is the rate of divergence at the frog: the diverging route moves one unit sideways for every N units forward. A number 6 moves one inch sideways over six inches of travel. Higher numbers are shallower, gentler and longer; lower numbers are sharper and shorter. The number is scale independent, so a number 6 is the same angle in N, HO and O.
What is the frog angle of a number 6 turnout?
About 9.53 degrees. The angle is twice the arctangent of one divided by twice the number, which is exact geometry rather than a manufacturer figure. A number 4 is 14.25 degrees, a number 8 is 7.15 degrees and a number 10 is 5.72 degrees. The same angles apply in every scale.
What turnout number should I use on a main line?
Number 6 as a minimum and number 8 where long passenger equipment regularly crosses over at speed. The cost is length: a number 8 crossover in HO occupies about 28.6 inches against 21.5 for a number 6. On a small layout that difference often decides whether the crossover fits at all.
Are number 4 turnouts acceptable?
Yes, in the right place. A number 4 is the correct choice for industrial spurs and cramped yards where everything moves at walking pace and short cars are the norm. It is the wrong choice on a main line where long equipment takes the diverging route at speed, because it will lurch and can pull couplers apart under load.
How far apart should parallel tracks be?
About 1.79 inches in HO, 0.98 inches in N and 3.25 inches in O on straight track, scaled from roughly 13 prototype feet. Curves need more, because long cars overhang inward at the middle and outward at the ends and can sideswipe. NMRA Standard S-8 covers track centres and clearances and is the document to check.
Should I buy powered frog or insulated frog turnouts?
Insulated frog turnouts are simpler to wire and cannot short, at the cost of a short dead spot that can stall a short wheelbase locomotive. Powered frog turnouts eliminate the stall and must be wired correctly or they short. Modern locomotives with all wheel pickup usually cope with a dead frog, and small switchers frequently do not.
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.