Model Building Calculator

miniThrottle: A model railroad WiFi throttle.


this page contains calculations useful in building model trains, boats, cars, aircraft and buildings. These include scale conversions, speed, distance, wheel diameter and gearbox ratios. Many popular scales come from model railway building. Some of the scales use odd conversions such as 4mm (or 3.5mm) metric to the prototype's Imperial foot. G scale is shown for meter gauge, but one scale (1:32) may be useful if using standard gauge prototypes in G scale. Calculations below include:

While the intention is to keep conversions accurate, please double check your work. We bear no responsibilty for any errors or misuse of these conversions. As calculations are perfored using metric values, there may be some "rounding error" on Imperial values, eg you need to enter 22.507 miles before the equivalent G scale value shows an actual mile representing this distance.

Optional Settings

Use this section to select the decimal points required for calculations. NB Inch fractions are calculated as follows:

Decimal points

Optionally define a custom scale if the scale you want is not shown.

custom scale, 1:

Distance / Speed Conversion

use this for converions of larger distances or speeds, eg km/h or mph, or track length. Distances may be fractional, eg 16.09 km.


ScaleMeter EquivImperial Equiv

Decimal Feet

Use to convert from feet, inches and fractions to feet in decimal. Note the result also updates the succeeding length calculation. However, if required for wheel diameters, you will need to update these manually. Note entry is only to nearest 1/16 inch. Smaller values are likely to be insignificant once scaled. Equivalent values for other fractions are shown in the lookup table below:

1/2 inch1/4 inch1/8 inch1/16 inch



Length Equivalents

Use this to calculate component lengths, eg wall length, vehicle width, etc.


Scalemm EquivImperial Equiv

Wheel, speed and gearbox sizing

Use to calculate gearbox ratios, based on speed and wheel diameter. Note default values are selected to demonstrate the relationship between values, rather than representing any prototype. Using a lower ratio (eg 1:40 when 1:50 is indicated) will result in a model with a scale max speed faster than the prototype.

Wheel Diameter (mm)prototype
Max Speed (km/h)prototype
Max Motor Speed (rpm)model

Speed (meters/minute) prototype
Speed (meters/second) prototype
Distance per revolution (mm) prototype (Circumference)
Max wheel rpm both
Total gearing required (1:n) model

Wheel Diameter (mm)Speed (mm/s)Imp. DiameterSpeed/sec

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