Enter the flow rate in cubic feet per minute (CFM) and choose the air or gas condition to determine the approximate mass flow in pounds per minute (lb/min).
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CFM to lb/min Formula
The following formula is used to convert a volumetric flow rate in CFM to a mass flow rate in lb/min.
lb/min = CFM × ρ
The reverse relationship can also be written as:
CFM = (lb/min) / ρ
Variables:
- lb/min is the mass flow rate in pounds per minute
- CFM is the volumetric flow rate in cubic feet per minute
- ρ is the density of the air or gas in pounds per cubic foot (lb/ft³)
To calculate lb/min, multiply the flow rate in CFM by the assumed density of the fluid in lb/ft³.
What is CFM to lb/min Conversion?
CFM measures the volumetric flow of a fluid, while lb/min measures the mass flow. Because mass depends on density, the conversion from CFM to lb/min requires an assumed density for the air or gas being measured. This is why the same CFM value can produce different lb/min values for cool air, warm air, hot air, or natural gas.
For quick engineering estimates, common density assumptions are often used. Standard air at about 70°F and 1 atmosphere is commonly taken as 0.075 lb/ft³. Cooler air is denser and produces a slightly higher lb/min value at the same CFM, while warmer air is less dense and produces a lower lb/min value.
How to Convert CFM to lb/min?
The following steps outline how to convert CFM to lb/min.
- First, determine the volumetric flow rate in cubic feet per minute (CFM).
- Next, choose the appropriate air or gas condition and identify its density in lb/ft³.
- Multiply the CFM value by the density using the formula lb/min = CFM × ρ.
- After calculating the result, check your answer with the converter above.
Example Problem:
Use the following variables as an example problem to test your knowledge.
Flow Rate (CFM) = 100
Condition = Standard Air (70°F, 1 atm)
Density (ρ) = 0.075 lb/ft³
lb/min = 100 × 0.075 = 7.5 lb/min
CFM to lb/min Conversion Table
The table below shows example conversions for a 100 CFM flow rate using several common density assumptions.
| Condition | Density (lb/ft³) | 100 CFM in lb/min |
|---|---|---|
| Standard Air (70°F, 1 atm) | 0.075 | 7.5 |
| Cool Air (~40°F) | 0.080 | 8.0 |
| Warm Air (~90°F) | 0.071 | 7.1 |
| Hot Air (~120°F) | 0.068 | 6.8 |
| Natural Gas (Typical) | 0.044 | 4.4 |
These values are approximate and are most useful for quick estimates. If you need higher accuracy, use the actual density of the gas at the specific operating temperature and pressure.