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Heat Flux Density Converter

Science

Convert heat flux density between watts per square metre, BTU per hour-square foot, and calories per second-square centimetre instantly and free.

Reviewed by the thecalcu.com team · Last updated July 28, 2026

From
To
All conversionsfor 1 BTU per Hour-Square Foot (BTU/(hr·ft²))
Watts per Square Metre (W/m²)3.15459
Kilowatts per Square Metre (kW/m²)0.00315459
BTU per Hour-Square Foot (BTU/(hr·ft²))1
Calories per Second-Square Centimetre (cal/(s·cm²))0.000075396511

What is a Heat Flux Density?

The Heat Flux Density Converter converts heat flux density between watts per square metre (SI), BTU per hour-square foot (imperial), and calories per second-square centimetre (CGS). Heat flux density measures how intensely heat energy flows through a surface per unit area, used for everything from solar irradiance to electronic component cooling to building insulation performance.

Enter a value in any supported unit and the converter calculates the equivalent instantly. For the related quantity that relates heat flux to temperature difference, see the Heat Transfer Coefficient Converter.

Why Use a Heat Flux Density Converter?

Solar irradiance data, electronics cooling specifications, and building energy audits each tend to report heat flux density in whatever unit is conventional for that field, W/m² for solar and general engineering, BTU/(hr·ft²) for US HVAC and building science, and occasionally cal/(s·cm²) in older scientific references. Comparing values across these contexts requires accurate conversion.

Who Should Use This Converter?

  • Electronics thermal engineers working with component heat flux specifications from datasheets in mixed unit systems.
  • Solar energy engineers converting irradiance data between W/m² and other unit conventions.
  • Building energy auditors comparing heat flux measurements through walls, roofs, and windows across metric and imperial units.
  • HVAC engineers working with heating and cooling load calculations that combine multiple unit sources.

What Insights Does the Heat Flux Density Converter Give You?

The main output is the converted value in your chosen target unit, calculated precisely from the fixed multiplier relationship between units. Because heat flux density values span an enormous practical range, from everyday solar irradiance (~1,000 W/m²) to concentrated electronics heat flux (over 1,000,000 W/m²), the converter helps confirm a value is realistic for its context.

How to use this Heat Flux Density calculator

  1. Choose your starting unit from the source dropdown, for example, "BTU per Hour-Square Foot".
  2. Enter the numeric value you want to convert in the input field.
  3. Choose your target unit from the destination dropdown, for example, "Watts per Square Metre (W/m²)".
  4. Read the converted result, which updates instantly as you type or change units.
  5. Use the swap (⇅) button if you need to reverse the conversion direction.
  6. Use the copy button to grab the result for a thermal design calculation or energy audit report.
Show formula & methodology ↓Show less ↑

Formula & Methodology

The converter's base unit is watts per square metre (W/m²). Every supported unit has a fixed multiplier:

- 1 kilowatt per square metre (kW/m²) = 1,000 W/m²
- 1 BTU per hour-square foot (BTU/(hr·ft²)) ≈ 3.15459 W/m²
- 1 calorie per second-square centimetre (cal/(s·cm²)) = 41,840 W/m²

Any conversion follows:

Result = Input × (toBase of source unit ÷ toBase of target unit)

Worked example, converting typical solar irradiance of 1,000 W/m² to BTU/(hr·ft²):

Result = 1,000 × (1 ÷ 3.15459) = 317.0 BTU/(hr·ft²)

This matches the commonly cited imperial-unit equivalent for peak clear-sky solar irradiance.

Related Tools

You may also find these useful: Thermal Conductivity Converter, Illumination Converter.

Frequently Asked Questions

What is heat flux density?
Heat flux density measures the rate of heat energy transfer per unit area, expressed in watts per square metre or equivalent units, it describes how intensely heat is flowing through a surface, whether from a heater, solar radiation, or a hot component's casing.
How do I convert W/m² to BTU/(hr·ft²)?
Divide the W/m² value by 3.15459, since one BTU per hour-square foot equals approximately 3.15459 watts per square metre. Enter your value with 'Watts per Square Metre (W/m²)' as the source and 'BTU per Hour-Square Foot' as the target to apply this automatically.
Is heat flux density the same thing as solar irradiance?
Solar irradiance is a specific application of heat flux density, it measures the power of sunlight per unit area (commonly around 1,000 W/m² at Earth's surface on a clear day), using the exact same units and physical concept as heat flux density in a general engineering context.
How does heat flux density relate to heat transfer coefficient?
Heat flux density equals the heat transfer coefficient multiplied by the temperature difference driving the heat flow (q = hΔT), so once you know the heat transfer coefficient and temperature difference, you can calculate the resulting heat flux. See the [Heat Transfer Coefficient Converter](/heat-transfer-coefficient-converter/) for that related quantity.
What is a typical heat flux density value for a CPU or electronic component?
Modern high-performance CPUs can generate heat flux densities of over 100 W/cm² (1,000,000 W/m²) at the die surface, which is why sophisticated cooling solutions are required, for comparison, a typical incandescent light bulb's surface might be in the tens of thousands of W/m² range.
Why does the CGS unit cal/(s·cm²) appear in older scientific literature?
CGS-based heat flux units persist in some older scientific and meteorological literature, particularly measurements related to solar radiation and atmospheric heat transfer from before SI units became the universal scientific standard.
How is heat flux density measured in practice?
Heat flux density is commonly measured using a heat flux sensor (a thin device placed on a surface that generates a voltage proportional to the heat passing through it) or calculated indirectly from temperature gradient measurements combined with known thermal conductivity.
Why do building energy audits use heat flux density?
Measuring heat flux density through walls, roofs, and windows helps identify where a building is losing the most heat, guiding insulation upgrade priorities, areas with unexpectedly high heat flux density indicate poor insulation or thermal bridging.
What's the difference between heat flux and heat flux density?
Heat flux (or heat rate) is the total rate of heat energy transfer in watts, without reference to area, while heat flux density specifically normalises that rate per unit area, heat flux density is the more useful quantity for comparing materials or surfaces of different sizes.
Can this converter be used for radiant heating panel specifications?
Yes, radiant heating panels are often specified by their heat output per unit area, which is exactly what heat flux density measures, making this converter useful for comparing panel specifications across different unit conventions.
Also known as
heat flux density converterw/m2 to btu/hr ft2 convertersolar irradiance converterheat flux converterthermal flux converter