Calculation sequence
Step-by-step method
- Confirm the sensible-load boundary and target temperature difference.
- Select an air-property constant for the operating condition.
- Divide load by constant times temperature difference.
MechanicalBeginnerLoads
Required Airflow Calculator
Calculate airflow required for a sensible load using temperature difference and a user-entered air constant.
Why it matters: The required airflow depends on the actual load, temperature difference, and air properties.
Calculated result
Engineering evaluation
Required airflow (Q)5,000 cfm
- Entered sensible load (Qs)
- 108,000 Btu/h
- Entered air temperature difference (ΔT)
- 20 °F
- Entered air constant (C)
- 1.08 Btu/(h·cfm·°F)
Engineering interpretation
The entered sensible duty requires approximately 5000 cfm.
Important limitations
- Latent load, ventilation, pressurization, minimum airflow, equipment selection, altitude, and acoustics may govern instead.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Q = Qs/(C·ΔT)
Current substitution
Q = 108000/(1.08×20) = 5000 cfm.
Equation legend
Inputs and calculated quantities
- Qs
- Sensible load — Btu/h or kW
- ΔT
- Air temperature difference — °F or K
- C
- Entered sensible-heat air constant — Btu/(h·cfm·°F) or kW/(m³/s·K)
- Q
- Required airflow — cfm
- Qs
- Entered sensible load — Btu/h
- ΔT
- Entered air temperature difference — °F
- C
- Entered air constant — Btu/(h·cfm·°F)
Applications
How this calculation is used
Building design
Screen airflow for a sensible heating or cooling duty.
Construction
Compare equipment airflow with the entered load basis.
Commissioning
Relate measured sensible load and temperature difference to required airflow.
Quality controls
Assumptions and limitations
Assumptions
- Entered values represent one stable operating condition and use consistent measurement boundaries.
- Fluid and air properties are representative of the evaluated condition.
Limitations
- This is a preliminary evaluation and field-verification tool, not a final sizing or code-compliance determination.
- Manufacturer data, adopted codes, project criteria, and professional engineering review remain controlling.
Field use
Verification procedure
- Confirm measurement boundaries, operating mode, and instrument calibration.
- Stabilize the system and record simultaneous readings.
- Compare the calculation with project requirements and manufacturer data before disposition.
Common engineering mistakes
- Mixing values from different operating conditions.
- Using nominal rather than measured flow, temperature, power, or fluid properties.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsPsychrometrics, heat transfer, load relationships, and HVAC calculation context; verify the licensed current edition during professional review.
- NIST Guide for the Use of the International System of UnitsFederal unit-system and conversion reference used for SI/I-P presentation.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement