Compute drag using linear or quadratic resistance models. Pick units, presets, and advanced fluid properties. Get force, power, acceleration, and Reynolds number instantly here.
| Scenario | ρ (kg/m³) | v (m/s) | A (m²) | Cd | Drag F (N) |
|---|---|---|---|---|---|
| Small sphere | 1.225 | 20 | 0.030 | 0.47 | 3.45 |
| Cyclist upright | 1.225 | 10 | 0.50 | 0.88 | 26.95 |
| Passenger car | 1.225 | 30 | 2.20 | 0.29 | 351.83 |
Air resistance is the aerodynamic force opposing motion through air. Drag rises with speed because the flow separates and forms a wake. This tool estimates force, power, deceleration, and Reynolds number using consistent unit conversions and two resistance models.
Quadratic drag follows Fd=½ρCdAv². It is widely used for vehicles, sports projectiles, drones, and falling objects because it captures pressure and wake losses. For quick scaling, increasing speed increases drag strongly, so small speed changes can dominate performance.
Linear drag uses Fd=b·v. It can approximate motion in very gentle flows, small-scale experiments, or when you have measured a proportional damping constant. In air, the linear model is usually a convenience model rather than a universal law, so treat it as a fit to data.
The drag coefficient Cd depends on shape, surface roughness, and flow conditions. Streamlined bodies can be near 0.05–0.30, while bluff bodies such as plates or cubes can approach 1.0 or more. Alignment and posture matter because they change separation and wake size.
Air density ρ strongly affects drag. Lower density at higher altitude or warmer temperatures reduces drag force for the same speed and geometry. For quick estimates, sea-level density around 1.2 kg/m³ is common. If you are modeling high-altitude flight or wind-tunnel conditions, use a density value consistent with your scenario.
The power needed to overcome aerodynamic drag is P=Fd·v. With quadratic drag, power grows approximately with v³, which explains why high-speed travel becomes energy intensive. Use the power output to compare design changes like smaller area or improved Cd.
Reynolds number Re=ρvL/μ compares inertial to viscous effects. It helps you judge whether the flow is likely turbulent around a characteristic length L. Provide viscosity μ and a reasonable L (diameter, chord, or width) for a useful estimate.
Terminal speed is a steady value where drag balances weight in vertical motion. Quadratic: vt=√(2mg/(ρCdA)). Linear: vt=mg/b. Treat it as a benchmark because real motion can vary with orientation and density.
For best accuracy, use measured Cd and area, and treat outputs as steady-speed estimates rather than a full trajectory simulation.
Use quadratic drag for most objects moving through air at everyday speeds. Choose linear drag only if your experiment or data suggests force is proportional to speed, or you have a known damping coefficient.
Cd depends on shape and flow. Smooth spheres are often around 0.47, upright cyclists can be near 0.8–1.0, and typical passenger cars may be near 0.25–0.35. Use measurements when possible.
Frontal area is the projected area normal to the airflow. You can approximate it from width × height for a rough estimate, or trace a silhouette photo and scale it for better accuracy.
In quadratic drag, the dynamic pressure scales with v², so aerodynamic force rises quickly as speed increases. Power grows even faster because power equals force multiplied by speed.
Yes. Drag depends on relative airspeed. If you have a headwind, add wind speed to your object speed for the relative value. For a tailwind, subtract it, but keep the relative speed nonnegative.
Choose a length that represents the body scale facing the flow, such as diameter for a ball, chord for a wing, or width for a vehicle. Consistency matters more than perfection for a first estimate.
Reynolds number helps interpret the flow regime and the stability of Cd, but drag force can still be estimated without it if you already have a reasonable Cd for your conditions.