Heat Loss Calculator

Our Heat Loss Calculator tells you how much heating power you need to maintain a comfortable indoor temperature against continuous heat loss. Here's how to use it:

Step 1: Enter the total surface area through which heat is escaping — in square meters. This is the area of the wall, ceiling, floor, or window you are analyzing. For a full room, add up all exposed surfaces.

Step 2: Enter the U-value of the material in W/m²·K. U-value measures how easily heat passes through — lower is better. Typical values: single-glazed window ≈ 5.0, uninsulated solid brick wall ≈ 2.1, cavity wall with insulation ≈ 0.3–0.5, modern triple-glazed window ≈ 0.8.

Step 3: Enter the temperature difference (ΔT) in °C. This is the gap between your desired indoor temperature and the coldest expected outdoor temperature. For example, 21°C indoors and −2°C outdoors gives ΔT = 23°C.

Step 4: Click Calculate to see total heat loss in Watts and BTU/hr — the two standard units used when selecting boilers, radiators, and electric heaters.

Heat loss through a flat surface follows the standard steady-state conduction formula:

Q (Watts) = U × A × ΔT

Where: - Q = heat loss rate in Watts - U = U-value of the material (W/m²·K) - A = surface area (m²) - ΔT = temperature difference between inside and outside (°C)

Example: A 15 m² cavity wall (U = 0.45 W/m²K), indoor temperature 21°C, outdoor −3°C: ΔT = 21 − (−3) = 24°C Q = 0.45 × 15 × 24 = 162 Watts

To convert Watts to BTU/hr, multiply by 3.412: 162 × 3.412 ≈ 553 BTU/hr

This is the continuous heater output required to maintain the desired indoor temperature against heat loss through that surface alone. To size a heater for a full room, calculate the loss for each exposed surface separately and add the results together.

All materials conduct heat. While you can warm up your indoor space, heat will continuously flow from warmer to colder areas as long as the temperature outside is lower. This flow of heat is unstoppable, regardless of the insulation quality used in your home. To maintain a comfortable indoor temperature, you need to supply energy at a constant rate using a heating system. This calculator helps you determine the required power of a heater to compensate for that continuous heat loss.

Heat loss is determined by three main factors: the area of the surface the heat flows through, the insulating properties of the materials (measured as the U-value), and the temperature difference between the inside and outside. A larger surface area allows more heat to escape, while materials with a higher U-value transfer heat more easily. The temperature difference is the driving force—the colder it is outside compared to inside, the faster heat will be lost.

Using this calculator, you can quickly estimate the heat loss and the necessary heater power. Simply enter the total surface area, the U-value of your insulation, and the expected temperature difference. The result will give you the total heat loss in Watts and BTU/hr, helping you choose perfectly sized heating equipment.