Moment of Inertia Converter

Moment of inertia measures how much an object resists rotational acceleration. It depends on both the mass and how far that mass is from the axis of rotation.

All conversions use kg·m² as the base unit.

Metric units:

• 1 kg·m² = 10,000 g·cm² = 10,000,000 g·mm²
• 1 g·cm² = 0.0001 kg·m²

Imperial units:

• 1 lb·ft² = 0.042140 kg·m²
• 1 lb·in² = 0.00029264 kg·m²
• 1 slug·ft² = 1.35582 kg·m²
• 1 oz·in² = 0.000018290 kg·m²

Cross-system:

• 1 kg·m² = 23.7304 lb·ft²
• 1 kg·m² = 0.73756 slug·ft²

Common examples:

• Bicycle wheel: ~0.1 kg·m²
• Car wheel + tire: ~0.5–1.0 kg·m²
• Flywheel (industrial): 10–1000 kg·m²

Mass moment of inertia is the rotational version of mass: it measures how hard it is to start or stop something spinning. Just as a heavier object is harder to push, an object with more moment of inertia is harder to angularly accelerate. The twist is that it depends not only on how much mass there is, but on how far that mass sits from the axis, so mass out at the rim counts for far more than mass near the center.

That distance dependence explains a lot of design. A flywheel is built with a heavy rim precisely to maximize its moment of inertia and store rotational energy, while a figure skater pulls their arms in to reduce theirs and spin faster. Don’t confuse this with the second moment of area, also loosely called “moment of inertia,” which describes a beam’s stiffness and is measured in m⁴, not kg·m². Same nickname, completely different quantity and units, so always check which one a problem means.