About the Drag Force Calculator
This drag force calculator uses the standard drag equation, F = ½ρv²C_dA, to estimate the aerodynamic resistance on a moving object. Enter the speed, the frontal area facing the flow, and a drag coefficient — pick a typical shape such as a car, cyclist, sphere or flat plate, or type your own C_d — and the calculator returns the drag in newtons and pounds-force, the dynamic pressure, and the power needed just to overcome drag at that speed.
It is useful for physics and engineering students, cyclists comparing riding positions, drivers wondering why fuel economy falls on the motorway, drone and model-rocket builders, and anyone estimating terminal velocity. The chart shows how drag grows with the square of speed: double the speed and drag quadruples, while the power needed rises eight-fold.
The drag equation is accurate when the drag coefficient is known for the relevant shape and Reynolds number, which is the regime of everyday objects moving through air or water at moderate speeds. Default air density is 1.225 kg/m³ (sea level, 15 °C); use about 1,000 kg/m³ for fresh water or the air density calculator for other conditions. Real C_d values come from wind-tunnel testing and vary with surface roughness and orientation, so treat the presets as typical values.
With the default inputs, the drag force is 311.92 N. Change any value above to recalculate instantly.
How to use the drag force calculator
- 1Enter the speed relative to the air (include headwind) and choose its unit.
- 2Pick an object type, or choose custom and enter your own drag coefficient.
- 3Enter the frontal area in m² or ft².
- 4Adjust fluid density for altitude, temperature or water if needed.
- 5Read the drag force, dynamic pressure and the power needed to overcome drag.
Formula and method
The drag equation says aerodynamic drag equals the dynamic pressure ½ρv² multiplied by a reference area A and a dimensionless drag coefficient C_d. The coefficient bundles all the effects of shape, surface and flow pattern; for bluff bodies such as a flat plate it is above 1, for a smooth sphere around 0.47 and for a teardrop shape well under 0.1.
Because drag rises with the square of speed, the power needed to keep moving against it, P = F·v, rises with the cube of speed. The calculation assumes steady motion through still fluid and a drag coefficient that does not change with speed, which is a good approximation for most everyday objects outside the transonic range.
- F_d
- Drag force (N)
- ρ
- Fluid density (kg/m³)
- v
- Speed relative to the fluid (m/s)
- C_d
- Drag coefficient (dimensionless)
- A
- Frontal reference area (m²)
- P
- Power to overcome drag (W)
Worked examples
Car at 100 km/h
100 km/h is 27.78 m/s, giving dynamic pressure ½ × 1.225 × 27.78² = 472.6 Pa. Multiplying by C_d 0.30 and 2.2 m² gives about 312 N of drag, which takes 8.7 kW (11.6 hp) to overcome.
Same car at 70 mph
70 mph is 31.29 m/s. Drag rises to about 396 N (89 lbf) and the power to about 12.4 kW (16.6 hp) — only 12.6% faster than 100 km/h but needing 43% more power.
Skydiver in free fall at 55 m/s
At 55 m/s (about 198 km/h) a belly-down skydiver with C_d ≈ 1.0 and 0.7 m² of frontal area feels about 1,297 N of drag — close to the weight of a 132 kg jumper with gear, which is why speed stops increasing near terminal velocity.
Frequently asked questions
What is the drag equation?+
F_d = ½ρv²C_dA: drag equals half the fluid density times speed squared times the drag coefficient times the frontal area. It is the standard way to estimate air or water resistance on cars, aircraft, cyclists and projectiles.
What is a typical drag coefficient?+
Modern cars are about 0.25–0.35, SUVs around 0.35–0.45, a smooth sphere about 0.47, a cube about 1.05, a flat plate facing the flow about 1.28, and a streamlined teardrop shape as low as 0.04.
Why does drag increase with the square of speed?+
At higher speed the object pushes more air aside each second and gives that air more momentum. Both effects scale with velocity, so drag scales with v². The power needed, force times speed, therefore scales with v³.
How does drag relate to terminal velocity?+
A falling object speeds up until drag equals its weight. Setting ½ρv²C_dA = mg and solving gives terminal velocity v = √(2mg / (ρC_dA)). The terminal velocity calculator does this directly.
Does altitude change drag?+
Yes. Drag is proportional to air density, which falls with altitude and rises in cold air. At 2,000 m the air is about 18% less dense than at sea level, so drag at the same speed is about 18% lower.