Dry-air density changes with both pressure and temperature. For a first estimate within the ideal-gas model, use rho = p ÷ (R × T), with absolute pressure p, absolute temperature T in kelvin, and the specific gas constant R. The table provides calculated values for selected pressure and temperature combinations using NASA Glenn’s published air equation-of-state constant. It is a dry-air reference, not a humid-air calculator or a substitute for measured process data.

How the table values were calculated

Pressure values are absolute: 80, 101.325, and 120 kPa absolute. Celsius readings are converted to kelvin by adding 273.15. The NASA Glenn equation-of-state reference gives R as 0.286 kJ/(kg·K), equivalent to 286 J/(kg·K). The calculation converts pressure to pascals, applies the ideal-gas relation, and rounds density to 0.001 kg/m³.

As a plausibility check, the calculation near 101.3 kPa and 15°C gives about 1.23 kg/m³, consistent with NASA Glenn’s listed sea-level reference density of 1.229 kg/m³. Small differences are expected because the table uses a rounded gas constant and ideal-gas assumptions. The displayed values are computed from the formula; they are not copied from the old handbook table.

For example, at 101.325 kPa absolute and 20°C, the calculation uses p = 101,325 Pa and T = 293.15 K, giving about 1.209 kg/m³. At the same temperature, changing pressure to 80 kPa gives a proportionally lower estimate. This example is also a unit check: using pressure in kilopascals while R is in joules per kilogram-kelvin will make the units inconsistent and the result wrong by a factor of one thousand.

Use absolute pressure and temperature

A gauge-pressure reading is relative to local atmospheric pressure. Convert it before using this table: absolute pressure is approximately gauge pressure plus the surrounding atmospheric pressure. The actual atmospheric pressure changes with weather and elevation, so a gauge value alone does not determine air density. Vacuum readings also need to be converted to absolute pressure, with consistent reference conventions.

The equation requires absolute temperature. For Celsius, T(K) = T(°C) + 273.15. Do not put a Celsius number directly into the denominator. At the same pressure, warming the air lowers the calculated density; at the same temperature, increasing absolute pressure raises it. Those proportional changes follow directly from rho = p/(R T).

Local weather pressure is not necessarily the standard atmosphere. At a high-elevation site, a given gauge pressure corresponds to a lower absolute pressure than it would near sea level. For a sealed system, use the absolute pressure at the state being analyzed; for an open system, account for the actual surrounding atmospheric pressure. Keep the pressure source and instrument reference alongside any calculated density.

When a dry-air estimate is insufficient

Water vapor changes the mixture properties, while high pressures or specialized conditions may require a real-gas model. The table does not include humidity, changing composition, local atmospheric pressure correction, uncertainty in sensors, or thermal gradients. It also does not describe airflow, compressible flow losses, buoyancy in a particular enclosure, or the pressure rating of a component.

For a practical estimate, record the measured absolute pressure, air temperature, humidity condition, and measurement uncertainty. Use a psychrometric or moist-air property model when humidity matters. For calibrated instruments, combustion, process control, or safety-critical design, use the applicable validated property source and have the calculation reviewed. Never infer a permissible working pressure from an air-density lookup.

The listed grid is finite. For a direct calculation, use the measured inputs in the equation instead of rounding them first to a nearby table row. Report the assumed gas constant, pressure reference, temperature conversion, rounding, and whether humidity was included so another engineer can reproduce the estimate. The input sensors and assumptions determine the precision; displaying more digits does not improve the accuracy of rounded or uncertain measurements.

Sources: NASA Glenn Research Center, Equation of State for Air (air gas constant and absolute-temperature requirement) and Air Properties Definitions (sea-level plausibility check). Values in the table are independently calculated from the stated ideal-gas equation.