Flywheel Energy Calculator

Kinetic energy depends on angular velocity SQUARED (E = 0.5 x I x omega squared), and Outer Radius is ALSO squared inside the moment of inertia -- so per unit of proportional change, RPM and Outer Radius are equally powerful levers, both stronger than mass, which enters only linearly. RPM dominates on THIS calculator not because of a physics asymmetry but because its usable slider range (1 to 100,000 RPM, a 100,000x span) is far wider than Outer Radius's (0.5 to 60 in, a 120x span) -- in practice, rim stress at high RPM, not the energy equation itself, is what actually limits how large a flywheel's radius can get.

Counterintuitively, no -- at the same total mass, increasing the Inner Radius (Bore) INCREASES stored energy. The moment of inertia of a hollow cylinder is I = 0.5 x mass x (Outer Radius squared plus Inner Radius squared), so widening the bore while holding mass fixed pushes the material further from the spin axis on average, which raises moment of inertia and therefore kinetic energy. This is why flywheel designers favor rim-heavy, ring-like shapes over solid disks when they want to maximize energy storage per pound.

Outer Radius enters the moment-of-inertia formula as a SQUARE (Outer Radius squared), so doubling it roughly quadruples the outer-radius contribution to stored energy. Mass, by contrast, is a simple linear multiplier -- doubling mass only doubles energy. For the same percentage increase, growing the outer radius is a much more effective way to add storage capacity than adding mass, though larger radii also increase stress on the rim at high RPM.

Kinetic Energy (ft-lb) and Kinetic Energy (J) are the same quantity in different unit systems, useful for comparing against mechanical torque and work specifications. Energy (Wh) converts that same kinetic energy into watt-hours -- the unit batteries are rated in -- so you can directly compare a flywheel's storage capacity to an equivalent battery pack. All three outputs move together and scale with RPM squared.