Convert Joules per Square Meter to Btu (th) per Square Foot
Joule per Square Meter (J/m²) to Btu (th) per Square Foot (Btu(th)/ft²) heat density conversion — enter any value below to get an instant result, or use the table for common values.
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
1 J/m² = 8.8114E-05 Btu(th)/ft²
1 Joule per Square Meter = 8.8114E-05 Btu (th) per Square Foot
1 Btu (th) per Square Foot = 11348.93 Joules per Square Meter
1 J/m² in every supported unit
Conversion chart: Joule per Square Meter to Btu (th) per Square Foot
Conversion table
| Joule per Square Meter (J/m²) | Btu (th) per Square Foot (Btu(th)/ft²) |
|---|---|
| 0.01 J/m² | 8.8114E-07 Btu(th)/ft² |
| 0.1 J/m² | 8.8114E-06 Btu(th)/ft² |
| 1 J/m² | 8.8114E-05 Btu(th)/ft² |
| 2 J/m² | 0.00017622807 Btu(th)/ft² |
| 3 J/m² | 0.0002643421 Btu(th)/ft² |
| 5 J/m² | 0.00044057017 Btu(th)/ft² |
| 10 J/m² | 0.00088114034 Btu(th)/ft² |
| 20 J/m² | 0.0017622807 Btu(th)/ft² |
| 50 J/m² | 0.0044057017 Btu(th)/ft² |
| 100 J/m² | 0.0088114034 Btu(th)/ft² |
| 1000 J/m² | 0.088114034 Btu(th)/ft² |
Joule per Square Meter (J/m²)
Definition: The SI derived unit of heat (or radiant energy) density — the joules of energy that have been delivered onto one square meter of surface.
History: Formed directly from the joule and square meter once both were standardized as SI units, giving radiation and heat-transfer science a coherent energy-per-area unit.
Current use: Used in scientific literature and engineering calculations of accumulated solar radiation, laser fluence, and cumulative heat exposure on a surface.
Btu (th) per Square Foot (Btu(th)/ft²)
Definition: Nearly identical to the IT-Btu version, using the slightly different thermochemical Btu definition instead of the International Table Btu.
History: Reflects the same historical split between the International Table Btu and the thermochemical Btu found throughout US heat-engineering units.
Current use: Occasionally appears in thermochemistry-oriented US engineering references that specify the thermochemical Btu explicitly.
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 Joule per Square Meter → Btu (th) per Square Foot: multiply by 8.8114E-05. For example, 1 J/m² × 8.8114E-05 = 8.8114E-05 Btu(th)/ft².
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 1 J/m² equals 8.8114E-05 Btu(th)/ft². 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
How many Btu (th) per Square Foot are in 1 Joule per Square Meter?
1 Joule per Square Meter (J/m²) equals exactly 8.8114E-05 Btu (th) per Square Foot (Btu(th)/ft²).
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.