Convert Btu (th) per Square Foot to Btu (IT) per Square Foot
Btu (th) per Square Foot (Btu(th)/ft²) to Btu (IT) per Square Foot (Btu/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 Btu(th)/ft² = 0.99933078 Btu/ft²
1 Btu (th) per Square Foot = 0.99933078 Btu (IT) per Square Foot
1 Btu (IT) per Square Foot = 1.0006697 Btu (th) per Square Foot
1 Btu(th)/ft² in every supported unit
Conversion chart: Btu (th) per Square Foot to Btu (IT) per Square Foot
Conversion table
| Btu (th) per Square Foot (Btu(th)/ft²) | Btu (IT) per Square Foot (Btu/ft²) |
|---|---|
| 0.01 Btu(th)/ft² | 0.0099933078 Btu/ft² |
| 0.1 Btu(th)/ft² | 0.099933078 Btu/ft² |
| 1 Btu(th)/ft² | 0.99933078 Btu/ft² |
| 2 Btu(th)/ft² | 1.9986616 Btu/ft² |
| 3 Btu(th)/ft² | 2.9979923 Btu/ft² |
| 5 Btu(th)/ft² | 4.9966539 Btu/ft² |
| 10 Btu(th)/ft² | 9.9933078 Btu/ft² |
| 20 Btu(th)/ft² | 19.986616 Btu/ft² |
| 50 Btu(th)/ft² | 49.966539 Btu/ft² |
| 100 Btu(th)/ft² | 99.933078 Btu/ft² |
| 1000 Btu(th)/ft² | 999.33078 Btu/ft² |
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.
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.
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 (th) per Square Foot → Btu (IT) per Square Foot: multiply by 0.99933078. For example, 1 Btu(th)/ft² × 0.99933078 = 0.99933078 Btu/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 Btu(th)/ft² equals 0.99933078 Btu/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 (IT) per Square Foot are in 1 Btu (th) per Square Foot?
1 Btu (th) per Square Foot (Btu(th)/ft²) equals exactly 0.99933078 Btu (IT) per Square Foot (Btu/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.