What is heat flux density?

Heat flux density is the rate of heat energy transfer through a surface, per unit area. It's measured in power per area, such as watts per square meter (W/m2), and describes how intensely heat is flowing across that surface.

How do I convert W/m2 to BTU/(h.ft2)?

Divide the W/m2 value by about 3.1546. For example, 1000 W/m2 equals about 317.0 BTU/(h.ft2).

What is the Stefan-Boltzmann law?

The Stefan-Boltzmann law states that the radiant heat flux emitted by a surface equals its emissivity times the Stefan-Boltzmann constant times its absolute temperature to the fourth power: q = epsilon * sigma * T^4, where sigma is about 5.67 x 10^-8 W/(m2.K4).

What is the solar constant?

The solar constant is the average solar radiant heat flux reaching the top of Earth's atmosphere, about 1,361 W/m2.

How do I convert kW/m2 to W/cm2?

Multiply the kW/m2 value by 0.1. For example, 10 kW/m2 equals 1 W/cm2, since 1 kW/m2 equals 1,000 W/m2 and 1 W/cm2 equals 10,000 W/m2.

A langley is a historical unit of solar radiation equal to 1 thermochemical calorie per square centimeter. It was widely used in meteorology and solar energy research before SI units became standard.

What is the difference between heat flux and heat flux density?

Heat flux (or heat transfer rate) is total power in watts. Heat flux density normalizes that by area, giving power per unit area, which is what this converter works with.

Why is heat flux density important in HVAC and building design?

It determines how much heat passes through walls, windows and roofs, which is central to sizing heating and cooling systems and calculating insulation performance.

Is this heat flux density converter free to use?

Yes, it is completely free, requires no registration, and runs entirely in your browser with no usage limits.

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Heat Flux Density Conversions

11 units · Stefan-Boltzmann calculator

Heat Flux Density Converter

Convert instantly between W/m², kW/m², BTU/(h·ft²), kcal/(h·m²) and more, then calculate radiant heat flux from surface temperature using the Stefan-Boltzmann law.

11Heat flux units

1361W/m² solar constant

0msServer round-trips

From

W/m²

To

BTU/(h·ft²)

316.9979919272BTU/(h·ft²)

BTU/(h·ft²) = W/m² / 3.154590745

Surface temperature

Temperature unit

Emissivity (0–1)

Show flux in

459.3 W/m²

q = ε × σ × T⁴ where σ = 5.670374419×10⁻⁸ W/(m²K⁴)

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Heat flux concepts & formulas

The math behind every conversion above, and the terms that come up most in thermal engineering.

Quick conversion table

Common values for your selected units in the converter above, updated automatically.

W/m²	BTU/(h·ft²)

Heat flux density unit reference

What each unit means, its exact size in W/m², and where you'll typically see it used.

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Frequently asked questions

Straight answers to the questions people ask most about heat flux density and radiant heat.

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