Heat Density Converter
Convert between heat density (radiant energy per area) units — joules per square meter, langleys, and Btu per square foot.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Result
500 Ly = 20920 kJ/m²
1 Langley = 41.84 Kilojoules per Square Meter
1 Kilojoule per Square Meter = 0.023900574 Langleys
500 Ly in every supported unit
What is a Heat Density Converter?
Heat density (also called radiant exposure or, in solar contexts, insolation) measures how much thermal or radiant energy has accumulated on a given area of surface over some period — not a rate of flow, but a running total. It's the figure behind "how much solar energy hit this roof today" or "how much laser energy struck this target," expressed as energy divided by area.
Every unit here is defined relative to the SI joule-per-square-meter unit by a fixed multiplier, letting a solar-energy report in langleys, a US engineering datasheet in Btu per square foot, and a physics paper in J/m² all be compared directly.
Conversion chart: Langley to Kilojoules per Square Meter
Conversion table
| Langley (Ly) | Kilojoule per Square Meter (kJ/m²) |
|---|---|
| 0.01 Ly | 0.4184 kJ/m² |
| 0.1 Ly | 4.184 kJ/m² |
| 1 Ly | 41.84 kJ/m² |
| 2 Ly | 83.68 kJ/m² |
| 3 Ly | 125.52 kJ/m² |
| 5 Ly | 209.2 kJ/m² |
| 10 Ly | 418.4 kJ/m² |
| 20 Ly | 836.8 kJ/m² |
| 50 Ly | 2092 kJ/m² |
| 100 Ly | 4184 kJ/m² |
| 1000 Ly | 41840 kJ/m² |
Supported Units
| Unit | Symbol | In J/m² |
|---|---|---|
| Joule per Square Meter | J/m² | 1 J/m² |
| Kilojoule per Square Meter | kJ/m² | 1000 J/m² |
| Calorie (th) per Square Centimeter | cal(th)/cm² | 41840 J/m² |
| Langley | Ly | 41840 J/m² |
| Btu (IT) per Square Foot | Btu/ft² | 11356.53 J/m² |
| Btu (th) per Square Foot | Btu(th)/ft² | 11348.93 J/m² |
About These Parameters
- Value
- The heat density value you want to convert, expressed in the "From" unit. Accepts decimals.
- From Unit
- The unit your input value is currently measured in — a climatology report's langleys, or a US datasheet's Btu/ft².
- To Unit
- The unit you want the result converted into. Use the swap button to flip From and To instantly.
How Heat Density Conversion Works
The Formula
Every unit here is defined by a fixed multiplier relative to joules per square meter. To convert a value from one unit to another:
result = value × (factor of "From" unit ÷ factor of "To" unit)
For Langley → Kilojoule per Square Meter: multiply by 41.84. For example, 500 Ly × 41.84 = 20920 kJ/m².
Heat Density vs. Heat Flux Density
Heat density (this page) is a cumulative total — how much energy has landed on a surface altogether. Its close cousin, heat flux density (covered on this site's separate Heat Flux Density Converter), is a RATE — how much energy is landing per second, right now. The relationship is the same as distance versus speed: heat flux density integrated over time gives you heat density, the same way speed integrated over time gives you distance traveled.
The Langley and Solar Science
The langley was, for decades, the standard unit meteorologists and agricultural scientists used to report daily accumulated solar radiation ("langleys per day"), named after solar radiation pioneer Samuel Pierpont Langley. Modern science has largely shifted to SI's joules per square meter, but historical climate records and some regional agricultural datasets still report figures in langleys, which is why converting between the two remains useful.
Example
A heat density of 500 Ly equals 20920 kJ/m². For scale, a clear summer day at mid-latitudes can deliver roughly 500-600 langleys of solar radiation onto a horizontal surface — equivalent to about 20,900-25,100 kJ/m².
Frequently Asked Questions
Is a langley the same as a calorie per square centimeter?
Yes, exactly — one langley is defined as one thermochemical calorie of energy delivered per square centimeter of surface. "Langley" is simply the more common name used in solar and atmospheric science, honoring solar-radiation pioneer Samuel Pierpont Langley.
Why would I need to convert heat density instead of heat flux density?
Use heat density when you're working with an accumulated total over a period of time — a day's worth of solar exposure, or the total energy a laser pulse delivered. Use heat flux density (a separate converter on this site) when you need an instantaneous rate, like the current solar intensity hitting a panel right now.
How accurate are these conversions?
Every conversion factor used here is the exact, internationally recognized relationship between the underlying energy and area units (e.g. 1 langley = exactly 41,840 J/m²) — results are limited only by floating-point display precision, not by rounded conversion constants.