Convert heat flux density between W/m², kW/m², cal/(s·cm²) and BTU/(h·ft²).
Heat • 6 units
All 6 units on the Heat Flux Density Converter are defined against the Watt/meter² (W/m²), so each result is one conversion factor away from a single reference rather than the end of a chain of roundings.
The conversion this page is most often opened for is ready before you type anything: 1 W/m² = 0.316998305 BTU/(h·ft²). Change either side and every row in the table recalculates with it.
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This page opens on watts per square metre into BTU per hour per square foot, and the factor is worth remembering in both directions: 1 W/m² is 0.317 BTU/(h·ft²), and 1 BTU/(h·ft²) is 3.155 W/m². Building-fabric work in the United States is still published in the imperial form while the underlying physics is taught in SI, so this single conversion sits between most American envelope calculations and most of the literature behind them.
Two anchor values bracket the scale in engineering practice. Radiative emission from a surface follows the Stefan–Boltzmann law, so an ideal radiator at 1,000 K emits about 56.7 kW/m² — which is why furnace and kiln design is dominated by radiation rather than convection. At the other extreme sits the critical heat flux of boiling water at atmospheric pressure, around 1 MW/m²: beyond it the surface is blanketed by vapour, heat transfer collapses rather than improves, and the metal temperature runs away. That single limit shapes boiler, reactor and cooling-channel design.
Measured heat flux and calculated heat flux are not interchangeable. A thin-film heat-flux sensor is a thermopile reading the small temperature drop across a known thermal resistance, so it responds to whatever is passing through it at that instant — including solar gain, air movement and stored heat released from the wall behind it. In-situ envelope measurement standards therefore call for continuous logging over days and averaging across the record, because a flux reading taken over an afternoon describes the weather rather than the construction.
Quick reference — 1 Watt/meter² (W/m²) is equal to:
| Kilowatt/meter² | kW/m² | 0.001 |
| Watt/centimeter² | W/cm² | 0.0001 |
| Calorie/(s·cm²) | cal/(s·cm²) | 0.0000239006 |
| BTU/(h·ft²) | BTU/(h·ft²) | 0.316998305 |
| Kilocalorie/(h·m²) | kcal/(h·m²) | 0.859845228 |
6 units of heat flux density, each a fixed multiple of the W/m². The table spans 41,840:1, from W/m² (1 W/m²) to cal/(s·cm²) (41840 W/m²). Conversion is one multiplication into the base unit and one division out of it, with no lookup table and no approximation.
value_to = value_from × (factor_from ÷ factor_to)1 W/m² = 1 W/m²1 kW/m² = 1000 W/m²1 W/cm² = 10000 W/m²1 cal/(s·cm²) = 41840 W/m²1 BTU/(h·ft²) = 3.15459 W/m²1 kcal/(h·m²) = 1.163 W/m²where:
Assumptions: Factors are exact definitions. Full double precision is carried internally and rounding is applied only for display.
Result25 W/m² = 7.92496 BTU/(h·ft²)
Heat flux density is power crossing unit area, in watts per square metre (W/m²). It is the quantity that determines whether a surface can actually shed the heat reaching it, independent of how large that surface is.
One reference value anchors the whole scale: the solar constant, about 1,361 W/m² at the top of the atmosphere and roughly 1,000 W/m² at ground level on a clear day. Building envelopes are analysed in the same units, electronics cooling in watts per square centimetre, and older HVAC work in BTU per hour per square foot. A processor die dissipating 100 W over 2 cm² is running at 500,000 W/m², which is why the heatsink, not the chip, sets the design.