Engine Torque Calculator – Power, RPM, Gearbox and Shaft Stress in One Tool
Torque and power are at the heart of every engine and rotating machine, but the relationships between them can be confusing. This Engine Torque Calculator brings together seven practical modes so you can move between horsepower, kilowatts, torque, RPM, gearbox ratios and shaft stresses without juggling separate tools or formulas.
Whether you are tuning an engine, checking dyno numbers, sizing a shaft or estimating wheel torque in a particular gear, the calculator uses standard engineering formulas behind the scenes and presents the results in clear, consistent units.
1. Torque from Power and RPM
The first tab lets you start from power and engine speed to find torque. You enter power in either horsepower or kilowatts, along with RPM and preferred decimal precision. Internally, the calculator uses:
- Imperial: HP = torque(lb-ft) × RPM ÷ 5252
- SI: P(kW) = torque(N·m) × RPM ÷ 9550
It then reports torque in both N·m and lb-ft, plus power in both kW and hp so you can cross-check your inputs and outputs.
2. Power from Torque and RPM
The second tab reverses the calculation. You enter torque (in N·m or lb-ft) and RPM, and the tool outputs power in kW and hp. This is useful for estimating engine output from torque curves or for sanity-checking published performance data.
3. Torque–Power–RPM Triangle Solver
The triangle tab is a compact solver for the core relationship P(kW) = T(N·m) × RPM ÷ 9550. You provide any two of:
- Torque (N·m)
- Power (kW)
- Rotational speed (RPM)
The calculator identifies the missing variable and solves for it. This mode is ideal for engineering students and quick design checks.
4. Torque from Force and Radius
Sometimes it is easier to think in terms of a force applied at a radius, such as a wrench on a nut or a force at the wheel radius. The torque tab uses the fundamental formula T = F × r and supports:
- Force in N or lbf
- Radius in meters or feet
Results are reported in N·m and lb-ft, making it simple to compare with engine torque values or tool specifications.
5. Shaft Shear Stress – Solid and Hollow Circular Shafts
For mechanical design, it is not enough to know torque; you also need to know how much stress that torque produces in a shaft. The shaft tab lets you choose between:
- Solid circular shaft: J = π r⁴ / 2
- Hollow circular shaft: J = π (D⁴ − d⁴) / 32
You enter torque in N·m and dimensions in millimeters (outer and inner diameter for hollow). The calculator converts torque to N·mm, computes J and returns maximum torsional shear stress at the outer surface in MPa, along with the polar moment of inertia.
6. Torque from Work and Angle of Rotation
When you know how much work is done over a certain angle, you can infer the average torque using W = T × θ. In this mode you enter:
- Work or energy (J or ft·lb)
- Angle of rotation (degrees or radians)
The calculator converts the angle to radians and outputs torque in N·m and lb-ft. This is a useful way to relate energy storage, such as in torsion springs, to torque.
7. Gearbox Output Torque and Wheel Force
The final tab focuses on drivetrains. Given engine torque, gear ratio, final drive ratio and drivetrain efficiency, it estimates:
- Gearbox output torque
- Wheel torque (after final drive)
- Wheel contact force (if you provide wheel radius)
You can use this tab to compare how different gears or final drive ratios change wheel torque and off-the-line traction.
How to Use This Engine Torque Calculator Effectively
- Use the power–torque tabs to interpret dyno charts and spec sheets.
- Use the triangle solver when you know any two of power, torque and RPM.
- Use the force–radius tab for simple torque problems and tool sizing.
- Use the shaft stress tab for preliminary mechanical design and checks.
- Use the gearbox tab to understand how gearing multiplies torque to the wheels.
The calculator is meant for learning and quick engineering estimates. For critical components or high-risk applications, always include appropriate safety factors and check against relevant design standards.
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Engine Torque & Power FAQs
Frequently Asked Questions About Engine Torque, Horsepower and Gear Torque
Quick answers to common questions before you size components or interpret dyno data.
Power measures how quickly work is done. For rotating systems, power is proportional to torque multiplied by rotational speed. If torque stays roughly constant while RPM increases, the engine does more work per unit time, and power increases accordingly.
Neither is “better” in isolation. Torque reflects the twisting force available, while horsepower reflects how quickly that torque is applied. Vehicles with high torque at low RPM often feel strong off the line, while high horsepower at high RPM is important for top speed and sustained acceleration.
Yes. The basic relationships between torque, power and RPM apply to any rotating machine, including electric motors and industrial drives. Just make sure you use appropriate units and nameplate ratings for your specific equipment.
Appropriate safety factors depend on material, loading type, fatigue requirements and applicable design codes. This calculator provides stress estimates, but choosing safety factors should follow your organization’s standards or published mechanical design guidelines.