Heat flux (also called thermal flux or thermal flux density) is the rate of heat energy transfer through a given surface per unit area. It is measured in watts per square meter (W/m²) in the SI system. Heat flux represents how much thermal energy flows through a surface each second and is a fundamental concept in thermodynamics, HVAC engineering, and heat transfer analysis. It can be measured using heat flux sensors (thermopiles), calorimeters, or calculated from temperature gradients using Fourier's Law.
SI Units Converter
Convert between W/m², kW/m², MW/m², mW/m² and μW/m² — the International System of Units for heat flux measurement.
Conversion Formula
1 W/m² = 1000 mW/m²
Fourier's Law of Heat Conduction
Heat flux is fundamentally described by Fourier's Law, which states that the rate of heat transfer through a material is proportional to the negative gradient of temperature and to the area. This law forms the foundation of all thermal engineering calculations.
q = −k × (dT/dx)
q
Heat flux (W/m²)
k
Thermal conductivity (W/m·K)
−
Heat flows from hot to cold
dT/dx
Temperature gradient (K/m)
For convective heat transfer, Newton's Law of Cooling applies: q = h × (Tₛ − T∞), where h is the convective heat transfer coefficient (W/m²·K), Tₛ is surface temperature, and T∞ is ambient temperature.
Common Conversions
Quick reference table| From | To | Result |
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What is Heat Flux?
Heat flux (also called thermal flux or thermal flux density) is the rate of heat energy transfer through a given surface per unit area. It is a vector quantity, meaning it has both magnitude and direction, and is measured in watts per square meter (W/m²) in the SI system.
Heat flux represents how much thermal energy flows through a surface each second. A higher heat flux means more thermal energy is being transferred through the same area in the same time. This concept is fundamental in thermodynamics, HVAC engineering, electronics cooling, building insulation, and materials science.
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SI Unit Watt per square meter (W/m²) — one joule per second per square meter
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Symbol Typically denoted as q, q″, or φ (phi) in engineering literature
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Vector Nature Heat flux has direction — it flows from higher to lower temperature
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Three Modes Conduction, convection, and radiation — each with distinct mechanisms
Applications of Heat Flux Measurement
Heat flux measurement and conversion is critical across numerous engineering disciplines and industries. Understanding and converting between different heat flux units enables professionals to work with international standards, legacy systems, and specialized equipment.
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HVAC Engineering Heating, ventilation, and air conditioning system design and load calculations
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Building Insulation Thermal performance analysis of walls, roofs, and windows for energy efficiency
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Electronics Cooling Thermal management of semiconductors, CPUs, and power electronics
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Aerospace Re-entry vehicle thermal protection and spacecraft thermal control
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Metallurgy Heat treatment processes, casting, and welding thermal analysis
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Solar Energy Solar irradiance measurement and photovoltaic system design
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Fire Safety Fire heat release rate and flame spread analysis
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Biomedical Human thermal comfort, burn injury analysis, and medical device design
How to Use This Converter
Our heat flux converter is engineered for speed, precision, and ease of use. Follow these steps to get accurate conversions instantly.
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Select Unit Family Choose from 6 categories using the sidebar (SI, British Thermal, Calorie-Based, Mechanical, etc.)
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Choose Source Unit Pick the heat flux unit you want to convert from in the first dropdown
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Choose Target Unit Select the unit you want to convert to in the second dropdown
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Enter Value Type your heat flux value — the result appears instantly as you type
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Swap Units Click the swap button to instantly reverse the conversion direction
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View Formula The exact conversion formula is displayed below for verification
Why Professionals Choose ASLI FORM
Built with precision engineering and premium design, our converter meets the demands of thermal engineers, HVAC professionals, and researchers.
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Scientific Precision Results accurate to 10 significant digits using NIST and ISO standard conversion factors
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Instant Results Real-time conversion as you type — no buttons to click, no waiting
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Works Offline All calculations run locally in your browser. Perfect for field work and labs
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Universal Compatibility From basic keypad phones to 4K displays — works on every device and OS
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Free Forever No subscriptions, no hidden fees, no usage limits, no advertisements
Frequently Asked Questions
The SI unit of heat flux is the watt per square meter (W/m²). One W/m² represents one joule of heat energy passing through one square meter of surface area per second. Other common units include kilowatts per square meter (kW/m²), BTU per hour per square foot (BTU/h·ft²), and calories per second per square centimeter (cal/s·cm²). The W/m² unit is used in most scientific and engineering applications worldwide.
Heat flux is calculated using Fourier's Law of Heat Conduction: q = −k × (dT/dx), where q is heat flux (W/m²), k is thermal conductivity (W/m·K), and dT/dx is the temperature gradient. For convection, Newton's Law of Cooling is used: q = h × (Tₛ − T∞), where h is the convective heat transfer coefficient. For radiation, the Stefan-Boltzmann Law applies: q = ε × σ × (T⁴ − Tₛ⁴), where ε is emissivity and σ is the Stefan-Boltzmann constant.
Heat flux is heat transfer rate per unit area (W/m²), while heat transfer rate (or heat flow) is the total energy transferred per unit time (W). Heat flux is intensive (independent of area), while heat transfer rate is extensive (depends on total area). Multiply heat flux by area to get total heat transfer rate: Q = q × A. For example, a heat flux of 1000 W/m² over 2 m² gives a total heat transfer rate of 2000 W.
To convert from W/m² to BTU(IT)/h·ft², multiply by 0.316998. To convert from BTU(IT)/h·ft² to W/m², multiply by 3.15459. For example, 100 W/m² = 100 × 0.316998 = 31.7 BTU/h·ft². Our converter handles this automatically — just select the source and target units and enter your value.
The three fundamental modes of heat transfer are:
- Conduction — heat transfer through direct contact (Fourier's Law)
- Convection — heat transfer through fluid motion (Newton's Law of Cooling)
- Radiation — heat transfer through electromagnetic waves (Stefan-Boltzmann Law)
Our converter delivers results accurate to 10 significant digits using conversion factors from official metrology institutes including NIST, ISO, and the International Bureau of Weights and Measures (BIPM). This level of precision is suitable for engineering calculations, HVAC design, scientific research, and professional applications. All conversion factors are based on internationally accepted standards.
Yes, the ASLI FORM Heat Flux Converter is 100% free forever. There are no registration requirements, no usage limits, no subscription fees, and no hidden charges. We do not display advertisements and we do not collect any personal data. Our mission is to provide a premium engineering tool that everyone can use without barriers.
Yes. Once the page loads, all calculations happen locally in your browser. There is no server-side processing, no API calls, and no internet dependency for conversions. This makes the tool perfect for field work, remote locations, laboratories, and anywhere connectivity is limited or unavailable.
Absolutely. Our converter is designed with a mobile-first responsive approach and works perfectly on every device — from basic keypad phones to 4K desktop displays. The interface adapts intelligently: desktops get a sidebar, tablets get a collapsible sidebar, and mobile devices get optimized dropdowns. Touch interactions, scroll snapping, and momentum scrolling are all optimized for mobile.
BTU(IT) (International Table BTU) is defined as exactly 1055.056 joules, while BTU(th) (thermochemical BTU) is defined as exactly 1054.350 joules. The difference is small (about 0.07%) but matters in precise engineering calculations. Our converter supports both variants so you can use whichever standard your application requires.
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