Duct Heat Loss Calculator
Estimate conductive heat loss from a round duct, equivalent monthly energy and cost using editable duct length, diameter, temperature difference, insulation value and operating schedule.
Default project preview
1,152 BTU/h
Duct surface area
157.1 ft²
Thermal transfer
338 W
Equivalent system input
0.38 kW
Monthly energy
90 kWh
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Duct length
Enter this project assumption in ft. The current imperial default is 50 ft.
Round duct diameter
Enter this project assumption in in. The current imperial default is 12 in.
Air-to-space temperature difference
Enter this project assumption in °F. The current imperial default is 40 °F.
Installed insulation resistance
Enter this project assumption in h·ft²·°F/BTU. The current imperial default is 6 h·ft²·°F/BTU.
Joint and installation allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Heating/cooling efficiency
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 1. Maximum: 100.
Operating hours
Enter this project assumption in hours/day. The current imperial default is 8 hours/day. Minimum: 0. Maximum: 24.
Operating days
Enter this project assumption in days/month. The current imperial default is 30 days/month. Minimum: 0. Maximum: 31.
Energy price
Enter this project assumption in per kWh. The current imperial default is 0.18 per kWh. Minimum: 0.
How to read the outputs
Duct surface area
round duct exterior. Default preview: 157.1 ft².
Thermal transfer
conductive estimate. Default preview: 338 W.
Equivalent system input
efficiency included. Default preview: 0.38 kW.
Monthly energy
entered schedule. Default preview: 90 kWh.
Monthly energy cost
entered energy rate. Default preview: $16.20.
Annualized cost
same schedule for 12 months. Default preview: $194.45.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Round-duct surface area = π × diameter × length. Conductive heat transfer = area × temperature difference ÷ insulation resistance, adjusted by the entered installation allowance.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use the Duct Heat Loss Calculator to estimate a planning quantity from verified duct area, temperature difference, insulation value and run time. It supports scope comparison, purchasing and operating-cost planning, but it does not select equipment, approve code compliance or replace project-specific design.
Calculation steps
- 1Measure or obtain each design input from the actual project and keep one consistent unit system.
- 2Enter duct area, temperature difference, insulation value and run time using product data or approved design information rather than generic nominal values.
- 3Review the unrounded engineering or usage result before applying allowances, duty cycle, efficiency or tariff assumptions.
- 4Apply each adjustment once, then round only the final purchasing, equipment-count or scheduling output.
Worked numerical example
If 120 ft² of duct surface has an effective U-value of 0.08 Btu/h·ft²·°F and a 40°F temperature difference, steady heat loss is about 384 Btu/h.
Purchasing and rounding
Use the installed insulation assembly and exposed duct surface area, not nominal duct length alone. Material purchasing should be based separately on wrap/board coverage, seams and fittings.
Interpret the result
The result is a steady-state conductive heat-transfer estimate for the entered temperature difference. Air leakage, thermal bridges and changing ambient conditions are separate losses.
Page-specific assumptions
- Inputs describe the actual connected system or measured project scope.
- One operating condition, product rating and unit system are used per calculation.
- Allowances and efficiency losses are not already embedded in the entered values.
- Code, health, structural, electrical and hydraulic requirements are verified independently.
Common mistakes
- Using nominal equipment ratings instead of delivered performance at design conditions.
- Mixing instantaneous flow or power with daily or annual energy.
- Applying the same allowance more than once.
- Treating a quantity or cost estimate as equipment selection or professional approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Duct Heat Loss Calculator questions
Does this include duct leakage?+
No. Leakage is a separate air-loss mechanism and should be evaluated independently.
Why does duct location matter?+
The temperature difference between duct air and surrounding space drives conductive heat transfer.
Can this size duct insulation?+
It compares entered insulation values but does not determine condensation control or required insulation.