How to use this calculator
Use indicated power and crankshaft brake power from the same engine at the same speed, load and stabilized test condition. Record the measurement method and accessory configuration alongside the result. Indicated power represents cylinder work per unit time; brake power is shaft output. An advertised horsepower rating or a wheel dyno reading cannot supply both quantities. This tool compares supplied measurements; it does not reconstruct cylinder pressure data.
Positive-power, nonnegative-loss accounting only: idle, motoring, externally assisted operation and negative residuals are outside this tool. The difference is often called friction power, but its contents depend on the indicated-work and accessory boundaries. It is not a measurement of bearing friction alone, fuel conversion efficiency, engine condition or drivetrain loss. A plausible ratio does not validate the underlying measurements.
The formula
A worked example
Suppose a matched test gives 100 kW indicated power and 80 kW brake power. Efficiency = 100 × 80 ÷ 100 = 80%. The difference is 20 kW, or 20% of indicated power. With a second hypothetical test of 120 kW indicated and 90 kW brake power, the result is 75% and 30 kW. A larger brake output alone therefore does not establish a higher mechanical efficiency.
Matched-power examples
Original hypothetical pairs in kW. Each row is a separate test, not a prediction for an engine.
| Indicated power | Brake power | Efficiency | Difference |
|---|---|---|---|
| 100 kW | 70 kW | 70% | 30 kW |
| 100 kW | 80 kW | 80% | 20 kW |
| 120 kW | 90 kW | 75% | 30 kW |
Common questions
What is the difference between indicated power and brake power?
Indicated power comes from work inside the cylinders. Brake power is the output available at the engine shaft. Enter a matched pair, rather than maximum values taken from different engine speeds.
Does the difference include pumping and accessories?
That depends on the definitions used in the test. Accounting based on compression and expansion work can include gas-exchange pumping, rubbing friction and accessories in the residual. If pumping is already included in the indicated-work measurement, do not subtract it again. Document the boundary before comparing results.
Can I use wheel horsepower instead of brake horsepower?
Not for this engine-only calculation. A wheel measurement adds transmission and other vehicle effects between the crankshaft and the measurement point. The calculator cannot separate those effects from the engine power difference.
Is this the same as brake thermal efficiency?
No. Mechanical efficiency compares brake output with indicated cylinder power. Brake thermal efficiency compares brake output with fuel energy input. Use the related thermal-efficiency tool when your inputs are BSFC and fuel heating value.
Why are zero and negative inputs rejected?
This calculator is scoped to positive engine output. Division by zero is undefined, and motoring or idle measurements need a different interpretation. The equality case returns 100% with zero difference as a mathematical boundary, not a realistic performance claim.
Can I mix kW and horsepower?
No. Choose one unit and express both numbers in it before calculating. A unit mismatch changes the ratio and may still produce a seemingly reasonable result. The selector does not perform a conversion.
How should I compare repeated tests?
Keep speed, load, temperature, measurement method and accessory configuration consistent. Keep the original power readings and their uncertainty. Small changes in a ratio may reflect measurement variation; this tool supplies neither uncertainty propagation nor a pass/fail diagnostic threshold.
Sources & calculation method
Sathyabama: Thermal Systems, Unit 4, power and mechanical efficiency defines the brake-to-indicated power ratio and their difference. National Research Council: SI Engine Definitions and Efficiency Fundamentals describes the pumping, rubbing and accessory components of a total friction accounting boundary. The calculator applies the ratio to user-supplied measurements; its examples are original arithmetic, not measured engine data.
For a different energy boundary, use Brake Thermal Efficiency. Use Horsepower & Torque to convert measured shaft torque and speed to power, or BMEP to relate torque to engine displacement.
References accessed October 11, 2026. No independent professional review or endorsement is claimed.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.