Flywheel Kinetic Energy Calculator

A spinning flywheel stores rotational kinetic energy KE = ½Iω², where ω is the angular velocity in radians per second (rotational speed in RPM × 2π ÷ 60) and I is the moment of inertia — the rotational equivalent of mass, which depends on both total mass and how far that mass sits from the spin axis. I = k × m × r², where k is a fixed coefficient set by the shape: ½ for a solid disk or cylinder, 1 for a thin hoop with all its mass at the rim, ⅖ for a solid sphere, and ⅔ for a thin hollow sphere. Verified via web search against standard rotational-dynamics references rather than relied on from memory.

For the same mass and radius, a hoop stores more energy at a given RPM than a solid disk, and a solid disk more than a solid sphere — because more of a hoop's mass sits far from the axis, while a sphere's mass is concentrated closer to its center. Mass and radius are converted to kilograms and meters (the units these formulas are published in) before either formula runs, then moment of inertia and kinetic energy are converted back to the active unit system for display.