Air Density Calculator and CFM to Mass Flow Converter
Air gets thinner with altitude, heat and humidity. Find its density, the correction factor to standard air, and the mass flow behind a CFM reading.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Moist-air density
- 0.075lb/ft³
- Density ratio to standard air (0.075 lb/ft³)
- 0.994
Air at 70 °F and 50 % RH weighs 0.0745 lb/ft³, 99.4 % of standard air.
- Specific volume (per lb dry air)
- 13.52ft³/lb
- Humidity ratio
- 54.5gr/lb
- Barometric pressure used
- 14.7psia
Show the working
- Barometric pressure at elevation (standard atmosphere)101.3 kPa
- Specific volume v = 0.287042 (T + 273.15)(1 + 1.607858 W) ÷ p13.5202 ft³ per lb of dry air (W = 54.5 gr/lb)
- Density ρ = (1 + W) ÷ v0.07454 lb/ft³
- Correction factor = ρ ÷ 0.075 (standard air behind the 1.08 and 4.5 constants)0.9939
Why air density matters
Fans move a volume of air (CFM), but heat is carried by mass. The 1.08 and 4.5 constants used throughout HVAC assume standard air at 0.075 lb/ft³. In Denver, or in a hot attic, the same CFM carries noticeably less mass and therefore less heat. The density ratio shown here is the factor to apply to those constants.
Method
Barometric pressure comes from the standard atmosphere at your elevation. The specific volume of moist air (per pound of dry air) comes from the ideal-gas form in the ASHRAE Handbook—Fundamentals, as implemented in PsychroLib, and density is (1 + W) ÷ specific volume, where W is the humidity ratio.
- Dry-air mass flow (lb/h) = CFM × 60 ÷ specific volume
- Moist-air mass flow = dry-air mass flow × (1 + W)
- CFM = dry-air mass flow × specific volume ÷ 60
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Calculate
- Air density and correction factor
- Air temperature (dry-bulb)
- 70 °F
- Relative humidity
- 50 %
- Site elevation
- 0 ft
Result
- Moist-air density
- 0.075 lb/ft³
- Density ratio to standard air (0.075 lb/ft³)
- 0.994
- Specific volume (per lb dry air)
- 13.52 ft³/lb
- Humidity ratio
- 54.5 gr/lb
- Barometric pressure used
- 14.7 psia
Air at 70 °F and 50 % RH weighs 0.0745 lb/ft³, 99.4 % of standard air.
Assumptions
- Ideal-gas moist air at the standard-atmosphere pressure for the elevation (ASHRAE 2017 equations, as in PsychroLib).
- Standard air is 0.075 lb/ft³, the density behind the 1.08 and 4.5 air-side constants.
Limitations
- Weather changes barometric pressure by a few percent; enter the station pressure-equivalent elevation if you need better.
Frequently asked questions
Does humid air weigh more than dry air?
No — at the same temperature and pressure, humid air is slightly lighter, because water vapour molecules are lighter than the nitrogen and oxygen they replace.
What is standard air?
In HVAC formulas, standard air is 0.075 lb/ft³, roughly dry air at 70 °F at sea level. The 1.08 sensible-heat and 4.5 total-heat constants are built on it.
Should I use dry-air or moist-air mass flow?
Psychrometric calculations (moisture and enthalpy per pound of dry air) use the dry-air flow. Moist-air flow is the total mass actually moving.
Sources
- PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
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