Calculate drag force, density, area, velocity, or drag coefficient from the drag equation with SI and imperial unit conversions for fluids and objects.

Enter the known values to solve the drag equation.

Use speed relative to the fluid. The reference area must match the drag coefficient’s area convention.

Drag Equation Formula

The drag equation estimates the resistive force on an object moving through a fluid, such as air or water. The calculator uses SI units internally, then converts the result back to the unit you selected.

Fd = 0.5 × rho × A × v² × Cd
rho = (2 × Fd) / (A × v² × Cd)
A = (2 × Fd) / (rho × v² × Cd)
v = sqrt((2 × Fd) / (rho × A × Cd))
Cd = (2 × Fd) / (rho × A × v²)
  • Fd = force of drag, usually in newtons (N)
  • rho = fluid density, usually in kilograms per cubic meter (kg/m³)
  • A = reference area matching the drag coefficient’s area convention, usually in square meters (m²)
  • v = object velocity relative to the fluid, usually in meters per second (m/s)
  • Cd = drag coefficient, unitless

Select the value to solve for, enter the other four values, and choose the result unit when available. Because velocity is squared, doubling velocity makes the drag force four times larger if the other values stay the same.

Common Drag Coefficients and Fluid Densities

Use these values as rough references when you do not have measured data. Actual drag coefficients depend on shape, surface texture, orientation, and flow conditions.

Object or shape Typical drag coefficient, Cd Notes
Streamlined body 0.04 to 0.10 Low drag shape, such as an airfoil-like form
Modern car 0.25 to 0.35 Depends on vehicle shape and frontal area
Sphere About 0.47 Common approximation for a ball
Flat plate facing flow About 1.17 High drag because it blocks the flow directly
Cube About 1.05 Approximate value for broadside flow
Fluid Typical density SI value to enter
Air at sea level About 1.225 kg/m³ 1.225 kg/m³
Fresh water About 1000 kg/m³ 1000 kg/m³
Seawater About 1025 kg/m³ 1025 kg/m³
Engine oil About 850 to 900 kg/m³ 850 to 900 kg/m³

Example Drag Equation Calculations

Example 1: Calculate drag force

You have an object moving through air with these values:

  • Density: 1.225 kg/m³
  • Area: 0.50 m²
  • Velocity: 20 m/s
  • Drag coefficient: 0.80
Fd = 0.5 × 1.225 × 0.50 × 20² × 0.80
Fd = 98 N

The drag force is 98 N.

Example 2: Calculate velocity

An object has a drag force of 50 N in air. The density is 1.225 kg/m³, the area is 0.30 m², and the drag coefficient is 0.70.

v = sqrt((2 × 50) / (1.225 × 0.30 × 0.70))
v = 19.72 m / s

The velocity is about 19.72 m/s.

FAQ

Why does velocity have such a large effect on drag?

Velocity is squared in the drag equation. If velocity doubles, drag force becomes four times larger. If velocity triples, drag force becomes nine times larger, assuming density, area, and drag coefficient stay the same.

What area should you use in the drag equation?

Use the reference area for which the drag coefficient is defined. Cars and spheres commonly use projected frontal area; airfoils may use wing planform area. The area and coefficient must use the same convention.

Is the drag coefficient always constant?

No. The drag coefficient can change with shape, surface roughness, angle, Reynolds number, and flow speed. For many basic calculations, you can use a typical value, but measured or tested values are better for precise work.