Temperature Drop Calculator for Air Ducts  

by the National Mechanical Insulation Committee (NMIC)


Calculates the Temperature Change of Air Flowing in a Duct (Reference: 2009 ASHRAE HoF, Chapter 4, Equation 48, (pg 4.21))

This calculator estimates the temperature drop (or rise) of air flowing in a duct.


(Input information may generally be be obtained from design documents or field measurements)

  • Line 1. Enter the temperature of the air entering the duct
  • Line 2. Enter the ambient temperature (the average temperature of the air surrounding the duct)
  • Line 3. Enter the flow rate of the air carried by the duct in cubic feet per minute
  • Line 4. Enter the length of the duct run in feet
  • Line 5. Enter the perimeter of the duct in inches
  • Line 6. Enter the R-value of the selected duct insulation. Available R-values will vary depending on insulation products selected.

Note: NA = Insulation Size Not Available. Some sizes and thicknesses may not be commercially available.


  • One dimensional, steady state heat transfer
  • Specific heat of air assumed constant at 0.24 Btu/(lbm·°F)
  • U-factor of duct walls assumed constant
  • Air film resistances (inner + outer) assumed constant at 0.85 h·ft2·°F/Btu
  • The impact of duct leakage is neglected