Load a pickup with the toolbox behind the cab and it steers straight; hang the same toolbox off the tailgate and the steering goes light and twitchy. The difference is the center of mass — the single point where you can pretend all of an object's weight is concentrated. For a set of point masses it is the weighted average of their positions: multiply each mass by its position, add those products, then divide by the total mass. Heavier objects drag the balance point toward themselves, which is why a 220-pound linebacker and a 150-pound receiver balance a seesaw only when the linebacker sits closer to the pivot.

The stakes are real. Trailer manufacturers print a rule that about 60 percent of cargo weight should sit ahead of the axle, because a trailer whose balance point falls behind its wheels will sway at highway speed. Aircraft pilots run a weight-and-balance sheet before every flight for the same reason. This calculator evaluates the pieces of that weighted average: enter each mass and its distance from your chosen zero point, and use the results to find where everything balances.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Center Mass Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.