Overview
Air is the most commonly handled gas in engineering — used in HVAC, pneumatics, compressed air systems, combustion, cooling, and aerodynamics. Dry air is approximately 78% nitrogen, 21% oxygen, and 1% argon by volume, with trace amounts of other gases. It behaves as an ideal gas at standard temperatures and pressures.
Standard Air Conditions
| Standard | Temperature | Pressure | Density | Use |
|---|---|---|---|---|
| ISO (standard) | 15°C / 288.15 K | 101.325 kPa | 1.225 kg/m³ | Aerospace, aerodynamics |
| NTP (normal) | 20°C / 293.15 K | 101.325 kPa | 1.204 kg/m³ | HVAC, general engineering |
| STP (chemistry) | 0°C / 273.15 K | 101.325 kPa | 1.293 kg/m³ | Gas chemistry, physics |
| SATP | 25°C / 298.15 K | 100.000 kPa | 1.184 kg/m³ | Thermodynamics |
Physical Properties (Dry Air at 20°C, 1 atm)
| Property | Value |
|---|---|
| Density | 1.204 kg/m³ (0.0752 lb/ft³) |
| Dynamic Viscosity | 1.81 × 10⁻⁵ Pa·s (0.018 cP) |
| Kinematic Viscosity | 1.51 × 10⁻⁵ m²/s (15.1 cSt) |
| Specific Heat (c_p) | 1005 J/kg·K |
| Specific Heat (c_v) | 718 J/kg·K |
| Ratio of Specific Heats (γ) | 1.40 |
| Gas Constant (R) | 287 J/kg·K |
| Thermal Conductivity | 0.026 W/m·K (good insulator) |
| Speed of Sound | 343 m/s |
| Molecular Weight | 28.97 g/mol |
| Prandtl Number | 0.71 |
Specific Gravity
For gases, specific gravity is referenced to air itself (not water): the ratio of gas density to dry air density at the same conditions. By definition, dry air has SG = 1.000.
| Gas | Specific Gravity (vs air) |
|---|---|
| Air (dry) | 1.000 |
| Methane | 0.554 |
| Natural gas (typical) | 0.60-0.70 |
| Steam (100°C, 1 atm) | 0.60 |
| Carbon dioxide | 1.52 |
| Propane | 1.52 |
Air is denser than methane and natural gas, which is why leaked natural gas rises and dissipates — a key ventilation design consideration for compressor rooms and gas handling areas.
Unit Weight
Unit weight (weight density) is the weight per unit volume: γ = ρ × g.
| Condition | Density (kg/m³) | Unit Weight (N/m³) | Unit Weight (lb/ft³) |
|---|---|---|---|
| 0°C (STP) | 1.293 | 12.7 | 0.0807 |
| 15°C (ISO) | 1.225 | 12.0 | 0.0765 |
| 20°C (NTP) | 1.204 | 11.8 | 0.0752 |
| 25°C (SATP) | 1.184 | 11.6 | 0.0739 |
The low unit weight of air explains why hydrostatic pressure in air systems is negligible — a 100 m air column exerts only ~1.2 kPa, versus ~981 kPa for water.
Unit Conversion Reference
| Quantity | Conversion |
|---|---|
| Density | 1.204 kg/m³ = 0.001204 g/cm³ = 0.0752 lb/ft³ |
| 1 kg/m³ | = 0.06243 lb/ft³ |
| 1 lb/ft³ | = 16.018 kg/m³ |
| 1 atm | = 101.325 kPa = 14.696 psi = 1.01325 bar |
| Standard airflow | 1 CFM = 0.4719 L/s = 0.000472 m³/s |
Volume flow conversions are critical in HVAC: a 10,000 CFM fan moves 4.72 m³/s. Mass flow = volume flow × density — always correct for temperature and pressure when converting standard flow (SCFM) to actual flow (ACFM): ρ_actual = ρ_standard × (P_actual/P_standard) × (T_standard/T_actual).
Density Variation with Conditions
Ideal gas law: ρ = P / (R·T) where P is absolute pressure (Pa), R = 287 J/kg·K, T is absolute temperature (K).
