About this converter
Heat density (or irradiation) is the total heat energy delivered per unit area over a period of time, measured in joules per square metre (J/m²). Unlike heat flux density (which is power per area, W/m²), heat density is the time-integrated quantity — the cumulative energy that has passed through a surface. Solar radiation scientists express it as langleys, while building engineers use Btu/ft².
Solar radiation scientists, meteorologists, and building energy analysts work with heat density to quantify daily and seasonal solar exposure. A langley (Ly) equals 1 cal(th)/cm² = 41,868 J/m² — a unit widely used in atmospheric science since the mid-20th century to report daily insolation. A typical mid-latitude sunny day delivers 400–700 Ly. Building envelope analysts use Btu/ft² to calculate cumulative heat gain through glazing and walls.
How to Use This Converter
- Enter the heat density value in the Value field.
- Select the source unit from the From dropdown.
- Select the target unit from the To dropdown.
- Read the converted result and formula line.
- Use Swap to reverse the conversion direction.
Units Covered
| Unit | Symbol | Common Use |
|---|---|---|
| Joule/m² | J/m² | SI unit; used in solar energy resource databases and building simulation software. |
| calorie(th)/cm² | cal/cm² | Thermochemical calorie per cm²; numerically equal to 1 langley. |
| Langley | Ly | Atmospheric science; 1 Ly = 1 cal/cm² = 41,868 J/m². Common in solar radiation atlases. |
| Btu(IT)/ft² | Btu/ft² | US building energy analysis; heat gain through glazing and opaque surfaces. |
| Btu(th)/ft² | Btu(th)/ft² | Thermochemical Btu variant; ~0.067% lower than IT Btu. |
J/m² to Btu(IT)/ft² Conversion Table
| From | To |
|---|---|
| 1,000 J/m² | 0.0881 Btu/ft² |
| 5,000 J/m² | 0.4403 Btu/ft² |
| 10,000 J/m² | 0.8806 Btu/ft² |
| 41,868 J/m² (1 Ly) | 3.687 Btu/ft² |
| 50,000 J/m² | 4.403 Btu/ft² |
| 100,000 J/m² | 8.806 Btu/ft² |
| 200,000 J/m² | 17.61 Btu/ft² |
| 500,000 J/m² | 44.03 Btu/ft² |
| 1,000,000 J/m² | 88.06 Btu/ft² |
| 10,000,000 J/m² | 880.6 Btu/ft² |
How to Convert J/m² to Btu/ft²
Joules per square metre to Btu(IT)/ft²
For example, 1,000,000 J/m² × 0.000088055 = 88.06 Btu/ft² — typical daily solar exposure in a sunny climate.
Btu(IT)/ft² to joules per square metre
For example, 10 Btu/ft² × 11,356.5 = 113,565 J/m² ≈ 113.6 kJ/m² — the heat gained through a south-facing window on a winter morning.
When You Need to Convert Heat Density
Solar radiation scientists and climate researchers use langleys to express daily insolation from historical weather station records and radiation atlases. A weather station reporting 600 Ly/day = 25.12 MJ/m²/day. Converting to J/m² or kWh/m² is necessary when feeding data into photovoltaic yield simulations (PVsyst, SAM) or comparing with modern satellite-derived solar resource datasets in SI.
Building energy analysts calculate solar heat gain through glazing using SHGC (dimensionless) and incident irradiation in either W/m² (instantaneous flux) or kWh/m² and Btu/ft² (cumulative daily or seasonal). A window with SHGC 0.4 facing a 200,000 J/m² daily insolation delivers 80,000 J/m² = 7.04 Btu/ft² of heat gain — inputs needed for ASHRAE 90.1 and EN ISO 52016 compliance checks.
Atmospheric scientists studying UV exposure and erythema dosimetry express cumulative solar UV radiation in J/m² (minimal erythemal dose = 200 J/m²) and compare against older datasets in langley units. Converting between these and Btu/ft² is also required in materials durability testing, where accelerated weathering standards (ASTM G154) specify cumulative radiant exposure in kJ/m² or Btu/ft².