The number of teeth necessary for the mating gears is equal to the number of teeth in the pinion multiplied by the square root of the desired train ratio. And the number of teeth in the pinion is the minimum number of teeth possible. Because it’s important to avoid interference between two gears.
The minimum number of teeth possible for gear depends upon the system of gear teeth and its angle.
| System of Gear Teeth | Minimum No. of Teeth |
| 14 ½ Composite | 12 |
| 14 ½ Full-depth involute | 32 |
| 20 Composite | 14 |
| 20 Full depth involute | 18 |
Now, to get the number of teeth necessary for mating gears in a compound gear train, you can use the formula below.
The number of teeth necessary for the mating gears = Number of teeth × SQRT of desired train ratio
