understanding gas flow rate conversion

US vs. Imperial Gallon

• What is the numerical difference between US and Imperial gallons, and why does it exist?

• In what industries does this mistake most commonly show up (fueling, HVAC, chemical dosing)?

• How big is the error if you mix them up in a mass or energy balance?

• Why does this matter more for liquids than gases—but still matters for gases?

• How do codes and standards specify which gallon they mean (if they do at all)?

SCFM to Mass Flow Rate

• What exactly does SCFM mean, and what assumptions are baked into it?

• What temperature and pressure define “standard,” and who decides?

• Why does SCFM not uniquely define mass flow without additional info?

• What is the clean, unit-consistent equation to convert SCFM → lbm/hr or kg/s?

• When should you not use SCFM at all?

SCFM Data to Different Gases

• Why can’t you directly reuse air SCFM data for another gas?

• When does specific gravity scaling work—and when does it fail?

• How do molecular weight and compressibility factor affect conversion accuracy?

• What assumptions do vendors usually make (and not tell you)?

• What level of error is acceptable for early design vs. final sizing?

Why This Is So Confusing

• Why do vendors publish SCFM instead of mass flow?

• How do mixed unit systems (psi, ft³, lbmol) make this worse?

• Why do engineers argue about “standard conditions” endlessly?

• What is the single safest way to communicate flow unambiguously?

1. How to convert SCFM to ACFM and back?

There's many sites and calculators online that cover this topic but I believe a number of them miss a few key points that help illustrate this.

When referring to compressor inlet conditions this relationship describes what the CFM at actual conditions would need to be in order to move the same mass flow at standard conditions.  IT DOES NOT GIVE YOU A CONVERSION OF THE COMPRESSOR PERFORMANCE!!!  Compressor CFM is the same regardless of the inlet conditions. After doing the calculation to determine ACFM you would need to find a compressor with that rated SCFM to meet that mass flow rate requirement. 

The following relationship can be used to describe the reduction in compressor output .  

Note: is the same for a compressor regardless of inlet conditions

2. How to convert SCFM to mass flow and back?

Mass flow @ standard conditions  

Mass flow @ actual conditions 

3. How to convert SCFM between different gases?

By using Graham’s law!  Graham’s law is an empirical relationship that states that the ratio of the rates of diffusion or effusion of two gases is the square root of the inverse ratio of their molar masses.

4. Confusion that inevitably happens

   1. What are standard conditions?

There is no agreed upon standard condition. Always ask/find the temperature/pressure/humidity (if applicable) of the volumetric flow rate that you are referring to.

2. How does humidity affect SCFM?

It is not intuitive but humid air is LESS dense than dry air. It affects the density and if you are using coolprop/refprop for material properties you can include the humidity when determining the density.

3. Compressor performance vs. condition conversion

COMING SOON!

5. Examples

COMING SOON!