How this calculation works
Density and speed determine dynamic pressure. Multiply by the coefficient and its matching reference area to find drag magnitude. Speed comparisons explicitly hold those other inputs constant.
Drag = ½ρCdAv²; drag power = drag × relative speedA worked example
At density 1.2 kg/m³, Cd = 1, reference area 0.5 m² and relative speed 10 m/s, drag is 30 N and drag power is 300 W.
Good to know
Cd and reference area must use the same definition. Cd can change with flow conditions and orientation; the comparison assumes it stays fixed. No rolling resistance, lift or propulsion efficiency is included.
Reference: NASA Glenn: the drag equation (opens in a new tab) ↗
Relative speed enters twice
The default values give dynamic pressure ½×1.2×10² = 60 Pa. Multiplying by Cd = 1 and area 0.5 m² gives drag 30 N. At 10 m/s, the associated drag-power magnitude is 300 W.
Holding Cd, density and area constant, doubling relative speed multiplies drag by four and drag power by eight. That cubic power relationship follows from multiplying a speed-squared force by another factor of speed.
The terminal settling speed calculator balances quadratic drag with the remaining downward force after buoyancy.
The area definition belongs to the coefficient
A coefficient based on frontal area cannot be combined casually with a different surface area. Cd and the reference area work together to represent the same measured or modeled drag. Changing one convention changes the numerical coefficient.
The terminal settling speed calculator uses a quadratic-drag balance with weight and buoyancy. This page evaluates drag at a speed you supply; it does not determine which speed a freely falling object will settle toward.
A constant coefficient is a scenario assumption
Cd can change with shape, orientation, Reynolds number and compressibility. The sensitivity table intentionally holds it fixed to isolate one arithmetic relationship. That is useful for understanding the model but does not predict all real operating points.
Use flow speed relative to the object. A headwind changes the relative speed of a ground-moving object; a stationary object can still experience drag in moving air. Rolling resistance, lift and propulsion efficiency are separate effects.
A few questions, answered
Does the result show the direction of drag?
It gives a magnitude. Drag opposes motion relative to the fluid.
Is drag power the motor’s electrical demand?
No. Other forces and propulsion or drivetrain efficiency must also be considered.
Can density be inferred from altitude?
This page takes density directly and does not fetch weather or an atmospheric profile.