Window Heat Loss Calculator
Estimate steady window heat loss, monthly heating energy and cost from area, U-factor and temperature difference.
Default project preview
2,800 BTU/h
Thermal loss
0.82 kW
Monthly delivered heat
295.4 kWh
Monthly input energy
328.2 kWh
Monthly heating cost
$59.08
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.
Total window area
Enter this project assumption in ft². The current imperial default is 200 ft².
Window U-factor
Enter this project assumption in BTU/h·ft²·°F. The current imperial default is 0.35 BTU/h·ft²·°F.
Indoor temperature
Enter this project assumption in °F. The current imperial default is 70 °F.
Outdoor temperature
Enter this project assumption in °F. The current imperial default is 30 °F. Minimum: -100.
Heating hours per day
Enter this project assumption in hours. The current imperial default is 12 hours. Minimum: 0. Maximum: 24.
Heating days per month
Enter this project assumption in days. The current imperial default is 30 days. Minimum: 0. Maximum: 31.
Heating months per year
Enter this project assumption in months. The current imperial default is 5 months. Minimum: 1. Maximum: 12.
Heating efficiency or COP
Enter this project assumption in a unitless value. The current imperial default is 0.9.
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
Thermal loss
steady-state estimate. Default preview: 0.82 kW.
Monthly delivered heat
through windows. Default preview: 295.4 kWh.
Monthly input energy
performance 0.9. Default preview: 328.2 kWh.
Monthly heating cost
window loss only. Default preview: $59.08.
Seasonal input energy
5 heating months. Default preview: 1,641 kWh.
Seasonal heating cost
entered rate. Default preview: $295.42.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Heat loss = area × U-factor × temperature difference. Input energy divides by heating efficiency or COP.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use this calculator to estimate steady-state window heat loss. It supports measurement, purchasing and preliminary budgeting, but it does not select assemblies, products, code details or installation methods for the project.
Calculation steps
- 1Measure each project section separately and keep every input in one unit system.
- 2Enter the verified values for window area, U-factor and indoor-outdoor temperature difference.
- 3Review the raw measured result before applying waste, overlap, labor or price assumptions.
- 4Apply one documented allowance, then divide by the exact package yield or stock length and round up only at the purchasing stage.
Worked numerical example
A 20 ft² window with U-factor 0.30 and a 35°F temperature difference loses about 210 Btu/h by the simplified U × A × ΔT calculation.
Purchasing and rounding
Use the whole-window U-factor for the exact unit where available and the design indoor-outdoor temperature difference appropriate to the comparison. No material purchase should be inferred directly from this heat-loss result; window selection should compare rated performance, size and installation conditions.
Interpret the result
The result is steady-state conductive heat transfer using U × area × temperature difference. It excludes solar gain, air leakage, curtains and dynamic weather, so it is best used for comparative load estimates rather than predicting a utility bill.
Page-specific assumptions
- Dimensions represent the actual installation area or run.
- One product size, package yield and installation condition are used per calculation.
- Allowances are added once and are not already embedded in the measurements.
- Structural, moisture, fire, energy and code requirements are verified separately.
Common mistakes
- Estimating from nominal dimensions instead of field measurements.
- Using package coverage from a different product or installation method.
- Applying waste in both the input dimensions and the allowance field.
- Treating a quantity or cost estimate as approval of the assembly or installation detail.
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
Window Heat Loss Calculator questions
Does a lower U-factor reduce heat loss?+
Yes. Lower U-factor means less heat transfer.
Are drafts included?+
No. Air leakage is separate.
Can a heat-pump COP be entered?+
Yes. Enter a representative operating value.