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.