How to use this calculator
Select km/h or mph and enter vehicle speed. Enter the drag coefficient and the frontal area used to define that coefficient, followed by an air-density assumption. The result isolates the force and mechanical power associated with air drag. Change speed to compare scenarios with the same Cd and area. The defaults are hypothetical inputs, not specifications for a named car.
This is not total road-load power, crankshaft power, battery draw or fuel consumption. Rolling resistance, gradients, acceleration, auxiliaries and drivetrain losses are excluded. Wind and changes in Cd with operating conditions are not modeled. It does not determine safe speed, top speed, vehicle stability or the benefit of a particular modification.
The formula
A worked example
With Cd 0.30, frontal area 2.2 m² and density 1.225 kg/m³, 100 km/h equals 27.7778 m/s. Drag is 311.92 N and power is 8.66 kW (11.62 hp). At 120 km/h with all other inputs held constant, power becomes 14.97 kW. This is a 72.8% increase in aerodynamic power, not a predicted increase in total fuel use.
Speed sensitivity in still air
Same illustrative Cd 0.30, area 2.2 m² and density 1.225 kg/m³.
| Speed (km/h) | Drag force (N) | Drag power (kW) |
|---|---|---|
| 80 | 199.63 | 4.44 |
| 100 | 311.92 | 8.66 |
| 120 | 449.17 | 14.97 |
Common questions
Why does power rise faster than drag force?
With density, Cd and area fixed, drag force rises with speed squared. Power also includes distance traveled per second, so still-air power rises with speed cubed. Doubling speed gives four times the drag force and eight times its power demand in this model.
Is Cd the same as CdA?
No. Cd is dimensionless, while CdA is drag coefficient multiplied by its reference area. This tool accepts Cd and area separately and displays their product. Do not enter CdA in the Cd field and multiply by the area again.
Can I use width times height as frontal area?
That rectangle is only a rough bounding area. Use a frontal projected area compatible with the quoted Cd when available. A coefficient based on a different reference area cannot be combined with an unrelated area without conversion.
Can I add a headwind to vehicle speed?
This tool assumes still air. With wind, force depends on relative air speed but road power uses vehicle speed, so simply adding wind to the speed input gives the wrong power calculation. Use a model that handles both speeds and wind direction.
Does the horsepower result match an engine’s rated horsepower?
No. It is mechanical power associated with aerodynamic resistance at the stated speed. Engine output must also cover other loads and losses. The hp unit here is mechanical horsepower, not metric PS.
Sources & calculation method
NASA Glenn: Drag Equation describes the force relationship and the need to pair a coefficient with its reference area. MathWorks: Longitudinal Vehicle applies aerodynamic resistance to vehicle motion separately from rolling and grade forces. This tool uses the still-air case and derives power as force times speed; the sources do not supply a rating for the entered vehicle.
References accessed September 14, 2026. Sources support the stated method, not an endorsement or independent professional review of AutomotiveCalc.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.