Heat Density Converter
Convert heat density, the amount of heat energy delivered per unit of area, between joule per square metre, calorie per square centimetre, langley and Btu per square foot. Pick two units, enter a value and read the result instantly.
Select two units to see the conversion factor.
Heat density here means energy per area (J/m²). It is not heat flux density (W/m², energy per area per time) and not volumetric energy density (J/m³).
Heat Density Formula
Heat density is heat energy divided by area: q = Q ÷ A, in J/m². Every unit is a fixed multiple of J/m². Result = value × (source factor ÷ target factor). Exact building blocks: 1 cal (th) = 4.184 J, 1 cm² = 10⁻⁴ m², 1 ft = 0.3048 m, 1 Btu (IT) = 1,055.05585262 J, and 1 langley = 1 cal (th)/cm² = 41,840 J/m².
What Is Heat Density?
Heat density in this converter is heat (or radiant) energy received or delivered per unit area, in joules per square metre. It tells how much energy has accumulated over a surface in total, not how fast it arrives. A typical sunny day delivers roughly 20 MJ/m² to a horizontal surface.
The langley is the classic meteorology unit: one thermochemical calorie per square centimetre, exactly 41,840 J/m². Solar radiation totals used to be reported in langleys per day. The Btu per square foot is the common US building and solar-collector unit.
Do not confuse it with related quantities. Heat flux density (W/m²) is a rate: energy per area per second. Multiply a flux of 200 W/m² by one hour (3,600 s) and you get a heat density of 720,000 J/m². Energy density is per volume (J/m³) or per mass (J/kg), and a heat transfer coefficient is W/(m²·K). This converter changes the energy-per-area units only. See the Heat Flux Density Converter for rates.
Supported Units
This converter supports the following 5 heat density units. The last column gives the value of each unit in the base unit J/m².
| Unit | Symbol | Value in J/m² |
|---|---|---|
| Joule per square meter | J/m² | 1 |
| Calorie (th) per square centimeter | cal(th)/cm² | 41,840 |
| Langley | Ly | 41,840 |
| Btu (IT) per square foot | Btu(IT)/ft² | 11,356.52668 |
| Btu (th) per square foot | Btu(th)/ft² | 11,348.93179 |
How to Use the Heat Density Converter
- Type the heat density in the value box. Decimals and scientific notation such as 0.0025 or 2.5e-3 are accepted.
- Choose the From unit and the To unit.
- The result appears instantly, or press Convert Now. Very large or very small results are shown in scientific notation.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, open Show this value in all units for a full list, and Clear to start again.
Step-by-step by hand
- Find the size of each unit in the common base unit (J/m²): 1 Ly = 41,840 J/m² and 1 J/m² = 1 J/m².
- Divide the source size by the target size to get the conversion factor: 41,840 ÷ 1 = 41,840, so 1 Ly = 41,840 J/m².
- Multiply by your value: 500 Ly × 41,840 = 20,920,000 J/m².
Heat density here is a non-negative quantity, so only zero and positive values are accepted.
Common Heat Density Conversions
- Daily solar radiation: 500 langleys equals 20,920,000 J/m².
- Langley to Btu: 500 langleys equals 1,842.112521 Btu/ft².
- Btu to joule: 1,000 Btu/ft² equals 11,356,526.68 J/m².
- Calorie per cm²: 20 cal(th)/cm² equals 836,800 J/m².
- IT vs thermochemical Btu: 100 Btu(IT)/ft² equals 100.0669216 Btu(th)/ft².
Langley to Other Units
Quick reference for frequently used values
| Langley | J/m² | Btu(IT)/ft² | cal(th)/cm² |
|---|---|---|---|
| 1 | 41,840 | 3.684225043 | 1 |
| 10 | 418,400 | 36.84225043 | 10 |
| 100 | 4,184,000 | 368.4225043 | 100 |
| 300 | 12,552,000 | 1,105.267513 | 300 |
| 500 | 20,920,000 | 1,842.112521 | 500 |
| 1,000 | 41,840,000 | 3,684.225043 | 1,000 |
Where and Why This Conversion Is Used
- Meteorology and agriculture: daily solar radiation totals.
- Solar energy: insolation reported in Btu/ft² or MJ/m².
- Thermal exposure testing: total energy delivered to a surface.
- Reading historical data: older reports in langleys.
Limitations
- The value is a total over time. It does not state the intensity or the time over which it was delivered.
- Absorbed heat depends on the surface; this tool changes units only.
Important Differences Between Related Heat Quantities
- Specific heat capacity, J/(kg·K): energy needed to warm one kilogram of a material by one kelvin. It is not the total heat capacity of an object (J/K), which also depends on how much material there is, and it is not molar heat capacity (J/(mol·K)).
- Heat density, J/m²: heat or radiant energy delivered per unit area over a period. Heat flux density, W/m² is the rate of that transfer, energy per area per time. They are not interchangeable.
- Heat flux density, W/m² is heat flow per area. The heat transfer coefficient, W/(m²·K) is heat flux per kelvin of temperature difference, so heat flux = coefficient × temperature difference.
- Thermal conductivity, W/(m·K) is a material property. The heat transfer coefficient is a property of a surface or an assembly, and thermal resistance, K/W belongs to one specific object. Resistance is not conductivity or resistivity: R = L ÷ (k × A) for a slab.
- Temperature interval vs temperature: a difference of 10 °C is a difference of 18 °F. A reading of 10 °C is 50 °F. Every unit here that contains “per degree” or “K” uses intervals, so no offset is ever applied.
Each converter changes units within one quantity only. Related tools: Temperature Interval, Thermal Expansion, Thermal Resistance, Thermal Conductivity, Specific Heat Capacity, Heat Density, Heat Flux Density, Heat Transfer Coefficient, Fuel Efficiency - Mass and Fuel Efficiency - Volume.