Torque After Gear Ratio Calculator
Estimate idealized torque multiplication after a reduction or overdrive ratio.
How to use this calculator
- Enter input torque at the operating point you are analyzing. Do not combine peak torque from one RPM with a gear-ratio calculation intended for a different RPM unless that is deliberate.
- Enter the numerical reduction ratio using GIIRR's convention: a 3:1 reduction is entered as 3. An overdrive can be represented by a value below 1.
- Enter mechanical efficiency as a percentage when you have a defensible estimate. A 100% setting gives ideal torque and intentionally ignores losses.
- Keep torque units consistent. The interface is labeled N·m; convert lb-ft to N·m before entry if you want the displayed result to remain correctly labeled.
- Do not interpret the output as shaft or tooth load capacity. Component strength, transient shock, duty cycle and service factor require separate engineering checks.
Formula and assumptions
Output torque ≈ input torque × numerical ratio × efficiency fraction. Ideal output power cannot exceed input power; extra torque from reduction is accompanied by proportionally lower rotational speed, with real losses reducing output power further.
GIIRR calculates idealized geometry and kinematics. Friction, slip, compliance, tire deformation, manufacturing tolerances and control-system behavior can move real measurements away from the theoretical result.
Worked example
With 100 N·m input, a 3.00:1 reduction and 95% efficiency, estimated output torque is 100 × 3 × 0.95 = 285 N·m. If that same stage changes from 3:1 to 2:1 while input torque and efficiency stay fixed, estimated output torque falls to 190 N·m but output RPM increases by 50% relative to the 3:1 case.
Frequently asked questions
Does this violate conservation of energy?
No. A reduction raises torque while lowering rotational speed. In a real system, losses also make output power lower than input power.
Can I use lb-ft?
This interface labels torque in N·m. Convert lb-ft to N·m before entry by multiplying by approximately 1.35582; the gear-ratio multiplier itself is dimensionless.
Does a 4:1 ratio always quadruple real torque?
Only in the ideal 100%-efficient case. Real gears and bearings lose some power, so the efficiency term reduces the output estimate.
Does this tell me whether a shaft or gearbox is strong enough?
No. Static torque is only one load case. Shaft diameter, material, tooth stress, bearings, shock loads, fatigue and service factor must be checked separately.
What happens with an overdrive ratio below 1?
The calculated output torque decreases while output speed increases. For example, a 0.8 ratio at the same efficiency passes less torque multiplication than a 1:1 stage.
For safety-critical, legal or fitment decisions, verify the result against manufacturer documentation and physical measurements.
Related calculators
Gear Ratio Calculator
Use tooth counts from a simple gear pair to see the reduction or overdrive ratio, output shaft speed and torque multiplication.
Open calculator →DrivetrainRPM to Speed Calculator
Estimate vehicle speed at a given engine RPM from the selected gear, differential ratio and rolling diameter.
Open calculator →DrivetrainSpeed to RPM Calculator
Work backward from road speed to engine RPM, useful when comparing highway gearing, tire sizes or differential changes.
Open calculator →