Heat Flux Converter

Convert between W/m², BTU/(h·ft²), kW/m², and cal/(s·cm²) — the rate of heat transfer per unit area, for thermal and solar engineering.

Heat Flux

W/m², BTU/(h·ft²)

From

Result

0.001

1 W/m² = 0.001 kW/m²

Popular conversions

What Is a Heat Flux Converter?

A heat flux converter translates the rate of heat transfer per unit area between units — watts per square metre to BTU per hour per square foot, and so on. Heat flux (or heat flux density) measures how much thermal power flows through a surface, and the SI unit is the watt per square metre (W/m²). It's central to insulation, solar energy, electronics cooling, and building thermal design.

This converter routes through W/m². The US thermal and HVAC industries use BTU/(h·ft²), solar engineers use W/m² and kW/m² (solar irradiance at the Earth's surface peaks near 1,000 W/m²), and older texts use cal/(s·cm²). The tool bridges these so a thermal load reads correctly in whatever unit a spec or calculation uses.

This is one category of the full Unit Converter — pair it with our percentage calculator or scientific calculator for related everyday maths.

How Heat Flux Conversion Works

Everything routes through W/m²

Each unit has a fixed W/m² factor. The converter normalises to watts per square metre, then projects to BTU/(h·ft²) and more.

Heat flux is power per area

W/m² is thermal power passing through each square metre of surface. Multiply by area to get total heat flow in watts.

Combines power and area factors

Because heat flux pairs a power unit with an area unit, its conversion factors fold both — which is why W/m² to BTU/(h·ft²) isn't a round number.

Solar irradiance benchmark

Full sun at the surface is about 1,000 W/m² (1 kW/m²), the reference used to rate solar panels and thermal collectors.

Core Heat Flux Conversion Factors

Multiply to reach W/m²; divide to come back.

BTU/(h·ft²) → W/m²

× 3.15459

One BTU per hour per square foot is 3.155 W/m². The US-to-SI thermal bridge.

kW/m² → W/m²

× 1000

One kilowatt per square metre is 1,000 W/m² — about one sun of irradiance.

Heat flux

q = Q / A

Heat flux is total heat power divided by the area it passes through.

How to Use the Heat Flux Converter

1
Enter the heat flux value
Type the heat flux you want to convert — a solar irradiance, an insulation rating, a cooling load per area.
2
Choose the 'from' unit
Pick W/m², kW/m², BTU/(h·ft²), or cal/(s·cm²) as your starting unit.
3
Choose the 'to' unit
Select the target unit, or swap the two to reverse direction.
4
Read every unit at once
The all-units table shows the heat flux across W/m², kW/m², and BTU/(h·ft²) together.

Key Heat Flux Concepts

Watt per square metre

The SI unit of heat flux: thermal power per unit area. Full sunlight is ~1,000 W/m²; building heat loss is far less.

Solar irradiance

Heat flux from the Sun, ~1,000 W/m² at the surface under clear skies (1,361 W/m² at the top of the atmosphere). The basis of solar ratings.

BTU/(h·ft²)

The US thermal unit for heat flux, used in HVAC and building codes. One BTU/(h·ft²) is about 3.155 W/m².

Heat flux vs heat

Heat flux (W/m²) is a rate per area; total heat (joules) or power (watts) needs the area and time. Don't conflate them.

Real-World Heat Flux Conversions

☀️

Solar panels

Standard test conditions use 1,000 W/m² (about 317 BTU/(h·ft²)). Panel output is rated against this irradiance.

🏠

Building heat loss

A wall might lose 30 W/m² in winter. Insulation performance is assessed as heat flux through the envelope.

💻

Electronics cooling

A CPU can dissipate 50+ W/cm² locally. Thermal design works in heat flux to size heat sinks and spreaders.

🔥

Furnace walls

Industrial furnace linings see high heat flux. Refractory design specifies W/m² the lining must withstand.

🛰️

Spacecraft thermal

Sunlit surfaces in orbit receive ~1,361 W/m². Thermal control balances absorbed flux against radiated heat.

🌡️

HVAC loads

A roof gain of 20 BTU/(h·ft²) is about 63 W/m². Building HVAC sizing converts between the two unit systems.

