Torque Calculator

Torque Calculator

Power from Torque

Torque Calculator — Force, Distance, and Power

Torque is the rotational equivalent of linear force. This free torque calculator computes torque from force and moment arm, converts between all common torque units (Nm, ft-lb, kgf-m, in-lb), and calculates mechanical power from torque and rotational speed. It's useful for mechanical engineering, automotive, robotics, and DIY projects.

What Is Torque?

Torque (also called moment of force) is the tendency of a force to rotate an object about an axis. It is defined as: τ = F × d, where F is the applied force in Newtons and d is the perpendicular distance (moment arm) from the axis of rotation to the line of action of the force in metres. The SI unit is the Newton-metre (Nm).

Torque Unit Conversions

Different industries use different torque units. Automotive engineers in North America commonly use foot-pounds (ft-lb) for engine torque specifications, while the metric world uses Newton-metres (Nm). Fastener torque specifications often use inch-pounds (in-lb) for small fasteners and Newton-metres for larger ones. Key conversion factors:

  • 1 Nm = 0.7376 ft-lb
  • 1 ft-lb = 1.3558 Nm
  • 1 Nm = 0.1020 kgf-m
  • 1 Nm = 8.851 in-lb

Power from Torque and RPM

Mechanical power is directly related to torque and rotational speed: P = τ × ω, where ω is angular velocity in radians per second (rad/s = 2π × RPM / 60). This relationship is used to determine engine power from dyno test torque measurements, to size motors for conveyor belts, and to calculate the power required to drive pumps, fans, and compressors.

Practical Applications

  • Automotive: Engine torque and horsepower ratings, wheel torque.
  • Fastening: Bolt torque specifications to achieve correct clamping force.
  • Robotics: Motor selection based on required joint torque.
  • Structural engineering: Moment calculations in beam and frame analysis.

Frequently Asked Questions

What is the difference between torque and power?
Torque measures the rotational force available at any given instant, independent of speed. Power measures how quickly work is done — it is torque multiplied by angular velocity. An engine can have high torque at low RPM (diesel engines) or lower torque at high RPM (petrol engines), but both can produce similar power at their respective operating speeds.
How do I convert foot-pounds to Newton-metres?
Multiply foot-pounds by 1.35582 to get Newton-metres. For example, 100 ft-lb = 135.58 Nm. To go the other way, multiply Nm by 0.73756.
Why does a longer wrench provide more torque?
Torque equals force times moment arm (τ = F × d). Doubling the wrench length doubles the moment arm, so for the same applied force you produce twice the torque. This is why breaker bars are longer than standard ratchet handles.
What is 1 horsepower in Watts?
One mechanical horsepower equals 745.7 Watts. One metric horsepower (PS, used in Europe) equals 735.5 Watts. When comparing international engine specifications, be aware of which definition is being used.
How is torque different from energy?
Both torque (Nm) and energy (Joules = Nm) have the same dimensional units, but they are conceptually different. Torque is a vector (it has a direction of rotation) while energy is a scalar. Torque does not involve motion — energy is the result of a torque acting through a rotation angle.