How to use this calculator
- Choose the inertia model. Pick solid disk, thick ring or thin rim depending on how the mass is distributed.
- Enter mass and diameter. Enter total rotating mass and outside diameter. For a ring, enter the inner diameter too.
- Enter RPM. Use the operating speed for the stored-energy check.
- Read energy and rim speed. Read stored energy, mass moment of inertia, rim speed and angular speed.
How it works
A flywheel stores rotational kinetic energy:
E = 1/2 · I · ω²
where I is the mass moment of inertia and
ω = 2πn/60 converts RPM to rad/s. The inertia model depends on mass
distribution: a solid disk uses I = 1/2mr², a thick ring uses
I = 1/2m(ro² + ri²), and a thin rim uses I = mr².
Worked example
Verified against the live calculator
A 10 kg, 400 mm solid disk at
1000 RPM has I = 0.2 kg·m² and stores about
1.10 kJ. With the same mass concentrated as a thin rim, inertia and
energy double.
Frequently asked questions
How do you calculate flywheel energy?
Rotational kinetic energy is E = 1/2·I·ω², where I is mass moment of inertia and ω is angular speed in radians per second. Convert RPM with ω = 2π·RPM/60.
Why does a rim flywheel store more energy than a disk?
Energy depends on mass moment of inertia. Moving the same mass farther from the center increases I, so a rim-heavy flywheel stores more energy at the same RPM.
Does this check flywheel burst speed?
No. This calculator estimates energy, inertia and rim speed only. Flywheel stress, burst speed, shaft attachment, balance and guarding need separate design checks.
What is rim speed?
Rim speed is tangential speed at the outside radius: v = ωr. It is a useful screening value because flywheel stress rises with speed.
Method & assumptions
- Uses simple axisymmetric inertia models; spokes, hubs and shafts should be added as separate inertia terms for final work.
- Energy scales with RPM squared, so doubling speed quadruples stored energy.
- This does not check hoop stress, burst speed, material defects, balance, keying, guarding or overspeed safety factors.