What this calculator works out
This calculator converts a model's actual speed into the scale speed it represents, shows that speed in centimetres per second, and gives the model speed needed to represent a 100 km/h prototype.
The centimetres-per-second figure is the practical one. Model speeds in kilometres per hour are hard to picture; 36 cm/s is something you can see against a ruler.
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Before you set a speed
- Common scale denominators: 76 for OO, 87 for HO, 148 for N, 43.5 for O, 22.5 for G.
- Models are almost always run too fast. A locomotive that looks brisk on a layout is frequently doing several hundred scale kilometres an hour.
- Measure actual speed over a known distance with a stopwatch — a metre of straight track is enough.
- Prototype speed limits vary enormously by line and era, and are worth checking for the subject being modelled.
At 1:76, a model covering one metre in three seconds is doing 91 scale kilometres an hour — and one metre in three seconds looks slow on a layout. The instinct to run faster comes from the layout being small relative to the eye's expectation, and correcting for it is the single change that makes a layout look most like a railway.
How this calculator works
Actual speed scaled up, and converted to cm/s:
Scale speed = actual speed × scale denominator
Actual speed in cm/s = actual km/h × 100,000 ÷ 3,600
For a 100 km/h prototype: model cm/s = 100 ÷ scale × 100,000 ÷ 3,600Scale speed is the speed the model represents, not a speed anything travels at. It is the figure to compare against prototype limits; the cm/s figure is what to compare against a stopwatch.
Worked example: 1.3 km/h at 1:76
Using the default figures — an actual model speed of 1.3 km/h at 1:76:
- Scale speed: 99 km/h
- Actual speed: 36.1 cm/s
- For a 100 km/h prototype: 36.5 cm/s
36 cm/s is just over a third of a metre a second — a model crossing a two metre baseboard in about six seconds. That is a realistic express train, and on most layouts it looks unhurried. A model crossing the same board in two seconds is doing nearly 300 scale kilometres an hour, which is faster than any train the layout is likely to represent.
Common mistakes
- Judging speed by eye. The instinct is consistently too fast.
- Confusing scale speed with actual speed. One is represented, the other is measured.
- Using the wrong scale denominator. OO and HO differ enough to matter.
- Running shunting movements at line speed. Yard work is very slow indeed in scale terms.
- Ignoring the prototype's limits. A branch line goods train did not travel at express speed.
Frequently asked questions
How do I measure my model's speed?
Mark a known distance — a metre of straight track is convenient — and time a locomotive over it with a stopwatch. Speed in cm/s is the distance in centimetres divided by the seconds. Several DCC systems report speed directly, and speedometer accessories exist for the purpose.
What is a realistic speed for my subject?
It depends entirely on the prototype and the era. A branch line goods train might have run at 40 km/h; a main line express at 145 or more; shunting movements at walking pace. Modelling a specific line and era gives specific answers, and running everything at the same speed is one of the things that makes a layout look wrong.
Does DCC help with realistic speeds?
Yes, considerably. Speed tables and momentum settings let a locomotive accelerate and brake gradually and cap its top speed at something prototypical, which is difficult with plain analogue control. Setting a locomotive's maximum to a realistic scale speed is one of the more worthwhile decoder adjustments.
Why does slow running look better?
Because the eye judges speed against the size of the scene, and a small layout compresses distances that would be miles in reality. Slow, smooth running with gradual starts and stops reads as weight and mass; fast running reads as a toy. It is also more demanding of track and mechanism, which is why it is a mark of a well-built layout.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — railway records and historical operating information
- MoneyHelper — budgeting for hobby equipment
Sources are checked at publication and can change — how I choose and check references.