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# How to Calculate Planetary Gear Ratio

Updated March 22, 2017

Planetary gear systems are composed of three main parts. The ring gear is the outermost gear on the gearset. The sun gear is the centre gear of the system and is encircled by the planetary gears. Planetary gears are able to function on one single axis instead of many. This makes calculating the gear ratio easier because you only have to count teeth in one specific section, whereas other gear ratios would require you to count multiple different gear units. It is important to note whether the gear is in overdrive or underdrive and the action of the carrier, as this will affect the process of calculating the gear ratio.

Count the number of sun gear teeth and the number of ring gear teeth if the planetary gear is set in underdrive with the carrier as the output. Add the two numbers together.

Divide the resulting number by the number of teeth on the driving member for the gear in underdrive when the carrier is the output. The total is the gear ratio for that gear setting.

Divide the number of teeth on the driving member by the sum of the sun gear and ring gear teeth on the planetary gear for a system set in overdrive with the carrier as the input. The resulting number is the gear ratio for that transmission setting.

Count the number of teeth on the driven gear and the number of teeth on the driving gear if the planetary gear is set in underdrive with the carrier held.

Divide the number of driven gear teeth by the number of driving gear teeth to obtain the gear ratio for this system setting.

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