Temperature Effect (at 1 atm)
| Temperature (°C) | Density (kg/m³) |
|---|---|
| -20 | 1.395 |
| 0 | 1.293 |
| 20 | 1.204 |
| 40 | 1.127 |
| 100 | 0.946 |
| 200 | 0.746 |
Pressure Effect (at 20°C)
| Gauge Pressure | Absolute Pressure | Density (kg/m³) |
|---|---|---|
| 0 (atmospheric) | 101.3 kPa | 1.204 |
| 1 bar (15 psig) | 202.6 kPa | 2.41 |
| 7 bar (100 psig) | 800 kPa | 9.5 |
| 10 bar (150 psig) | 1100 kPa | 13.0 |
Humidity Effect
Water vapor is less dense than air (molecular weight 18 vs 29), so moist air is less dense than dry air at same P/T:
| Relative Humidity | 20°C Density (kg/m³) | Change from dry |
|---|---|---|
| 0% (dry) | 1.204 | — |
| 50% | 1.198 | -0.5% |
| 100% (saturated) | 1.194 | -0.8% |
Humidity effect is small for most engineering calculations but matters for precision aerodynamics, engine tuning, and HVAC psychrometrics.
Engineering Applications
HVAC Systems
- Standard air for AC/heating design: 1.204 kg/m³, specific heat 1.005 kJ/kg·K
- Air flow measured in CFM, L/s, or m³/s; heating/cooling load = ṁ × c_p × ΔT
- Typical duct velocities: 3-10 m/s (residential), 8-15 m/s (commercial)
- Pressure drops in ducts are small (100-500 Pa) due to low density
Pneumatic/Compressed Air
- Compressed air at 7-10 bar gauge is ~10x denser than atmospheric
- Storage receiver size based on pressure × volume / temperature
- Compressor power: ~0.1 kW per CFM at 7 bar for rotary screw
- Compressed air velocity: keep <6 m/s in piping to minimize pressure drop
Aerodynamics
- Dynamic pressure q = ½ρv² — air density directly affects lift and drag
- At altitude, lower density requires longer runways and higher true airspeed
- Wind loads on structures: q = 0.613 × v² (at sea level, in Pa with v in m/s)
Combustion
- Stoichiometric air-fuel ratio: ~14.7:1 for gasoline, ~14.5:1 for diesel by mass
- Combustion air requirements scale with fuel heating value
- Flue gas density differs from ambient (temperature, composition)
Related Calculator
Beyond the Density Calculator above, use the Fluid Density Calculator for air at non-standard temperature and pressure, the Flow Rate Calculator for duct and pipe flow, the Pressure Drop Calculator for duct and pneumatic line losses, and the Pipe Flow Calculator for compressible air line sizing.
Fluid Properties
Air is the reference fluid for gas calculations across the site. Compare air with other fluids: Natural Gas Density & Properties, Seawater Properties, and Water Hammer Analysis for liquid transients. For piping design, see Pipe Flow Engineering and the Reynolds Number Guide.
Frequently Asked Questions
What is the density of air at standard conditions? 1.204 kg/m³ at 20°C and 101.325 kPa (NTP), 1.225 kg/m³ at 15°C (ISO), and 1.293 kg/m³ at 0°C (STP). Use the value matching your standard — HVAC designers normally use 1.204 kg/m³.
What is the specific gravity of air? 1.000 by definition, since gas specific gravity is referenced to dry air. This makes it easy to compare: natural gas (0.6) is lighter than air; propane (1.52) is heavier.
What is the unit weight of air? About 11.8 N/m³ (0.0752 lb/ft³) at 20°C. Multiply density by 9.81 m/s². This low value makes hydrostatic pressure effects negligible in air systems.
How does air density change with temperature and pressure? By the ideal gas law ρ = P/(R·T). Density increases linearly with absolute pressure and decreases with absolute temperature: from 1.395 kg/m³ at -20°C to 0.746 kg/m³ at 200°C; from 1.204 kg/m³ at atmospheric to 13.0 kg/m³ at 10 bar gauge.
Why is moist air less dense than dry air? Water vapor (MW 18) is lighter than nitrogen/oxygen (MW 29). At 100% relative humidity and 20°C, air density drops about 0.8% (1.194 vs 1.204 kg/m³). Small for most calculations but significant for precision psychrometrics.
How do I convert air density kg/m³ to lb/ft³? Multiply by 0.06243: 1.204 × 0.06243 = 0.0752 lb/ft³. Conversely multiply lb/ft³ by 16.018 to get kg/m³.
Summary
Dry air at standard conditions has a density of 1.204 kg/m³ (NTP) or 1.225 kg/m³ (ISA). As an ideal gas, its density varies with P/RT — increasing with pressure and decreasing with temperature. This value is fundamental to HVAC load calculations, pneumatic system design, aerodynamic forces, and fan/pump sizing. Humidity effects are small for most engineering purposes except precision psychrometric calculations.