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Flywheel Energy Calculator

Work out the rotational kinetic energy stored in a spinning flywheel from its moment of inertia and rotational speed. Flywheels are used to smooth out torque pulses, store short-term energy and carry machinery through the dead spots of a cycle.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter the flywheel's moment of inertia I in kg.m2 (from a CAD model, manufacturer data or a hand calculation).
  2. Enter the rotational speed N in revolutions per minute (rpm).
  3. Read the stored energy in joules and kilojoules, plus the angular velocity in rad/s used in the calculation.

How it works

The angular velocity is omega = 2 x pi x N / 60, converting rpm to radians per second. The stored rotational kinetic energy is E = 0.5 x I x omega^2. Because energy scales with the square of speed, doubling the rpm quadruples the stored energy, which is why flywheel speed is the dominant design lever.

Worked example

Worked example. A flywheel with a moment of inertia of 10 kg.m2 spinning at 3000 rpm has omega = 2 x pi x 3000 / 60 = 314.16 rad/s, so E = 0.5 x 10 x 314.16^2 = 493480.22 J, or about 493.48 kJ.

Common mistakes

  • Confusing moment of inertia (kg.m2) with mass (kg) — the calculator needs the rotational inertia about the spin axis.
  • Entering speed in rad/s or Hz instead of rpm; this tool converts rpm internally.
  • Assuming the full stored energy is usable — only the energy between the maximum and minimum allowable speeds can actually be extracted.

Frequently asked questions

How do I find the moment of inertia?

For a solid disc I = 0.5 x m x r^2, and for a rim/ring I is closer to m x r^2. Real flywheels are usually taken from CAD mass properties or manufacturer data.

Why does speed matter so much?

Kinetic energy is proportional to the square of angular velocity, so a small increase in rpm gives a large increase in stored energy — but also a large increase in centrifugal stress.

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Tip: Enter any known values to calculate the remaining results.

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