Flywheel Energy Calculator

Storing Energy in Spin Instead of Chemistry

A flywheel stores energy the same way a swung hammer does — in the motion of a rotating mass — and releases it by slowing down. Unlike a battery, that energy has no chemistry to degrade, which is exactly why flywheels show up anywhere a system needs to smooth out short, sharp power demands: punch presses, engines, and increasingly, grid-scale energy storage.

The Formulas

KE = 0.5 × I × ω², where ω = 2 × π × N / 60
Solid Disk: I = 0.5 × m × r²

I is the moment of inertia, ω is angular velocity in radians per second, and N is rotational speed in rpm. For a solid disk flywheel, moment of inertia can be derived directly from mass and radius rather than measured separately.

Worked Examples

Stored kinetic energy by input method
Method	Inputs	Moment of Inertia	Stored Energy
From Inertia	I = 2.5 kg·m² at 1,200 rpm	2.5 kg·m² (given)	19,739.21 J
From Solid Disk	50 kg, 0.3 m radius, 1,800 rpm	2.25 kg·m²	39,971.90 J

Stored energy scales with the square of rotational speed — doubling a flywheel's rpm quadruples the energy it stores for the same moment of inertia, which is why speeding up a flywheel is a far more effective way to add capacity than adding mass.

Where This Calculation Matters

Energy storage sizing — determining how much usable energy a flywheel can deliver between a maximum and minimum operating speed.
Punch press and stamping equipment — sizing a flywheel to supply the brief energy spike a press stroke demands without stalling the drive motor.
Engine and drivetrain smoothing — evaluating how much a flywheel reduces speed fluctuation between power strokes.

How to Use This Calculator

Choose the mode: From Moment of Inertia or From Solid Disk (Mass + Radius).
For the inertia method, enter Moment of Inertia (kg.m^2) and Rotational Speed (RPM).
For the disk method, enter Mass (kg), Radius (m), and Rotational Speed (RPM).
Select Calculate to get the stored kinetic energy.

Related Calculations

To relate flywheel speed to the torque it can deliver during a discharge, see the Power Transmission Calculator.