Clutch Torque Capacity Calculator
A friction clutch transmits torque by clamping a disc between two surfaces, and the maximum torque it can carry before slipping follows a well-established mechanical equation. This calculator takes the clamp (spring) force, the coefficient of friction of your facing material, the outer and inner diameters of the friction lining and the number of friction faces, then returns the mean friction radius and the torque capacity in newton metres and pound-feet. The friction coefficient is your input because it depends entirely on the facing material.
Clutch torque capacity formula
Mean radius (m) = (outer diameter + inner diameter) / 4 / 1000
Torque (Nm) = clamp force * friction coefficient * mean radius * faces
Torque (lb-ft) = torque (Nm) * 0.737562
The mean radius is the average of outer and inner radii, equal to the sum of the diameters divided by four. Torque is the product of clamp force, friction coefficient, mean radius and number of friction faces. The conversion factor 0.737562 newton metres per pound-foot is an exact unit relation.
Clutch design context
- Single-plate dry clutches have two friction faces; twin-plate clutches have four.
- Organic facing coefficients are commonly around 0.3; sintered and ceramic facings differ, so use your supplier figure.
- Torque capacity scales directly with clamp force, friction coefficient, radius and face count.
- Designers size capacity above peak engine torque using a safety factor; apply yours to the result.
- Worn or contaminated facings reduce the effective friction coefficient and capacity.
Clutch torque: frequently asked questions
How is clutch torque capacity calculated?
Torque capacity equals the clamp force times the friction coefficient times the mean friction radius times the number of friction faces. This is the standard friction-clutch torque equation. Each input must be supplied for your specific clutch because materials and geometry vary.
What is the mean friction radius?
The mean radius is the effective radius at which the friction force acts. A common engineering approximation is the average of the outer and inner radii of the friction lining. This calculator computes it from the outer and inner diameters you enter.
Why must the friction coefficient be an input?
The coefficient of friction depends on the clutch facing material (organic, ceramic, sintered metal) and condition, so there is no universal value. Typical dry organic facings are around 0.3, but you should use the figure from your facing supplier. This tool never assumes it.
How many friction faces does a clutch have?
A single-plate dry clutch has two friction faces (both sides of the disc). A twin-plate clutch has four. Enter the number of friction faces for your clutch; the torque scales directly with this count.
Does this include a safety factor?
No. This returns the theoretical maximum torque the clutch can transmit before slipping. Designers apply a safety factor (often 1.2 to 1.5 or more) so the clutch capacity exceeds the engine's peak torque. Apply your own factor to the result as required.
Official sources
- U.S. National Institute of Standards and Technology: SI Units and Conversions.
- NASA Glenn Research Center: Torque (Moment) Fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.