What is Heat Flux Density?
Heat flux density is the rate at which heat passes through a unit of area. It answers the question "how many watts cross one square metre of this surface", so it is a power divided by an area, and its SI unit is the watt per square metre, W/m².
Three of the units in the list are the same thing written differently. J/(m²·s) is a joule per second per square metre, and a joule per second is a watt, so it equals W/m² exactly. kg/s³ is the same quantity reduced to base SI units, and it too equals W/m² exactly. They appear in the list because different documents write the unit in different ways, not because the numbers differ.
Where it Appears in Facade Work
Solar irradiance on a facade is a heat flux density; so is the heat lost through a square metre of wall, which is the U-value multiplied by the temperature difference. Radiant heat limits in fire separation distance calculations are given in kW/m², and the output of heating and cooling equipment per unit of floor or facade area is often quoted the same way.
The conversions matter because the documents do not agree on units. British and American specifications, ASHRAE data and equipment sheets from the United States use Btu/(h·ft²). European standards and product data use W/m². Older Turkish documents, and some HVAC practice, still use kcal/(h·m²). A value copied from one into a calculation set up in another is a common and expensive mistake.
The Conversions
The two that come up most often on a facade project:
1 kW/m² = 317.00 Btu/(h·ft²)
1 W/m² = 0.8598 kcal/(h·m²)
And in the other direction, 1 Btu/(h·ft²) = 3.1546 W/m² and 1 kcal/(h·m²) = 1.163 W/m².
The Btu figure follows from the definitions: one British thermal unit is 1055.05585262 joules, an hour is 3600 seconds and a square foot is 0.09290304 square metres, which gives 3.154591 W/m². The kilocalorie figure follows the same way from 1 kcal = 4186.8 J.
A Word on Btu and Calorie Definitions
Both the Btu and the calorie have several definitions that differ slightly, because they were originally defined by heating water and the answer depends on the starting temperature. This converter uses the international table values, which are the ones used in building services and in the standards a facade project is likely to reference. The thermochemical definitions differ by roughly a tenth of a percent, which is far below the accuracy of any facade heat calculation, but it is worth knowing why two sources can disagree in the fourth digit.
Disclaimer; This converter is a simple and informative tool for unit conversion only. It does not assess thermal performance, condensation risk or compliance with any standard. For detailed and official calculations please apply to FMT.