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This result excludes sensor uncertainty and does not certify a downstream design. Verify pressure basis, measurement traceability, model range, and the required safety margin.

The chart renderer is unavailable. The exact density components remain available in the report and ledger.

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The same cubic metre of air does not always contain the same mass. Pressure packs more molecules into the volume, heat spreads them farther apart, and water vapor changes the mixture because a water molecule is lighter than the average dry-air molecule it replaces. Air density therefore belongs to stated conditions, not to a room, altitude, or weather label by itself.

Density affects buoyancy, aerodynamic lift and drag, fan and compressor mass flow, combustion calculations, and precision mass measurements. Specific volume expresses the reciprocal view: how much volume one kilogram of the air mixture occupies. Higher density means lower specific volume.

Change with other inputs fixed Typical density effect Reason
Higher absolute pressure Raises density More gas amount is held in the same volume.
Higher temperature Lowers density The gas occupies more volume per unit mass.
Higher relative humidity Usually lowers density Water vapor displaces heavier dry-air molecules.
Higher carbon dioxide fraction Raises density slightly Carbon dioxide raises the dry-air molar mass.

Pressure must be absolute station pressure. A weather report adjusted to sea level is useful for comparing weather systems but does not describe the actual molecular pressure at a high-altitude site. Using sea-level-adjusted pressure in a local density calculation can make the air appear much denser than it is.

A single result is only as representative as the measurements. Temperature near a hot wall, humidity beside a wet process, or a default carbon dioxide assumption may not describe the air moving through a duct or around an instrument. Record the sensor location, time, and units whenever results will be compared.

How to Use This Tool:

Use measurements from the same place and time, with station pressure rather than a sea-level weather value.

  1. Enter Air temperature from 15°C to 27°C, or the equivalent 59°F to 80.6°F. Changing the unit preserves the physical temperature.
  2. Enter Absolute station pressure from 60 kPa to 110 kPa. Use the pressure at the measurement location.
  3. Set Relative humidity from 0% for dry air through 100% for saturated air at the entered temperature.
  4. Leave Carbon dioxide assumption at 400 ppm only when that estimate is suitable; otherwise enter a measured or justified value from 0 to 1000 ppm.
  5. Read Moist-air density with Specific volume and Sea-level density ratio, then review the condition ledger before comparing another run.

Interpreting Results:

Moist-air density is the combined dry-air and water-vapor mass per cubic metre. Specific volume is its reciprocal in cubic metres per kilogram. Sea-level density ratio divides the result by 1.225 kg/m³, a standard-atmosphere reference rather than a live local baseline.

Compare runs only when pressure type, sensor location, timing, and carbon dioxide treatment are consistent. A density ratio near 1 does not prove standard atmosphere conditions, and the separate water-vapor contribution should not be mistaken for the density change caused by humidity.

Technical Details:

CIPM-2007 models moist air as dry air plus water vapor and corrects ideal-gas behavior with a compressibility factor. Relative humidity first determines the water-vapor mole fraction from saturation vapor pressure and an enhancement factor. Carbon dioxide changes the molar mass assigned to the dry portion.

Formula Core:

The total density is the sum of the dry-air and vapor contributions at absolute pressure p and thermodynamic temperature T.

ρa = pZRT [ Ma(1-xv) + Mvxv ] xv = hfpsvp v = 1ρa

Z is the CIPM compressibility factor, R is 8.314472 J/(mol·K), Ma is the carbon-dioxide-adjusted dry-air molar mass, Mv is the water molar mass, xv is water-vapor mole fraction, h is relative humidity as a fraction, f is the enhancement factor, and psv is saturation vapor pressure.

Quantity Accepted range Internal form
Temperature 15°C to 27°C inclusive K = °C + 273.15
Absolute pressure 60 kPa to 110 kPa inclusive Pa; 1 inHg = 3386.389 Pa
Relative humidity 0% to 100% inclusive Fraction from 0 to 1
Carbon dioxide 0 ppm to 1000 ppm inclusive Mole fraction from 0 to 0.001

At 20°C, 101.325 kPa, 50% relative humidity, and 400 ppm carbon dioxide, the model gives approximately 1.199314 kg/m³ and a specific volume of 0.833810 m³/kg. The density ratio is about 0.979032 against the 1.225 kg/m³ reference.

Calculations retain full floating-point precision; display formatting rounds the result for reading. Water-vapor, dry-air, total-density, and condition outputs all come from the same unrounded model result.

Accuracy Notes:

CIPM-2007 is an interpolating model with a recommended validity range of 600 hPa to 1100 hPa and 15°C to 27°C. Instrument uncertainty in pressure, temperature, humidity, and carbon dioxide adds to model uncertainty.

  • The 400 ppm carbon dioxide value is an assumption, not a measured room concentration.
  • Relative humidity must describe the same air sample and temperature used in the calculation.
  • Do not extrapolate this result to very hot, cold, low-pressure, or high-pressure conditions outside the accepted ranges.

References: