Interactive Gear Ratio ↔ RPM Calculator
Use this tool to calculate output RPM, required input RPM, or overall gear ratio. You can enter a direct ratio or calculate ratio from gear teeth.
What a gear ratio to RPM calculator tells you
A gear ratio to RPM calculator helps you translate rotational speed between a driving gear (or motor shaft) and a driven gear (or output shaft). In practical terms, it answers questions like:
- “If my motor runs at 3,000 RPM and my ratio is 4:1, what is output RPM?”
- “What input RPM do I need to get 500 RPM at the output?”
- “What ratio am I effectively running based on measured input/output speeds?”
Core formulas
1) Ratio from gear teeth
Gear Ratio = Driven Gear Teeth ÷ Driver Gear Teeth
2) Output RPM from input RPM
Output RPM = Input RPM ÷ Total Ratio
Where Total Ratio can include gearbox ratio multiplied by final drive ratio.
3) Required input RPM
Input RPM = Desired Output RPM × Total Ratio
4) Ratio from measured RPMs
Overall Ratio = Input RPM ÷ Output RPM
How to use this calculator correctly
Method A: You already know the gear ratio
- Select Find Output RPM or Find Required Input RPM.
- Enter the gear ratio directly (for example, 3.2 for 3.2:1).
- Enter final drive ratio if applicable; otherwise leave it at 1.
- Click Calculate.
Method B: You know the tooth counts
- Leave direct ratio blank.
- Enter both Driver Teeth and Driven Teeth.
- The calculator computes ratio as Driven ÷ Driver automatically.
Worked examples
Example 1: Motor to wheel speed
Input RPM = 3600, gearbox ratio = 3:1, final drive = 2:1.
Total ratio = 3 × 2 = 6:1, so output RPM = 3600 ÷ 6 = 600 RPM.
Example 2: Back-calculating motor speed
Desired output = 800 RPM, ratio = 2.5:1, no final drive.
Input RPM needed = 800 × 2.5 = 2000 RPM.
Example 3: Finding effective ratio from test data
Measured input = 1500 RPM, measured output = 250 RPM.
Overall ratio = 1500 ÷ 250 = 6:1.
Quick reference table
| Total Ratio | Input RPM | Output RPM |
|---|---|---|
| 2:1 | 3000 | 1500 |
| 3:1 | 3000 | 1000 |
| 4:1 | 3000 | 750 |
| 6:1 | 3000 | 500 |
| 10:1 | 3000 | 300 |
Common mistakes to avoid
- Reversing tooth formula (it should be Driven ÷ Driver for reduction).
- Forgetting to include final drive when present.
- Mixing RPM measured under load with no-load assumptions.
- Using zero or negative values, which are invalid in this context.
FAQ
Does this include drivetrain efficiency losses?
No. This calculator gives ideal kinematic RPM relationships. Real output may vary due to slip, friction, and load.
Can I use this for bicycles, EVs, CNC, or robotics?
Yes. The same rotational-speed ratio math applies across these systems.
What if I have multiple stages?
Multiply stage ratios together to get total ratio, then use that value in the calculator.