Case 1
- Fluid density
- 1.225kg/m³
- Speed through the fluid
- 27.78m/s
- Drag coefficient (Cd)
- 0.3
- Frontal area
- 2.2m²
311.9712N
Open with these valuesDrag Force Calculator
311.9712N
Result: 311.9712 NDrag grows with the square of speed, so a car that fights 312 N at 100 km/h fights 647 N at 144 km/h. Enter the fluid density, the speed in metres per second, the drag coefficient of the shape and the frontal area. The coefficient is a wind-tunnel measurement, not a constant.
The numbers at a glance
Held fixed: Fluid density 1.225 kg/m³, Speed through the fluid 27.780 m/s, Drag coefficient (Cd) 0.300.
| Frontal area (m²) | Result (N) |
|---|---|
| 1.0000 | 141.8051 |
| 2.0000 | 283.6102 |
| 2.2000Your value | 311.9712 |
| 3.0000 | 425.4153 |
| 4.0000 | 567.2204 |
Worked examples
Case 2
- Fluid density
- 1.225kg/m³
- Speed through the fluid
- 40m/s
- Drag coefficient (Cd)
- 0.3
- Frontal area
- 2.2m²
646.8000N
Open with these valuesCase 3
- Fluid density
- 1.225kg/m³
- Speed through the fluid
- 10m/s
- Drag coefficient (Cd)
- 0.47
- Frontal area
- 0.01m²
0.2879N
Open with these valuesHow it's calculated
F = ½ × ρ × v² × Cd × A
- StepEnter the fluid density: 1.225 for air at sea level, about 1000 for water.
- StepEnter the speed in metres per second — divide km/h by 3.6.
- StepEnter the drag coefficient of the shape and the frontal area facing the flow.
- ResultRead the drag force in newtons.
Reference table
| Density, speed, Cd, area | What it is | Drag force |
|---|---|---|
| 1, 1, 1, 1 | The equation stripped to its factors | 0.5 |
| 1.225, 10, 0.47, 0.01 | A smooth sphere at 10 m/s | 0.287875 |
| 1.225, 27.78, 0.3, 2.2 | A car at about 100 km/h | 311.9712 |
| 1.225, 30, 0.5, 1.5 | A bluff body at 30 m/s | 413.4375 |
| 1.225, 40, 0.3, 2.2 | The same car at 144 km/h | 646.8 |
| 1000, 5, 0.04, 1 | A streamlined hull in water | 500 |
Questions
How do I calculate drag force?
Use F = ½ × ρ × v² × Cd × A: half the fluid density, times the speed squared, times the drag coefficient, times the frontal area. A car with ρ = 1.225, v = 27.78 m/s, Cd = 0.3 and A = 2.2 m² meets about 312 N of drag.
Why does drag grow with the square of speed?
Because the speed is squared in the equation. Doubling the speed multiplies the force by four and tripling it by nine, which is why fuel use climbs so steeply at motorway speeds.
What is the drag coefficient?
A dimensionless number for how streamlined a shape is, measured in a wind tunnel rather than derived. A smooth sphere in turbulent flow is about 0.47, a rough one 0.81, a modern car 0.25 to 0.3. The same car measured in a different tunnel can come out up to 5 per cent apart, so treat it as a measurement with spread.
What air density should I use?
About 1.225 kg/m³ for air at sea level and 15 °C, the standard-atmosphere value. It falls with altitude and rises in cold weather — 1.2923 at 0 °C, 1.1839 at 25 °C. For water use roughly 1000 for fresh and 1025 for salt.
Which units does this use?
Kilograms per cubic metre, metres per second and square metres, which gives newtons. The drag coefficient has no unit. If your speed is in km/h, divide by 3.6 first.
Sources and last check
Information, not professional advice.