Best Practices for Heat Flux Conversion

✓Use 1,000 W/m² as the sun benchmark. Full surface sunlight is about one kilowatt per square metre — a handy reference to sanity-check any solar heat-flux figure.
✓Combine power and area factors. Heat flux conversions fold a power ratio and an area ratio. The W/m² to BTU/(h·ft²) factor (3.155) isn't round for exactly this reason.
✓Don't confuse flux with total power. Heat flux is per area. Multiply by the surface area to get total watts; the flux figure alone doesn't give the whole load.
✓Match the unit to the field. Solar and SI engineering use W/m²; US HVAC and building codes use BTU/(h·ft²). Convert to the convention your spec follows.
✓Mind area units in electronics. Chip heat flux is often W/cm², far larger numerically than W/m². Confirm whether an area is in cm² or m² before comparing.

Common Heat Flux Conversion Mistakes

Confusing flux with power

Heat flux (W/m²) is per area, not total. Treating it as total watts ignores the surface area and undercounts the real load.

Mixing W/m² and W/cm²

They differ by 10,000. Electronics uses W/cm²; building science uses W/m². A wrong area unit is a huge error.

Using a round BTU factor

1 BTU/(h·ft²) is 3.155 W/m², not 3. The folded power-and-area ratio isn't a tidy number.

Ignoring the time basis

BTU/(h·ft²) is per hour; W/m² is per second. The conversion already accounts for this, but dropping it elsewhere misstates the flux.

Why Heat Flux Conversion Matters

Heat flux governs solar-panel ratings, building insulation, electronics cooling, and industrial thermal design — and it's quoted in W/m² for SI and solar work but BTU/(h·ft²) in US HVAC and building codes. Because it folds a power unit with an area unit, the conversion factor isn't round, and confusing flux with total power or mixing m² and cm² leads to big errors.

With energy efficiency and thermal management under constant pressure, accurate heat-flux units underpin everything from solar yield to chip reliability. A converter that routes through W/m² and bridges BTU/(h·ft²) and kW/m² lets thermal, solar, and building engineers compare heat-transfer rates across both unit systems.

Built for solar, HVAC, thermal, and electronics-cooling engineers converting between W/m², kW/m², and BTU/(h·ft²).

Linear unit factors follow the BIPM SI brochure, the NIST Guide to the SI, and ISO 80000. Currency rates load live from open.er-api.com; crypto prices from CoinGecko. See our methodology and editorial policy. Educational only — not certified for regulated trading, settlement, medical, or aerospace use.

Heat Flux Converter FAQs

Divide watts per square metre by 3.15459 to get BTU per hour per square foot, or multiply BTU/(h·ft²) by 3.15459 to get W/m². So 1,000 W/m² (full sun) is about 317 BTU/(h·ft²). The factor folds together the power (watt-to-BTU/h) and area (m²-to-ft²) conversions.

Heat flux (or heat flux density) is the rate of heat-energy transfer through a unit area of surface, measured in watts per square metre (W/m²) in SI units. It tells you how intensely heat is flowing through a wall, panel, or component — multiply by the area to get the total heat power in watts.

The solar irradiance at the top of Earth's atmosphere is about 1,361 W/m² (the solar constant). At the surface under clear skies it's roughly 1,000 W/m² (1 kW/m²), which is the standard test condition used to rate solar panels — equivalent to about 317 BTU/(h·ft²).

Heat (measured in joules) is a total amount of thermal energy; heat flux (W/m²) is the rate of heat transfer per unit area. To get total heat power you multiply heat flux by the surface area, and to get total energy you also multiply by time. They're related but distinct quantities.

Because chips concentrate large amounts of heat into tiny areas, the numbers are more manageable in watts per square centimetre. A CPU can exceed 50 W/cm², which equals 500,000 W/m². Always confirm whether a heat-flux figure uses cm² or m², since they differ by a factor of 10,000.

It uses exact derived factors (1 BTU/(h·ft²) = 3.154591 W/m², 1 kW/m² = 1,000 W/m²) and routes every conversion through W/m² at full precision, so the result is exact to your input precision.