Torque Units Demystified: Converting N·m to lb-ft, oz-in, and Beyond for Motor Applications

Why Torque Unit Conversion Matters More Than You Think

If you've ever specced a micro servo from a Japanese datasheet (torque in $\text{kg·cm}$), then tried to compare it against an American motor rated in $\text{oz·in}$, you know the pain. Torque is torque — it's always force times distance — but the sheer number of unit systems floating around motor datasheets can turn a five-minute design check into a frustrating unit-conversion rabbit hole.

Whether you're sizing a stepper for a CNC gantry, selecting a servo for a robotic gripper, or verifying that an automotive starter can crank a cold engine, getting the units right is non-negotiable. One misplaced decimal and your motor either stalls under load or you've over-specced by a factor of ten and blown your BOM budget.

That's exactly why we built the open the Torque Unit Converter — a quick, reliable way to jump between every torque unit you'll encounter in practice.

The Fundamental Relationship

Torque ($\tau$) is defined as:

where $F$ is the applied force and $r$ is the moment arm (perpendicular distance from the axis of rotation). The SI unit is the newton-metre ($\text{N·m}$), but in practice you'll encounter at least half a dozen others:

The key conversion factors from $1\;\text{N·m}$:

Worked Example: Sizing a Stepper Motor

Let's say you're designing a lead-screw linear stage. Your mechanical analysis tells you the stepper needs to deliver at least $1\;\text{N·m}$ of holding torque. You've found a promising NEMA 23 motor on a US distributor's site, but its torque is listed as $125\;\text{oz·in}$. Is it enough?

Convert the requirement to $\text{oz·in}$:

The candidate motor delivers $125\;\text{oz·in}$, which is only about $0.883\;\text{N·m}$. That's roughly 12% below your requirement — not enough margin, especially considering torque roll-off at speed. You'd need to step up to the next frame size or choose a geared variant.

This is exactly the kind of quick sanity check the converter handles instantly. Plug in $1\;\text{N·m}$, read off $141.6\;\text{oz·in}$, and make your go/no-go decision in seconds.

Real-World Ranges: From Micro Servos to Starter Motors

To give you a feel for the numbers, here's how common motor types map across units:

Micro servo ($0.05\;\text{N·m}$)

• $0.0369\;\text{lb·ft}$
• $0.443\;\text{lb·in}$
• $7.08\;\text{oz·in}$
• $0.510\;\text{kg·cm}$
• Typical of SG90-class hobby servos. When an RC datasheet says "$0.5\;\text{kg·cm}$," this is what they mean.

Small DC motor ($0.5\;\text{N·m}$)

• $0.369\;\text{lb·ft}$
• $4.43\;\text{lb·in}$
• $70.8\;\text{oz·in}$
• $5.10\;\text{kg·cm}$
• Common in power tool triggers, small pumps, and drone gimbals.

Stepper motor ($1\;\text{N·m}$)

• $0.738\;\text{lb·ft}$
• $8.85\;\text{lb·in}$
• $141.6\;\text{oz·in}$
• $10.2\;\text{kg·cm}$
• NEMA 17 to NEMA 23 range — the workhorses of 3D printers and CNC machines.

Automotive starter ($50\;\text{N·m}$)

• $36.88\;\text{lb·ft}$
• $442.5\;\text{lb·in}$
• $7{,}080.6\;\text{oz·in}$
• $509.9\;\text{kg·cm}$
• $5.10\;\text{kg·m}$
• $5 \times 10^8\;\text{dyn·cm}$
• Peak cranking torque for a typical 4-cylinder gasoline engine.

Common Pitfalls

kg·cm is not an SI unit. Strictly, kilogram-force-centimetres ($\text{kgf·cm}$) uses the kilogram as a force unit ($1\;\text{kgf} = 9.80665\;\text{N}$). Most servo datasheets drop the "f" and just write $\text{kg·cm}$. Don't confuse this with a mass-times-distance quantity — it's a gravitational force unit. oz·in vs. oz·ft. In hobby servo land, ounce-inches ($\text{oz·in}$) is standard. Ounce-feet is rarely used but occasionally appears. Always double-check the denominator. Static vs. dynamic torque. Unit conversion won't save you if you're comparing a stepper's holding torque (static) with a DC motor's rated torque (continuous dynamic). Make sure you're comparing like with like before converting. Dyne-centimetres at scale. Because $1\;\text{N·m} = 10^7\;\text{dyn·cm}$, even modest torques produce enormous numbers in CGS. This unit is mostly relevant in MEMS and micro-electromechanical contexts where torques are on the order of $10^{-6}$ to $10^{-3}\;\text{dyn·cm}$.

When to Reach for the Converter

• Cross-referencing datasheets from different regions or manufacturers
• Translating mechanical analysis results (typically in $\text{N·m}$) into vendor-spec units
• Sanity-checking hand calculations during design reviews
• Preparing documentation for international teams where unit preferences differ

Try It

Next time you're staring at a motor datasheet wondering whether $72\;\text{oz·in}$ is enough for your application, skip the mental arithmetic and open the Torque Unit Converter. Punch in your value in any unit, get every other unit back instantly, and get on with the actual engineering. It's free, runs in your browser, and doesn't require a sign-up.