How to Figure a Gear Ratio

While performing basic engineering projects, you may sometimes need to calculate the gear ratio of two or more cogs. You may need to know gear ratio so you can know how much power, torque or speed can be gained from a given assemblage of cogs, or to make comparisons between prospective cog setups for your project. Knowing how to calculate gear ratio on an engineering level also allows you to make the same calculations for the gears on a bicycle or similar.

Things You'll Need

• Gears to be calculated
• Pen and paper

Instructions

1. Link up the gears so that they are turning as they will in the finished project. The gear that has force applied to it is called the Driver, and the gear that it propels is called the Follower. For example, on a bicycle, the gear connected to the pedals is the Driver, and the gear on the rear wheel is the Follower.

2. Count the number of teeth on the Driver. Note this number.

3. Count the number of teeth in the Follower gear. Note this number also.

4. Place the numbers next to each other, Follower first. If the Follower has 18 teeth and the Driver has 8 teeth, for example, the gear ratio is 8/18.

Tips & Warnings

• Gear ratio can also be calculated by turns of the gears. Turn the Driver one full revolution and count how many revolutions the Follower completes. An 8/18 gear set will have a ratio of about 4/9 when expressed in this way.

References

• Duynamic Science: Gears