Heat converters
Heat Density Converter
Convert J/m², langleys, cal/cm² and BTU/ft² of heat per unit area.
Result
41868 J/m²
1 Ly = 41868 J/m²
All units for this value
| Unit | Symbol | Value |
|---|---|---|
| Joule per square meter | J/m² | 41868 |
| Kilojoule per square meter | kJ/m² | 41.868 |
| Megajoule per square meter | MJ/m² | 0.041868 |
| Joule per square centimeter | J/cm² | 4.1868 |
| Watt-hour per square meter | Wh/m² | 11.63 |
| Kilowatt-hour per square meter | kWh/m² | 0.01163 |
| Calorie (IT) per square centimeter | cal/cm² | 1 |
| Langley | Ly | 1 |
| Kilocalorie (IT) per square meter | kcal/m² | 10 |
| BTU (IT) per square foot | BTU/ft² | 3.68669 |
| BTU (IT) per square inch | BTU/in² | 0.025602 |
About this calculator
Heat density, also called radiant exposure, measures how much energy has been delivered to each unit of surface area over some period, such as solar energy landing on a rooftop over a day. This converter switches between SI units like J/m², the older langley unit from meteorology, and BTU/ft² used in US building energy work.
How it works in plain English
Heat density is an amount of energy divided by an area, so the converter anchors every unit to joules per square meter and rescales your entered value through that common base. Metric multiples such as kilojoules and megajoules per square meter follow the usual factor-of-1000 steps, while electrical units like watt-hours per square meter fold in the 3600-second conversion between a joule and a watt-hour.
The langley is a legacy unit from solar radiation and meteorology studies, defined as one thermochemical-style calorie per square centimetre; here it uses the International Table calorie of 4.1868 joules, giving 1 langley = 41,868 J/m². The BTU-based units use the International Table BTU of 1055.05585262 joules divided by a square foot of 0.09290304 m², which is why 1 BTU/ft² works out to roughly 11,356.5 J/m².
Choose your from and to units, type a value, and the converted figure appears immediately alongside a table listing that same heat density in every supported unit. A precision selector controls the number of decimal places shown, and a swap button flips the conversion direction in one click.
The formula
- value_in_target = value_in_source × (source factor to J/m²) ÷ (target factor to J/m²)
- 1 langley (Ly) = 41,868 J/m² (cal(IT)/cm²)
- 1 BTU(IT)/ft² = 1055.05585262 ÷ 0.09290304 = 11,356.5 J/m²
- 1 Wh/m² = 3600 J/m²
Worked example
Suppose a solar monitoring station records a day's total radiant exposure as 500 langleys, a common reading from an older archived weather dataset, and a researcher wants that in joules per square meter to compare against modern satellite records. Multiplying by the langley factor: 500 × 41,868 = 20,934,000 J/m², or about 20.93 MJ/m².
Entering 500 into the langley field with J/m² selected as the target shows 20,934,000 J/m² right away. As a sanity check, 500 langleys is a fairly strong but plausible full-day solar total, and converting it to about 5.8 kWh/m² lines up with typical daily solar energy figures used in solar panel sizing, so the scale of the answer looks right. The full unit table also reports this as roughly 5814.2 Wh/m², 5.8142 kWh/m², and about 1843.5 BTU/ft², all computed from the same single entry without further typing.
Heat density reference values
| Quantity | Value in J/m² |
|---|---|
| 1 langley | 41,868 |
| 1 cal(IT)/cm² | 41,868 |
| 1 BTU(IT)/ft² | 11,356.5 |
| 1 Wh/m² | 3,600 |
| 1 kWh/m² | 3,600,000 |
| Solar constant (per second at top of atmosphere) | 1,361 (as W/m²) |
Standard unit definitions and solar radiation references
Frequently asked questions
What exactly is heat density measuring?
It measures total delivered energy per unit area over some elapsed time, sometimes called radiant exposure, rather than a rate. A rooftop that receives strong sunlight for several hours accumulates a certain number of joules per square meter, and that cumulative total is what heat density units like J/m² or langleys describe.
Why is the langley used in older meteorology and solar records?
The langley became a standard unit in mid-20th-century solar radiation and meteorological studies before SI units were universally adopted in that field, and many historical datasets still report daily solar exposure in langleys. This converter includes it so older records can be compared directly against modern J/m² or Wh/m² figures.
How is heat density different from heat flux density?
Heat density is a total amount of energy accumulated per unit area, such as joules per square meter, while heat flux density is a rate, such as watts per square meter, describing energy delivered per unit area per unit time. Multiplying a heat flux density by a duration gives a heat density.
Which calorie does the langley use here?
This converter defines the langley using the International Table calorie of exactly 4.1868 joules, giving 1 langley = 41,868 J/m². Some older texts use the very slightly different thermochemical calorie of 4.184 joules, which would shift the langley value by less than 0.1%, a difference small enough to ignore for most practical comparisons.
Can I convert BTU/ft² figures from a US building energy report into metric units?
Yes. Select BTU/ft² as the from unit and J/m², MJ/m², or Wh/m² as the target, then enter the reported figure. The converter applies the exact BTU-to-joule and square-foot-to-square-meter factors automatically, so there's no need to look up or multiply those conversion constants by hand.
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