Thermal Mass Calculator
Heat stored in a concrete, brick, stone, adobe, water, steel or custom mass from its area, thickness and temperature rise, in kWh or BTU, for passive solar. How it works
Use the Thermal Mass Calculator
About the Thermal Mass Calculator
A 10 cm concrete slab stores 1.28 kWh per square meter when the sun warms it 20 °C, from Q = density × area × thickness × specific heat × temperature rise. Water of the same depth stores 2.32 kWh/m², brick 0.89 and adobe 0.82, because storage follows density × specific heat, not weight alone. This thermal mass calculator gives the total in kWh or BTU for concrete, brick, stone, adobe, water, steel or concrete block, or for a custom material whose density and specific heat you type.
Heat stored per m² by a 10 cm layer
| Material | 10 °C rise | 20 °C rise | Time scale at 5 W/(m²·K) |
|---|---|---|---|
| Water | 1.16 kWh/m² | 2.32 kWh/m² | 23.2 h |
| Steel | 1.06 kWh/m² | 2.12 kWh/m² | 21.2 h |
| Concrete | 0.64 kWh/m² | 1.28 kWh/m² | 12.8 h |
| Stone | 0.64 kWh/m² | 1.28 kWh/m² | 12.8 h |
| Brick | 0.44 kWh/m² | 0.89 kWh/m² | 8.9 h |
| Adobe | 0.41 kWh/m² | 0.82 kWh/m² | 8.2 h |
| Concrete Block | 0.26 kWh/m² | 0.51 kWh/m² | 5.1 h |
The calculator's Per area figure for a 10 cm layer starting at 15 °C, with its preset densities and specific heats. Storage scales in step with thickness and temperature rise, so 20 cm of concrete stores twice as much. The time scale does not change with the temperature rise.
How to use this tool
- Pick the material; the hint under it shows the density and specific heat the calculator uses. Choose Custom to type your own density in kg/m³ and specific heat in kJ/(kg·K).
- Enter the surface area and the thickness, in m² and cm or, with Imperial, in ft² and inches.
- Enter the starting temperature and the temperature the mass reaches after heating. The second must be higher.
Frequently asked questions
- Does water or concrete store more heat?
- Water, by volume. A 10 cm layer of water stores 2.32 kWh/m² for a 20 °C rise against 1.28 kWh/m² for concrete, because water's specific heat (4.18 kJ/(kg·K)) more than makes up for its lower density. Per kilogram the gap is larger still.
- How thick should a thermal mass floor or wall be?
- The calculator does not recommend a thickness. Stored heat rises in step with thickness, but heat moves slowly through masonry, so only the part of a slab the daily cycle reaches takes part each day. That depends on the material's conductivity and the room, which the tool does not model.
- Why do concrete and stone give the same answer?
- Their presets multiply to the same heat capacity per cubic meter: concrete at 2,300 kg/m³ × 1.00 kJ/(kg·K) and stone at 2,500 kg/m³ × 0.92 kJ/(kg·K) both come to 2,300 kJ per m³ per kelvin.
- What do the 50 W/m² load and the 5 W/(m²·K) time scale mean?
- They are fixed comparisons, not predictions. The load figure is how long the stored heat could supply a steady 50 W per m² of surface: 25.6 h for the 10 m² slab above. The time scale divides the stored heat by a surface loss of 5 W per m² per degree across the whole rise, 12.8 h for the same slab.
- Can I use Fahrenheit and square feet?
- Yes. Imperial takes ft², inches and °F and reports BTU, kBtu or MMBtu. 100 ft² of concrete 4 in thick warmed from 59 °F to 95 °F stores 41.2 kBtu.
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