How to Calculate RPM from Gear Ratio

⚙️ Ever wondered how spinning one gear affects another? Whether you’re working on a bike, a car, or a mechanical project, understanding RPM and gear ratios helps you figure out how fast things are moving. Let’s make it super simple! 🛠️

💡 What is RPM?

RPM stands for Revolutions Per Minute — it tells you how many times something spins in a minute. Higher RPM means it’s spinning faster.

🔁 What is a Gear Ratio?

A gear ratio compares the number of teeth (or size) between two meshing gears. It tells you how many times one gear (the driven gear) turns for every spin of the other gear (the driving gear).

Gear Ratio Formula

Gear Ratio = Teeth on Driven Gear ÷ Teeth on Driving Gear

🧮 How to Calculate RPM from Gear Ratio

Once you know the gear ratio and the RPM of one gear, you can easily find the RPM of the other gear.

RPM Formula

RPM (output) = RPM (input) × (Driving Gear Teeth ÷ Driven Gear Teeth)

✅ Example

Let’s say:

  • Input RPM = 1200
  • Driving gear = 20 teeth
  • Driven gear = 40 teeth

Step 1: Calculate Gear Ratio

Gear Ratio = 40 ÷ 20 = 2

Step 2: Calculate Output RPM

RPM (output) = 1200 × (20 ÷ 40) = 1200 × 0.5 = 600 RPM

🎉 So the second gear spins at 600 RPM — half as fast as the first one.

🔄 Key Rule of Thumb

  • If the driven gear is bigger, the RPM decreases
  • If the driven gear is smaller, the RPM increases
  • ⚙️ It’s all about torque vs speed: higher torque = lower speed, and vice versa

💡 Why This Matters

Understanding RPM from gear ratios helps in:

  • Engineering & mechanical design
  • Automotive repair & performance tuning
  • Robotics & RC vehicles
  • Bikes, scooters, and gear-driven machines