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Heat Loss Calculator

Estimate steady-state heat loss by building component. Enter each area and U-value, the indoor–outdoor temperature difference, and an optional simplified ventilation estimate.

Wall area / Wall U-value (W/m²·K)
Roof area / Roof U-value (W/m²·K)
Window area / Window U-value (W/m²·K)
Floor area / Floor U-value (W/m²·K)

Simplified steady-state estimate, not a heating-system size recommendation. Use U-values and air changes from reliable building data. This omits thermal bridges, solar/internal gains, moisture, wind, ducts and dynamic operation. Have a qualified energy professional review assumptions before making building or equipment decisions.

How do you calculate heat loss through a building component?

For transmission, multiply area by U-value and temperature difference: watts = m² × W/(m²·K) × K. A component with U = 1, area 10 m² and a 20 K difference contributes 200 W. Ventilation is estimated separately from room volume and air changes.

Frequently asked questions

How do you calculate heat loss through a building component?

For transmission, multiply area by U-value and temperature difference: watts = m² × W/(m²·K) × K. A component with U = 1, area 10 m² and a 20 K difference contributes 200 W. Ventilation is estimated separately from room volume and air changes.

Are my room measurements uploaded?

No. This estimate runs locally in your browser; your inputs are not sent to a server or saved by this tool.

Is a temperature difference in Fahrenheit converted like a normal temperature?

No. A Fahrenheit temperature difference is multiplied by 5/9 to convert to kelvin difference; an offset is not added.

Can I use this to choose a furnace or heat pump?

No. It is a simplified component estimate and does not include all design conditions or local standards. Heating equipment selection needs a complete, qualified load assessment.