How to use it
- Choose the Material of the wall, for example Styrofoam.
- Type the Area, m² and the Thickness, m, for example 0.95 and 0.025.
- Type the Temperature difference, °C between the two faces, for example 35.
- Read the Heat flow, W, the energy in 24 hours and the thermal resistance, then the steps.
How the heat flow is calculated
When the two faces of a slab stay at fixed temperatures, heat flows steadily from the warm face to the cold face. The rate is larger for a larger area, a larger temperature difference and a material that conducts better, and smaller for a thicker slab (OpenStax, College Physics 2e, Section 14.5).
Q/t = k × A × (T2 − T1) ÷ d
R = d ÷ (k × A)
Q/t = rate of heat flow, W (J/s)
k = thermal conductivity, W/(m·°C)
A = area of the slab, m²
T2 − T1 = temperature difference across the slab, °C
d = thickness of the slab, m
R = thermal resistance, K/W- Double the area and the heat flow doubles; double the thickness and it halves.
- The conductivities differ widely. The table gives 420 for silver and 0.010 for Styrofoam, so silver conducts 42,000 times better.
- The conductivities are given near 0 °C; a material conducts differently at other temperatures.
Thermal conductivities
The values are those of Table 14.3 of the OpenStax section, in watts per metre and degree Celsius, at temperatures near 0 °C. Wood is not included, because the table gives it as a range from 0.08 to 0.16.
| Substance | k, W/(m·°C) |
|---|---|
| Silver | 420 |
| Copper | 390 |
| Gold | 318 |
| Aluminium | 220 |
| Steel or iron | 80 |
| Stainless steel | 14 |
| Ice | 2.2 |
| Glass (average) | 0.84 |
| Concrete or brick | 0.84 |
| Water | 0.6 |
| Fatty tissue | 0.2 |
| Asbestos | 0.16 |
| Plasterboard | 0.16 |
| Dry snow | 0.10 |
| Cork | 0.042 |
| Glass wool | 0.042 |
| Wool | 0.04 |
| Down feathers | 0.025 |
| Air | 0.023 |
| Styrofoam | 0.010 |
What the calculator does not include
- The thin films of still air on the two surfaces, and heat carried away by convection or radiation from them.
- Windows, doors, corners and joints, which often lose more heat than the flat wall does.
- A wall made of several layers of different materials. The calculator takes one material for the whole slab.
Worked example: a Styrofoam ice box
A Styrofoam ice box has a total area of 0.950 m² and walls 2.50 cm thick. With k = 0.010 W/(m·°C) and a temperature difference of 35.0 °C, the heat flow is Q/t = 0.010 × 0.950 × 35.0 ÷ 0.0250 = 13.3 W. Over 24 hours that is 0.319 kWh, and the thermal resistance is 2.6316 K/W.
| Figure | Value |
|---|---|
| Heat flow | 13.30 W |
| Energy in 24 hours | 0.319 kWh |
| Thermal resistance | 2.6316 K/W |
Frequently asked questions
Why does a thicker wall lose less heat?
Heat has to travel farther through the material, so less of it passes in each second. The rate falls in proportion to the thickness, as the equation shows.
Why are ice boxes made of Styrofoam?
Its conductivity is very small, 0.010 W/(m·°C) in the table, so little heat gets through in each second. Metals such as silver have values around 420 in the same table.
Do the conductivities hold at any temperature?
No. The OpenStax table gives its values at temperatures near 0 °C, and conductivity changes with temperature, so use a value for the working temperature when accuracy matters.
Does the calculator include the air films on the surfaces?
No. The result is the heat flow through the slab alone, between two fixed temperatures. Surface films, edges and joints add to the real heat loss.
Is my data sent to a server?
No. The calculation runs in your browser, and nothing you type is sent to us or stored.
Sources
Figures and text last checked on Wednesday, November 11, 2026.