Torque Calculator

How to use this calculator

1. Enter any two valuesTorque and force gives the radius; force and radius gives the torque; torque and radius gives the force. Leave the unknown at zero.
2. Watch lb-ft against lb-inThey differ by a factor of twelve. Small fasteners are often specified in lb-in, and reading one as the other will badly over- or under-tighten.
3. Measure the arm from the axisThe radius is the perpendicular distance from the centre of rotation to where the force acts — the centre of the fastener to your hand on the spanner.
4. Add rpm for powerTorque alone does not describe an engine or motor. Power is torque times rotational speed, which is why the two peak at different revs.

Why the arm length does the work

Torque is the rotational equivalent of force, and it depends on where the force is applied as much as how hard it is applied.

The relationship is a straight product: torque equals force times the perpendicular distance from the axis. That means a small force at a long distance produces the same torque as a large force close in.

This is why a breaker bar works. Doubling the length of the spanner halves the force needed for the same torque. A seized wheel nut that will not shift with a 30 cm wrench moves readily with a 60 cm bar, using the same effort.

It is also why an extension bar changes what a torque wrench reads. A torque wrench measures at its own handle position; adding a crow-foot or an offset extension that lengthens the effective arm means the fastener receives more torque than the wrench indicates. Adding an extension in line with the socket, which does not change the arm length, has no effect.

The word perpendicular matters. Force must act at right angles to the arm to contribute fully. Pulling at an angle contributes only the perpendicular component, reduced by the cosine of that angle — so pulling at 60° to the spanner delivers only half the torque of the same force pulled squarely. That is a real effect in confined engine bays where a clean pull is not available.

Units reflect the product directly: newton-metres, pound-feet, kilogram-force-metres. All are a force unit multiplied by a length unit, which is why they convert straightforwardly.

One genuine confusion worth naming: torque and energy have the same dimensions, and a newton-metre is dimensionally identical to a joule. They are not the same physical quantity, which is why torque is always written N·m and never J.

Worked example: 200 N at 300 mm

Convert to SI first. The force is already in newtons, and 300 mm is 0.3 m.

Torque: 200 × 0.3 = 60 N·m.

In other units that is 60 ÷ 1.355818 = 44.25 lb-ft, or 531 lb-in, or 6.12 kgf·m.

Notice the lb-ft and lb-in figures. They are the same torque expressed twice, differing by exactly twelve. A fastener specified at 44 lb-in is not the same as one at 44 lb-ft — the second is twelve times tighter, and applying it would likely shear the bolt.

Now the arm length effect. To achieve the same 60 N·m with a 150 mm spanner instead, the force needed is 60 ÷ 0.15 = 400 N — double, as expected when the arm halves. With a 600 mm bar it drops to 100 N.

For context, 200 N is roughly the weight of a 20 kg mass, so this is a firm two-handed pull rather than anything extreme.

Now add rotation. At 3,000 rpm, angular velocity is 2π × 3,000 ÷ 60 = 314.16 radians per second.

Power: 60 × 314.16 = 18,850 W, or 18.85 kW, which is 25.3 horsepower.

That relationship explains something about engines that puzzles people. Peak torque and peak power occur at different revs because power is the product of torque and speed. An engine can be producing less torque at high revs and still more power, because the speed term has grown faster than the torque term has fallen.

Torque units and conversions

The kgf·m row catches people out because the number looks small — 6 kgf·m sounds modest next to 60 N·m, but they are the same torque. Older Japanese motorcycle and car manuals use it routinely, often alongside N·m in brackets. When a manual gives two figures that differ by roughly a factor of ten, that is usually what you are looking at rather than an error.

Torque wrenches and getting fasteners right

Torque specifications exist because bolt tension matters and cannot be judged by feel. Under-tighten and the joint works loose; over-tighten and you yield the bolt, strip the thread or distort what you are clamping.

A click-type torque wrench should be set, used, and then wound back to its lowest setting for storage — leaving the spring compressed for long periods affects calibration. It should also be recalibrated periodically, particularly if it has been dropped.

Use a torque wrench only for final tightening, never for loosening. Loosening a seized fastener can apply far more than the wrench's rated range and damages the mechanism.

Approach the setting in one smooth pull and stop at the click. Repeatedly clicking a fastener adds torque each time, because static friction has been broken and the same setting now turns it further.

The specification usually assumes a particular thread condition. A dry specification applied to an oiled thread produces substantially more clamping force than intended, because lubrication reduces friction and more of the applied torque goes into stretching the bolt. This is a common cause of over-tightening, and it is why manuals specify whether threads should be dry, lightly oiled or treated with a compound.

Tightening sequence matters on anything with multiple fasteners. Cylinder heads, wheels and flanges are tightened in a crossing or spiral pattern and often in stages — a proportion of final torque on every fastener, then another pass — so the clamped component is drawn down evenly rather than distorted.

Angle-tightening, common on modern engines, specifies an initial torque followed by a further rotation of a stated number of degrees. This takes the bolt into its plastic range deliberately, giving a more consistent clamp load than torque alone, and such bolts are usually single-use.

Wheel nuts deserve particular attention. They should be checked after around 50 miles following any wheel removal, because seating settles and the initial torque relaxes.

Assumptions and limitations

• Assumes the force acts perpendicular to the arm — an angled force contributes only its perpendicular component
• Does not account for friction, which determines how much applied torque becomes bolt tension
• Does not convert torque into clamping force, which depends on thread form, pitch and lubrication
• Power conversion assumes constant torque at the stated speed
• Does not model angle-tightening or torque-to-yield fasteners
• Does not account for the effect of crow-foot or offset extensions on a torque wrench reading
• Always use the manufacturer's specification and stated thread condition

Questions people ask about this in United Kingdom