Heat Density Converter: J/m², Langley, Btu/ft² & More
Convert heat density and solar radiation (energy per area), radiant exposure, irradiance dose, between SI, langley, and imperial.
Convert energy per unit area between J/m², langley (cal/cm²), and Btu/ft². Used in solar radiation, meteorology, and surface heating. Five units; free and instant.
What is heat density?
Heat density here means energy per unit area (J/m²), the total energy that has been received, stored, or delivered per square meter. It’s not power per area (that’s heat flux); it’s total energy per area, so it shows up in radiant exposure (e.g. solar insolation over a day) and in some building and HVAC contexts. The langley (Ly) equals 1 cal/cm² and is widely used in solar radiation and atmospheric science; US building specs often use Btu/ft² for surface energy.
Meteorologists use it for solar radiation totals; engineers use it for surface heating and fire load. This converter covers J/m², langley, cal/cm² (th), and both Btu (IT) and Btu (th) per square foot.
Key units and solar radiation
Joule per square meter (J/m²)
Factor: 1 (SI base)
Standard unit for energy per area. Used in physics and engineering.
Langley (Ly)
Factor: 1 Ly = 41,840 J/m²
1 Ly = 1 cal/cm². Common in solar radiation and meteorology for daily insolation.
Btu (IT)/ft²
Factor: 1 ≈ 11,357 J/m²
US building and HVAC for surface energy (e.g. radiant floor, solar gain).
Who uses this converter?
Solar and renewable energy work with langley or J/m² for irradiance dose and panel exposure. Meteorologists report solar radiation in Ly or kJ/m². Building and fire engineers use Btu/ft² for surface energy. Students need to move between SI and langley for atmospheric and solar courses. All conversions run in your browser.
Common conversions at a glance
| From | To | Formula |
|---|---|---|
| Langley | J/m² | × 41,840 |
| Btu/ft² | J/m² | × 11,357 |
Heat Density FAQ
What is a langley used for?
The langley (Ly) is 1 cal/cm², energy per unit area. It’s used for solar radiation totals (e.g. daily insolation in Ly) and in meteorology. 1 Ly = 41,840 J/m² (thermochemical calorie). Convert to J/m² or Btu/ft² with the tool above.
What is the difference between heat density and heat flux density?
Heat density is energy per area (J/m²), a total. Heat flux density is power per area (W/m²), rate of flow. Integrate flux over time to get energy per area. Use our Heat Flux Density converter for W/m², Btu/(ft²·h), etc.
How do I convert langley to J/m²?
Multiply by 41,840. One langley (1 Ly = 1 cal/cm²) equals 41,840 J/m² when using the thermochemical calorie. Use the converter above for exact values and for Btu/ft², cal/cm² (th), and Btu (th)/ft².
How do I convert Btu/ft² to J/m²?
Multiply Btu (IT)/ft² by about 11,357. So 1 Btu/ft² ≈ 11,357 J/m². Use the tool for Btu (th)/ft² and other energy-per-area units. Common in US building and HVAC for radiant floor or solar gain totals.
What is radiant exposure or solar insolation in heat density units?
Radiant exposure is total energy per area (J/m² or Ly) over a period, e.g. daily solar insolation. Meteorologists often report it in langleys; engineering may use kJ/m² or Btu/ft². This converter lets you switch between J/m², langley, and Btu/ft² for the same exposure.
When should I use heat density (J/m²) instead of heat flux (W/m²)?
Use heat density (J/m²) when you care about total energy received or stored per area, daily solar dose, fire load, or radiant heating totals. Use heat flux (W/m²) when you care about rate, instantaneous irradiance or power through a surface. This tool is for energy per area; use the Heat Flux Density converter for power per area.