Convert Btu (IT) per Square Foot to Kilojoules per Square Meter
Btu (IT) per Square Foot (Btu/ft²) to Kilojoule per Square Meter (kJ/m²) 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 Btu/ft² = 11.35653 kJ/m²
1 Btu (IT) per Square Foot = 11.35653 Kilojoules per Square Meter
1 Kilojoule per Square Meter = 0.088055066 Btu (IT) per Square Foot
1 Btu/ft² in every supported unit
Conversion chart: Btu (IT) per Square Foot to Kilojoules per Square Meter
Conversion table
| Btu (IT) per Square Foot (Btu/ft²) | Kilojoule per Square Meter (kJ/m²) |
|---|---|
| 0.01 Btu/ft² | 0.1135653 kJ/m² |
| 0.1 Btu/ft² | 1.135653 kJ/m² |
| 1 Btu/ft² | 11.35653 kJ/m² |
| 2 Btu/ft² | 22.71306 kJ/m² |
| 3 Btu/ft² | 34.06959 kJ/m² |
| 5 Btu/ft² | 56.78265 kJ/m² |
| 10 Btu/ft² | 113.5653 kJ/m² |
| 20 Btu/ft² | 227.1306 kJ/m² |
| 50 Btu/ft² | 567.8265 kJ/m² |
| 100 Btu/ft² | 1135.653 kJ/m² |
| 1000 Btu/ft² | 11356.53 kJ/m² |
Btu (IT) per Square Foot (Btu/ft²)
Definition: A US customary heat-density unit — British thermal units of accumulated energy delivered per square foot of surface.
History: Combines the British thermal unit with the square foot, giving US engineers a heat-density figure consistent with everyday US building and solar-collector area measurements.
Current use: Used in US solar-collector performance ratings and building-envelope heat-gain calculations, where accumulated solar exposure over a roof or window area needs to be quantified.
Kilojoule per Square Meter (kJ/m²)
Definition: A more practically sized heat-density unit, since raw joule figures for real accumulated solar or thermal exposure run into the thousands or millions.
History: A natural SI multiple of joule per square meter, used once daily and hourly solar-exposure figures needed a more convenient magnitude for reporting.
Current use: Common in solar-energy engineering and building physics reports summarizing daily or hourly accumulated solar radiation on a surface.
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 Btu (IT) per Square Foot → Kilojoule per Square Meter: multiply by 11.35653. For example, 1 Btu/ft² × 11.35653 = 11.35653 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 1 Btu/ft² equals 11.35653 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
How many Kilojoules per Square Meter are in 1 Btu (IT) per Square Foot?
1 Btu (IT) per Square Foot (Btu/ft²) equals exactly 11.35653 Kilojoules per Square Meter (kJ/m²).
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.