Window Heat Loss Calculator
Estimate steady conductive heat transfer through glazing for an entered temperature difference and time interval.
How to use this calculator
- Enter window area (m²), window u-value (w/m²·k), indoor-outdoor temperature difference (k or °c) and the remaining stated inputs; keep their units consistent.
- Review the result and intermediate values; change one input at a time when comparing scenarios.
- Keep the stated formula and assumptions with any result you share.
Formula and method
Heat-loss rate (W) = U-value × area × temperature difference. Energy over the entered duration (kWh) = heat-loss watts × hours ÷ 1,000.
- Estimated conductive heat flow is 105.84 W; energy over 8 hours is 0.847 kWh.
- This excludes air leakage, solar gain, frames not represented by the U-value, thermal bridges and changing temperatures.
Worked examples
Worked example
4.2 m² of glazing at U = 1.4 W/m²·K and an 18 K temperature difference for 8 hours.
Estimated conductive heat flow is 105.84 W; energy over 8 hours is 0.847 kWh.
Estimated conductive heat flow is 105.84 W; energy over 8 hours is 0.847 kWh.
Understanding the result
Estimated conductive heat flow is 105.84 W; energy over 8 hours is 0.847 kWh.
This excludes air leakage, solar gain, frames not represented by the U-value, thermal bridges and changing temperatures.
Important notes
This excludes air leakage, solar gain, frames not represented by the U-value, thermal bridges and changing temperatures.
Use measured inputs and verify that the model matches the situation before making an operational, financial or technical decision.
Frequently asked questions
Is this the required heating-system capacity?
No. Whole-building heat-loss design needs other walls, ventilation, thermal bridges and local design conditions.
What U-value should I enter?
Use the rated whole-window value where possible, not only the centre-of-glass value.
What should I check before relying on this result?
Verify units, input definitions and assumptions against your real data or the relevant product, service or professional guidance.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
- How to Calculate Percentage Change — Learn the percentage change formula, how to calculate increases and decreases, and why the original value matters.
- How to Calculate Percent Error — Learn the percent error formula and how to compare an experimental value with a reference or accepted value.
- Percentage Change — Learn what percentage change means, how to calculate increases and decreases, and how the original value affects the result.
This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.