Heat Flux Density Converter

Convert Heat Flux Density Between Units

Heat Flux Density Formula

Formula

\[q_{\text{to}}=q_{\text{from}}\times\frac{C_{\text{from}}}{C_{\text{to}}}\]

Where:

\(q_{\text{from}}\) — the heat flux density entered in the starting unit
\(q_{\text{to}}\) — the converted heat flux density in the target unit
\(C_{\text{from}}\) and \(C_{\text{to}}\) — the converter’s factors for expressing each unit in W/m²

The calculator first expresses the entered value in W/m², then divides by the factor for the target unit. The result represents the same heat flux density in a different unit.

Heat Flux Density – Calculation Example

Example: convert the default value of 100 W/m² to kW/m². The converter uses factors of 1 for W/m² and 1,000 for kW/m².

\[100\times\frac{1}{1{,}000}=0.1\ \text{kW/m}^2\]

Result: 100 W/m² equals 0.1 kW/m².

How to use the calculator

Enter a nonnegative number in Heat flux value. The number must be expressed in the unit you select next.
Choose that unit in From unit. For example, select W/m² if your value is measured in watts per square meter.
Choose the unit you need in To unit. You can select the same unit to leave the number unchanged.
Read Converted Heat Flux Density for the converted number and its unit. Change either selection to compare units.

The available units are W/m², kW/m², cal/s·cm², BTU/h·ft², and BTU/s·ft². Ergs per second per square centimeter are not an option in this calculator.

What heat flux density means

Heat flux density is a rate of heat transfer per unit surface area. A value of 100 W/m² means that heat is crossing each square meter of a surface at a rate of 100 joules per second, provided the rate is uniform over that area. It is also commonly called heat flux.

Area matters when interpreting the number. A heat flow of 100 W across 1 m² corresponds to an average of 100 W/m²; the same heat flow across 2 m² corresponds to an average of 50 W/m². If the heat flux varies across the surface, a single value may represent an average rather than the conditions at every point. For calculations involving total heat flow, see the Heat Transfer converter.

Heat flux density can describe heat passing through a wall, entering a solar collector, or reaching a component’s surface. It is distinct from thermal conductivity: conductivity describes a material property related to heat conduction, while heat flux density describes heat transfer per area under particular conditions. Use the Thermal Conductivity converter when you need to convert the material property instead.

Units and conversion factors

The calculator uses W/m² as its reference unit. Its factors state how many W/m² correspond to one unit of each type:

W/m²: 1. A watt is one joule per second, so this is the SI unit for heat flux density.
kW/m²: 1,000 W/m². This is useful when a value would otherwise require a large number of watts per square meter.
cal/s·cm²: 41,868 W/m², using this calculator’s factor.
BTU/h·ft²: 3.15459 W/m², using this calculator’s factor.
BTU/s·ft²: 11,336.722 W/m², using this calculator’s factor.

For example, converting from kW/m² to W/m² multiplies the value by 1,000. Converting in the opposite direction divides it by 1,000. For any other pair, the formula above applies the two listed factors in sequence; you do not need to convert to W/m² manually.

Assumptions and practical limits

This tool converts units; it does not calculate heat flux density from temperature difference, wall thickness, material properties, or surface area. A change of units also does not establish the direction of heat flow. The Heat flux value field accepts nonnegative values, so enter a magnitude rather than a signed value.

Results are rounded to eight decimal places. A very small nonzero conversion may therefore display as zero. Keep your original value and its unit when you need more precision than the displayed result provides.

The listed BTU/s·ft² and BTU/h·ft² factors do not have the exact 3,600-to-1 ratio expected from seconds and hours alone. The calculator uses the factors shown above, so check the applicable BTU definition and conversion standard before relying on a BTU-based result for precision engineering work. Heat flux density is also different from mass flux density, which measures mass flow per unit area rather than heat transfer.

Frequently Asked Questions

How can I calculate heat flux density from energy, time, and area?

Divide the energy transferred by the elapsed time and the area it crosses: \(q=E/(tA)\). For example, 90,000 joules passing through 3 square meters in 5 minutes gives \(90{,}000/(300\times3)=100\) W/m². Convert minutes to seconds first if you want a result in W/m². This calculation gives an average over the stated time and area.

What is a normal or good heat flux density?

There is no universal normal value; it depends on the process and which surface you measure. For context, sunlight on a surface facing a clear midday sun can be around 1,000 W/m², but the heat actually absorbed depends on the surface and conditions. For a wall or component, compare your value with the design requirements and operating conditions rather than a single general benchmark.

How do I convert BTU/h·ft² to kW/m² by hand?

Using this converter’s factors, multiply the BTU/h·ft² value by 3.15459 to get W/m², then divide by 1,000 to get kW/m². For example, 10 BTU/h·ft² becomes 31.5459 W/m², or 0.0315459 kW/m². Keep the area unit squared: converting square feet as though they were linear feet is a common source of errors.

What does a negative heat flux density mean?

A negative value usually indicates heat flowing opposite to a chosen positive direction; it does not mean a negative amount of heat exists. For example, a conduction calculation may assign opposite signs to heat flowing inward and outward. This converter accepts magnitudes only, so convert the positive magnitude and retain the direction separately in your notes.

Is heat flux density the same as a heat transfer coefficient?

No. Heat flux density has units of W/m², while a heat transfer coefficient has units of W/(m²·K). In a simple convection model, they are related by \(q=h\Delta T\), where \(h\) is the coefficient and \(\Delta T\) is the temperature difference between the surface and fluid. Converting units of \(q\) cannot determine \(h\) without that temperature difference and an applicable model.

Which area should I use when calculating heat flux density?

Use the area through which the heat flow is being described, and state what that area represents. For heat crossing a wall, this may be the relevant wall face; for sunlight, it may be the illuminated surface area or its projected area facing the sun. Mixing those area definitions can produce different values even when the total incoming power is unchanged.