Moment of Inertia Converter

Moment of inertia is a measure of an object's resistance to angular acceleration around a given axis — the rotational equivalent of mass in ordinary (linear) motion. Just as a heavier object needs more force to accelerate at a given rate, an object with a larger moment of inertia needs more torque to achieve the same angular acceleration. It depends not just on an object's mass but on how that mass is distributed relative to the axis of rotation — mass concentrated far from the axis contributes far more to the moment of inertia than the same mass near the axis.

Our Moment of Inertia converter supports 14 units. The SI unit, kilogram square meter (kg·m²), and its metric variations — kilogram square centimeter, kilogram square millimeter, gram square centimeter, and gram square millimeter — are what you'll find in physics coursework and most modern engineering references. Imperial units include pound square foot and pound square inch, which use pound-mass directly, and slug square foot, which uses the slug (the imperial unit of mass, roughly 32.174 pounds-mass) — a distinction that matters because slug-based values come out roughly 32 times larger than pound-based ones for the same physical object.

The converter also includes older "technical" or "gravitational system" units: kilogram-force meter square second, kilogram-force centimeter square second, pound-force foot square second, and pound-force inch square second. These units were built around a system where force (not mass) was the primary defined quantity, with standard gravity (9.80665 m/s²) used to relate force-based units back to mass-based ones — which is why, for example, 1 kilogram-force meter square second equals exactly 9.8066499998 kg·m². You'll still encounter these units in older engineering drawings, legacy technical manuals, and some European engineering traditions.

To convert, select your starting and target units from the dropdown lists and enter a value — the result is calculated instantly, to up to 12 significant digits. For example, converting 1 kilogram square meter to pound square feet gives about 23.73 lb·ft², a common check when comparing a European component specification against a US design standard.

Moment of inertia is one half of the rotational form of Newton's second law — torque equals moment of inertia multiplied by angular acceleration — so a precise moment of inertia figure, correctly converted into the same unit system as your torque and angular acceleration values, is essential before combining them in any hand calculation or simulation input file elsewhere in your workflow.

Moment of inertia conversions come up constantly in mechanical and structural engineering: flywheel and rotor design (how much energy a spinning mass can store), robotics (calculating the torque a motor needs to move a robotic arm through a given angular acceleration), automotive engineering (wheel and crankshaft rotational dynamics), and aerospace engineering (spacecraft attitude control, where moment of inertia about each axis determines how much torque thrusters must apply). Physics and engineering students also use these conversions constantly when relating torque, angular acceleration, and moment of inertia through the rotational form of Newton's second law.

Every conversion factor in this tool is based on precise, standard unit definitions, so results are reliable for coursework, design calculations, and technical documentation. The converter is free, requires no account, and works equally well on desktop and mobile.