Flywheel Energy Calculator

Multiply half the moment of inertia by the angular velocity squared: E = ½ × I × ω². Use kg·m² for the moment of inertia and rad/s for the angular velocity to get the answer in joules. For example, a flywheel with I = 5 kg·m² spinning at 100 rad/s stores ½ × 5 × 100² = 25,000 J (25 kJ).

Flywheel energy is the rotational kinetic energy a spinning flywheel stores because of its rotation. A flywheel banks energy as it speeds up and releases it as it slows down, which is why it is used to smooth out engines and to store energy. It is measured in joules (J), the same unit used for every other form of energy.

Because the angular velocity is squared in the formula. Energy scales with ω², so doubling the spin speed multiplies the stored energy by 2² = 4, and tripling it multiplies the energy by nine. This is why flywheels are spun as fast as the material safely allows — speed stores far more energy than mass does.

Both raise the stored energy, but not equally. The moment of inertia, set by the mass and how far it sits from the axis, raises the energy in proportion — double it and the energy doubles. The angular velocity is squared, so it has a far stronger effect, which is why high spin speed beats heavy mass for storing energy compactly.

Use SI units: kg·m² for the moment of inertia and radians per second (rad/s) for the angular velocity, which gives the stored energy in joules. If your speed is in rev/s, multiply by 2π to convert to rad/s; for rev/min, multiply by 2π/60. One joule equals one kg·m²/s², and 1000 joules make one kilojoule (kJ).