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Heat Loss Calculator

Calculate fabric heat loss through walls, roofs, and windows using the U·A·ΔT formula.

Heat loss
770 W
Fabric heat loss through the surface via Q = U · A · ΔT. Lower U-values mean better insulation and less heat escaping.
In kilowatts
0.77 kW
In BTU/h
2,627 BTU/h
Per °C rise
35 W/K

Estimates fabric heat loss only — add ventilation and infiltration losses for a full heating-load calculation. Verify against local building codes and a qualified HVAC engineer before sizing equipment.

How it works

  1. 1Enter the U-value of the surface (W/m²·K) — lower means better insulation.
  2. 2Enter the surface area and the inside–outside temperature difference.
  3. 3The calculator applies Q = U·A·ΔT and shows heat loss in W, kW, and BTU/h.

Use cases

  • Estimating heat loss through a wall or roof during a heat-load survey.
  • Comparing insulation upgrades by adjusting the U-value.
  • Cross-checks when specifying a boiler or heat-pump output.

Frequently asked questions

What is the U-value and where do I find it?

The U-value (W/m²·K) is the overall thermal transmittance. Typical values: solid brick ≈ 2.1, insulated cavity wall ≈ 0.3, double glazing ≈ 1.4. Check the product datasheet or a building-regulations table.

Worked example?

Q = U·A·ΔT. A 15 m² window with U = 1.4 and ΔT = 20 °C loses 1.4 × 15 × 20 = 420 W. Sum each surface for the whole-building fabric loss.

Does this give the full heating load?

No — fabric (conduction) loss only. Add ventilation and infiltration losses for a full heating-load estimate, and follow standards like BS EN 12831 or Manual J.

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