What does a spring scale measure?

Spring scales measure the magnitude of forces, most commonly weight, which is the downward force of gravity on an object. It works from the principle laid down by Isaac Newton in his third law of universal motion, that for every action there is an equal and opposite reaction.

Hooke's Law describes the relationship between the force on a spring and the resulting extension or compression of the spring; the force on the spring is equal to the product of negative 1 times the spring's constant "k" (dependent on the material, thickness, shape, etc.), times the displacement from the springs resting point "x", written as F = -kx.