Duct Heat Gain Calculator
Cooling season heat gain into ducts.
Enter your values
Enter the duct diameter in in.
Enter the duct length in ft.
Enter the insulation r-value in R.
Enter the attic temperature in °F.
Enter the supply air temperature in °F.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Duct surface area
- 83.8 sq ft
- Heat gain into duct
- 1,047 BTU/hr
- Equivalent cooling capacity lost
- 0.09 tons
Quick answer
Supply duct heat gain in a hot attic follows the same Q = A x ΔT / R relationship as heat loss, but adds unwanted heat to cooled air.
Q (BTU/hr) = surface area x ΔT / R-value
At a glance
| What it does | Cooling season heat gain into ducts. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct diameter, Duct length, Insulation R-value, Attic temperature, Supply air temperature |
| Main output | Duct surface area |
| Formula | Q (BTU/hr) = surface area x ΔT / R-value |
| Cost | Free — no sign-up, no download |
How to use the Duct Heat Gain Calculator
- 1Enter duct size, length and insulation R-value.
- 2Enter the hot attic temperature and the cold supply air temperature.
- 3The result shows how much of your AC capacity is wasted reheating the duct's contents.
Inputs explained
- Duct diameter(in)
- Enter the duct diameter in in.
- Duct length(ft)
- Enter the duct length in ft.
- Insulation R-value(R)
- Enter the insulation r-value in R.
- Attic temperature(°F)
- Enter the attic temperature in °F.
- Supply air temperature(°F)
- Enter the supply air temperature in °F.
Worked example
Using the values the calculator loads with:
Inputs
- Duct diameter8 in
- Duct length40 ft
- Insulation R-value6 R
- Attic temperature130 °F
- Supply air temperature55 °F
Results
- Duct surface area83.8 sq ft
- Heat gain into duct1,047 BTU/hr
- Equivalent cooling capacity lost0.09 tons
Frequently asked questions
Why does this matter more for AC than heating?
The temperature difference between a 130°F attic and 55°F supply air can be larger than typical winter conditions, driving high gain.
Best fix?
Move ducts into conditioned space where possible, or add insulation and seal leaks if they must stay in the attic.
What do I need to enter into the Duct Heat Gain Calculator?
Just 5 values: duct diameter, duct length, insulation r-value, attic temperature, supply air temperature. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Duct Heat Gain Calculator work out the answer?
Supply duct heat gain in a hot attic follows the same Q = A x ΔT / R relationship as heat loss, but adds unwanted heat to cooled air. It applies the formula Q (BTU/hr) = surface area x ΔT / R-value and shows the working so you can check each step by hand.
Is the Duct Heat Gain Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.