A figure skater pulls her arms in and spins into a blur — same angular momentum, smaller radius, furious speed. The quantity that resists that spin-up is the moment of inertia: for a point mass, mass times radius squared, I = m × r². Double the distance from the axis and the inertia quadruples, which is why the skater's outstretched arms are so hard to get moving and so easy to fold in.

Flywheels exploit this ruthlessly. A heavy rim at a large radius stores enormous rotational energy — old steam engines, pottery wheels, and modern grid-scale flywheel batteries all bank energy in spinning inertia. Car designers fight it instead: lighter wheels with mass near the hub accelerate faster because less inertia stands between the engine and the road.

This calculator evaluates the point-mass formula. Type the mass in the Variable A box — kilograms for metric, slugs for imperial — and the radius squared in the Variable B box (square your radius first: a 0.5 m radius gives 0.25). The Result box gives the moment of inertia in kg·m² or slug·ft². Real shapes use cousins of this formula — ½mr² for a solid disk, ⅔mr² for a hollow sphere — but every one of them scales with mass times radius squared.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Inertia 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.