Gear Ratio Calculator
Calculate simple external gear ratio and ideal output speed from tooth counts and input rpm.
Use the Gear Ratio Calculator
Calculate simple external gear ratio and ideal output speed from tooth counts and input rpm.
Calculations happen locally in your browser. ToolLott does not send these inputs to a server for this tool.
Preliminary planning or arithmetic calculation only. Confirm project inputs, units, operating assumptions and applicable requirements before procurement, construction or operational decisions.
How to use it
Replace the worked-example values with actual project data. Review the intermediate quantities and visible assumptions before using the result elsewhere.
How the Gear Ratio Calculator works
Calculate simple external gear ratio and ideal output speed from tooth counts and input rpm. The methodology below exposes the production equation, a QA-verified worked example and the boundary between this screen and detailed design.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
Simple gear ratio and ideal output speed
The production engine evaluates ratio = driven teeth / driver teeth; output rpm = input rpm / ratio. It begins by divide driven gear teeth by driver gear teeth., then divide input speed by the ideal ratio., and finally treat the result as an ideal simple gear-pair relationship. This keeps the calculation auditable instead of hiding design assumptions behind a single number.
ratio = driven teeth / driver teeth; output rpm = input rpm / ratioToolLott evaluates the relationship using the entered units and full numeric precision before display rounding.
| Symbol / input | Meaning | Unit |
|---|---|---|
driverTeeth | Driving gear teeth | user input |
drivenTeeth | Driven gear teeth | user input |
inputRpm | Input speed (rpm) | user input |
Step-by-step method
- Divide driven gear teeth by driver gear teeth.
- Divide input speed by the ideal ratio.
- Treat the result as an ideal simple gear-pair relationship.
Worked example
Aisha, a project engineer, has a 20-tooth driving gear and 60-tooth driven gear on a 1,500 rpm motor and needs expected output speed.
Example inputs
- Driving gear teeth: 20
- Driven gear teeth: 60
- Input speed (rpm): 1500
Calculation / processing
Ratio = 60 / 20 = 3:1.Output speed = 1500 / 3.Ideal output speed = 500 rpm.
Use the result as a screening or quantity-planning value and compare it with the project criteria, manufacturer data and applicable engineering requirements before making a design or procurement decision.
Assumptions
- The entered values are representative of the condition being screened and use the units shown on the page.
- The equation models only the stated relationship; omitted system effects are not silently estimated.
Limitations
- Gear tooth counts must be whole positive integers; fractional teeth are rejected.
- This page is not engineering certification and does not replace applicable standards, design loads, manufacturer data, site investigation or competent professional review.
- Results can change materially when real geometry, losses, transients, material properties, duty cycles, safety factors or regulatory criteria differ from the simplified inputs.
Common questions
Is this a final engineering design?
No. It is a transparent screening calculation using the entered assumptions and the stated equation.
Why does the page show assumptions and limitations?
Engineering formulas are only valid within their modelling assumptions; exposing them helps users decide what additional design checks are required.
Should I use the rounded displayed value in later design work?
Use suitable calculation precision and the governing project/standard requirements rather than relying on display rounding alone.
Methodology sources
Related ToolLott tools
A realistic way Aisha could use this tool
Aisha is a project engineer.
Aisha has a 20-tooth driving gear and 60-tooth driven gear on a 1,500 rpm motor and needs expected output speed.
For this case the entered data is driver teeth = 20; driven teeth = 60; input rpm = 1500. The page evaluates that exact set and reports Gear ratio: 3:1 - Output speed: 500 rpm; the example therefore demonstrates the method with real values rather than describing a vague before-and-after story.
This produces Gear ratio: 3:1 - Output speed: 500 rpm. In practical terms, Aisha can expected output speed using a result tied to the shown inputs instead of an unexplained recommendation. The page also keeps this limitation explicit: Preliminary screening/planning calculation only.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this calculator is for
Use it to calculate simple external gear ratio and ideal output speed from tooth counts and input rpm.
It sits within ToolLott’s Mechanical collection, where you can also calculate bearing life, belt length, gear ratio, power transmission, shaft checks, thermal expansion and torque.