Tool for quick thermal analysis of assemblies
Heat Loss Calculator
Heat loss equals U-value multiplied by area and temperature difference.
Working calculator
Enter the core dimensions and check the result directly on this page.
A 20 m2 wall with U 0.25 W/m2K and indoor 21 C, outdoor -9 C has deltaT 30 K. Q = 0.25 * 20 * 30 = 150 W.
Heat loss equals U-value multiplied by area and temperature difference.
A 200 ft2 assembly with R-10 and indoor 70 F, outdoor 30 F has deltaT 40 F. Q = (1 / 10) * 200 * 40 = 800 BTU/h.
Practical note
Before you use the result
Heat Loss Calculator is intended for quick option checks and technical discussion before detailed execution. The result depends on the selected units, declared material values, and chosen surface resistances, so each change in layer or thickness should be treated as a separate variant.
The calculator does not automatically verify every local rule, thermal bridge, moisture condition, structural connection, or installation tolerance. If the result is close to a requirement, treat it as a reason for deeper verification rather than a final decision.
For better comparisons, test several realistic thicknesses, check current product data sheets, and review the complete assembly. A calculated value is most useful when the assumptions are clear: material, thickness, layer order, units, and data source.
For insulation or U-value tools, layer order and correct units are especially important. For concrete, electrical, plumbing, or heating tools, the result should be read as a quick quantity or plausibility check before standards and execution conditions are reviewed.
Save the result with the date, material name, and assumptions. If the product, diameter, cable section, or thickness changes later, do not compare the numbers alone without checking which inputs changed.
For calculator pages, clear separation between inputs and result is essential. If a value looks surprising, check units and default fields first, then review the project assumptions.
Heat Loss Calculator
Heat Loss Calculator
Heat loss equals U-value multiplied by area and temperature difference.
Assembly thickness
Assembly thickness
Use Q = U * A * deltaT for steady-state heat transfer through one building element. Use R-value input by converting U to 1 / R.
U-value
U-value
Does not include air leakage. Does not include thermal bridging. Does not include solar gains. Does not model moisture. Does not correct for real-world installation quality. Uses a steady-state simplification.
CategoryLambdaLayerMaterialThickness
How it works
How to read the result
Use this for early comparison of walls, roofs, windows, or other assemblies where U-value or R-value is known.
Important limitation
Does not include air leakage. Does not include thermal bridging. Does not include solar gains. Does not model moisture. Does not correct for real-world installation quality. Uses a steady-state simplification.
Heat Loss Calculator
Use Q = U * A * deltaT for steady-state heat transfer through one building element. Use R-value input by converting U to 1 / R.
Use this for early comparison of walls, roofs, windows, or other assemblies where U-value or R-value is known.
Add layerRestore default setupRemove
Assembly nameThickness unitInternal Rsi (m2K/W)External Rse (m2K/W)
Calculation assumptions
The calculators use visible formulas and explicit unit conversions. Treat the result as a preliminary check, not a complete building design.
- SI and US units are converted separately; R, RSI, U-value, and U-factor are not mixed without the unit factor.
- Enter positive values and compare the result with the selected product datasheet.
- Local codes, thermal bridges, fasteners, and installation quality can change the requirement.
- Last formula review: 2026-04-27.
See how formulas, unit conversions, rounding, and limitations are handled. Methodology details.
Frequently Asked Questions
Lambda is the thermal conductivity of a material, usually shown in W/mK. Lower lambda means heat moves through that material more slowly, but the final assembly still depends on thickness, surface resistances, and every other layer.
U-value describes heat loss through the whole building element, not just one material. A low U-value usually means a better insulated wall, roof, or floor, provided the real build-up matches the layers entered in the calculator.
Yes. Use the comparison section to keep thickness constant, then compare materials by lambda and calculated thermal resistance. This is useful when two products look similar on paper but behave differently at the same installed depth.
Yes. You can print the result or export it to CSV, Excel, or PDF for reports and documentation.
Yes. It is designed for layered assemblies such as external walls, flat roofs, pitched roofs, floors, and slabs. For unusual assemblies, add every relevant layer and treat the result as a planning check before formal verification.
Yes. It is intended for fast concept-stage calculations, insulation comparison, and envelope optimisation before detailed design. It is best used to narrow choices, not to replace a code check or project-specific thermal bridge assessment.
Yes. You can switch between millimeters, centimeters, and inches, and the calculator keeps the values consistent. For fewer mistakes, choose one unit system at the start of a project and review converted thicknesses before export.
Use Heat Loss Calculator as a first-pass reference. Before specifying anything, compare the result with the heat source output, actual project dimensions, product data sheet, and local requirements.