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Motor

Torque Unit Converter

Convert torque between Newton-metres, pound-feet, pound-inches, oz·in, kg·cm and dyne·cm — covers all motor datasheet units.

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Formula

1N⋅m=0.7376lb⋅ft=8.851lb⋅in=141.6oz⋅in1 N·m = 0.7376 lb·ft = 8.851 lb·in = 141.6 oz·in

How It Works

This calculator converts torque between engineering unit systems including N·m, lb·ft, lb·in, kgf·cm, and oz·in. Mechanical engineers, automotive technicians, and international procurement specialists use it to compare motor specifications across different regional standards. Unit conversion errors cause torque mismatches—a common source of motor undersizing in cross-border projects.

Per NIST SP 811 (Guide for Use of SI Units), the SI unit of torque is the newton-metre (N·m), which equals 1 kg·m²/s². Common conversion factors are exact mathematical relationships: 1 N·m = 0.737562 lb·ft = 8.85075 lb·in = 10.1972 kgf·cm = 141.612 oz·in. The lb·ft unit dominates US automotive applications, kgf·cm appears in Asian servo datasheets, and oz·in is standard for US hobby servos and small motors.

Distinguishing torque from energy is critical: both have SI dimensions of N·m, but torque is a vector (rotational force) while energy is a scalar (work done). Per physics conventions, 1 N·m of torque applied through 1 radian produces 1 joule of work. Confusing these units leads to dimensional errors in power calculations: Power (W) = Torque (N·m) × Angular velocity (rad/s), not Torque × RPM (which has wrong dimensions).

Worked Example

A Japanese servo motor datasheet specifies 35 kgf·cm stall torque. Convert to N·m, lb·in, and oz·in for US and European design teams.

Step 1 — Convert kgf·cm to N·m: 1 kgf = 9.80665 N (exact per ISO 80000-4) 1 kgf·cm = 9.80665 N × 0.01 m = 0.0980665 N·m 35 kgf·cm × 0.0980665 = 3.432 N·m

Step 2 — Convert N·m to lb·ft: 1 N·m = 0.737562 lb·ft 3.432 × 0.737562 = 2.531 lb·ft

Step 3 — Convert N·m to lb·in: 1 N·m = 8.85075 lb·in 3.432 × 8.85075 = 30.37 lb·in

Step 4 — Convert N·m to oz·in: 1 N·m = 141.612 oz·in 3.432 × 141.612 = 486.0 oz·in

Step 5 — Verify against design requirement: US spec requires 25 lb·in minimum holding torque Servo provides 30.37 lb·in—meets requirement with 21% margin

Result: 35 kgf·cm = 3.43 N·m = 2.53 lb·ft = 30.4 lb·in = 486 oz·in. The servo exceeds the 25 lb·in requirement, providing adequate margin for dynamic applications.

Practical Tips

  • ✓For quick field estimates per engineering rules of thumb: 1 N·m ≈ 10 kgf·cm (1.8% error) and 1 lb·ft ≈ 1.36 N·m (0.1% error)—accurate enough for initial feasibility checks
  • ✓Per NEMA MG-1, US motor datasheets may list torque in both SI and imperial units; when discrepancies exist, trust the SI value as primary and verify the conversion factor used
  • ✓Standardize on N·m early in multinational projects per ISO recommendations—this eliminates conversion errors and aligns with IEC 60034 motor standards used globally

Common Mistakes

  • ✗Confusing kgf·cm with N·cm: Per SI definitions, 1 kgf = 9.81 N, so kgf·cm is 9.81× larger than N·cm—this order-of-magnitude error causes severe motor undersizing when misread
  • ✗Mixing lb·ft and lb·in without conversion: 1 lb·ft = 12 lb·in; automotive torque specs use lb·ft while small motor specs use lb·in—confusing them causes 12× error in either direction
  • ✗Treating torque and energy as interchangeable: While both have units of N·m, torque×angle gives energy (work done); equating 10 N·m torque to 10 J without angular context is a dimensional error

Frequently Asked Questions

Per NIST historical analysis: Torque units evolved from regional engineering traditions. N·m from SI metric system (international standard since 1960). lb·ft from US/UK gravitational system (automotive, aerospace). lb·in from US small-motor and fastener industries. kgf·cm from pre-SI metric practice, still common in Asian specifications. oz·in from US RC hobby market. IEC 60034 and NEMA MG-1 now specify SI units, but legacy equipment and regional markets perpetuate older units.
Per NIST SP 811: Dimensionally identical (both kg·m²/s²), but semantically different. A joule measures energy (scalar work done). A newton-metre measures torque (vector rotational force at a radius). The distinction matters: 10 N·m torque does not equal 10 J energy unless applied through 1 radian of rotation. Power calculations must use proper dimensional analysis: P(W) = T(N·m) × ω(rad/s), where radians are dimensionless.
Per physics fundamentals: Power (W) = Torque (N·m) × Angular velocity (rad/s). To convert RPM to rad/s: ω = RPM × π/30 = RPM × 0.10472. Example: 5 N·m at 3000 RPM → P = 5 × (3000 × 0.10472) = 5 × 314.16 = 1571 W = 1.57 kW. For imperial units: HP = (Torque_lb·ft × RPM) / 5252. This 5252 constant derives from unit conversions: 33,000 ft·lb/min per HP ÷ 2π × conversion factors.

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