Many people ask us, “What is the difference between a scale and a balance?” Although many people use the words interchangeably, the two are quite different. For history buffs, the balance beam came first, well before the scale, in fact, most will say it is the first mass measurement instrument ever invented.

Typically a balance has a pivoted horizontal lever of equal length arms, called the beam, with a weighing pan, also called scale (that’s why the two words started becoming confused with each other) suspended from each arm. The item to be measured is placed in one pan, and standard masses are added to the other pan until the beam is as close to equilibrium as possible.

In a specific type of balances called precision balances, a slider weight is moved along a graduated scale. You no doubt remember this from a recent visit to your doctor’s office. So even though they probably asked you to step on a scale, you were actually stepping on a balance.

Balances are credited usually with being more accurate than scales, because unlike spring scales their accuracy is not affected by differences in the local gravity, which can vary by almost 0.5%[2] at different locations on Earth. The change in gravity does not affect a balance in that the effect of gravity is felt equally on both the item being measured and the mass at the other end of the balance.

A commercial scale is quite different. A scale works by measuring the difference between the unflexed state and the flexed state of a spring when the spring is stretched due to a weight being applied.

Most commercial scale and commercial balance owners don’t need to know this, but, the equation describing this relationship is:

F = K * (X^2)

where:

F is the force
K is the spring constant, and
X is the diistance the spring is flexed.

When you put an object on a spring scale, the object has a certain force (gravity pulling it toward the earth). Every spring has its own unique spring constant (K) which depends on its manufacture and material. Therefore the spring is the most important element of a commercial scale. A spring scale can be damaged if the spring is pulled too far.