Coefficient of Friction Calculator

What Is the Coefficient of Friction?

The coefficient of friction (μ) is a dimensionless number that describes how much friction force exists between two surfaces in contact. A higher coefficient means more friction. It has no units because it is a ratio of two forces.

There are two types: static friction (μs) describes the resistance to starting movement, while kinetic friction (μk) describes the resistance during sliding motion. Static friction is always greater than kinetic friction for the same surface pair — it takes more force to start an object sliding than to keep it sliding.

How to Calculate Coefficient of Friction

Coefficient of Friction Formula

Where:

• μ = coefficient of friction (dimensionless)
• F_friction = friction force (N)
• F_normal = normal force (N)

On a flat, level surface, the normal force equals the object's weight: F_normal = mg. So you can also write μ = F_friction / (mg).

Worked Example

You push a 20 kg box across a concrete floor with a steady 100 N of horizontal force. The box moves at constant velocity (zero acceleration), so the friction force must equal the pushing force: 100 N.

μ = 100 / (20 × 9.81) = 100 / 196.2 = 0.51

Static vs. Kinetic Friction

Static friction (μs) prevents an object from starting to move. Kinetic friction (μk) acts on an object that is already sliding. For rubber on dry concrete: μs ≈ 1.0, μk ≈ 0.8. For steel on steel: μs ≈ 0.74, μk ≈ 0.57. This difference is why cars are harder to control once the tires start skidding.