Overview
Heat moves through materials in ways that depend on several distinct physical properties, each with its own unit and its own US-versus-SI unit split. Thermal conductivity describes how well a material conducts heat; thermal expansion describes how much a material grows as it warms; thermal resistance describes how much an assembly resists heat flow overall; and heat flux, heat transfer coefficient, specific heat capacity, and heat density each describe a different piece of how energy moves and accumulates. This guide walks through all seven, with the exact conversion factors connecting the SI and US customary versions of each.
A recurring theme across these units is the difference between an interval-based measurement (like a temperature difference) and an absolute one. Because a change of 1°C equals a change of 1 Kelvin, several of these units convert cleanly between °C-based and K-based forms with no factor at all, it's only the Fahrenheit-based US customary units that introduce the 1.8 scaling factor from the different degree size.
These seven properties also chain together in real heat transfer problems. A wall's overall thermal resistance depends on the thermal conductivity of each layer and its thickness; the rate heat actually crosses that wall (heat flux density) depends on the resistance and the temperature difference across it; and how quickly a room's air responds to that heat gain depends on the air's specific heat capacity and mass. Understanding each unit individually makes it much easier to follow, and sanity-check, a multi-step calculation that combines several of them.
Step 1: Convert Thermal Conductivity
Thermal conductivity (k) measures how readily a material conducts heat through its bulk, in watts per metre-kelvin, W/(m·K). Metals sit at the high end, copper at about 401 W/(m·K), aluminum around 205 W/(m·K), while insulating materials like fiberglass batting sit near 0.04 W/(m·K), a difference of four orders of magnitude. The US customary unit, BTU/(hr·ft·°F), relates by 1 W/(m·K) = 0.5778 BTU/(hr·ft·°F), while the CGS unit cal/(s·cm·°C) is also occasionally used in materials science literature. The Thermal Conductivity Converter also handles the insulation-industry unit BTU·in/(hr·ft²·°F), commonly seen on foam board and batt insulation packaging, where a lower number always indicates a better insulator regardless of which unit system the packaging uses.
Step 2: Convert Thermal Expansion Coefficients
The coefficient of thermal expansion (α) describes the fractional change in a material's length (or volume, for volumetric expansion) per degree of temperature change. Steel expands at roughly 12 × 10⁻⁶ per °C (often written as 12 ppm/°C), while aluminum expands faster at about 23 × 10⁻⁶ per °C, a difference that matters when bolting dissimilar metals together across large temperature swings. Because °C and K intervals are identical in size, per-°C and per-K values are numerically the same; converting to per-°F requires dividing by 1.8, since each Fahrenheit degree represents a smaller temperature change. The Thermal Expansion Converter handles per-K, per-°C, per-°F, per-°R, and ppm/°C forms.
Step 3: Convert Thermal Resistance
Thermal resistance (R, in K/W or the numerically identical °C/W) describes how much a specific component or assembly resists heat flow for a given power input, a heat sink rated at 2°C/W will rise 2°C above ambient for every watt of heat it dissipates. The US customary equivalent, °F·hr/BTU, relates by 1 K/W ≈ 0.5275 °F·hr/BTU (equivalently, 1 °F·hr/BTU ≈ 1.896 K/W). This unit is central to electronics cooling design, where a CPU's thermal design power is matched against a heat sink's rated thermal resistance to keep junction temperature within spec. The Thermal Resistance Converter converts between all three forms.
Step 4: Convert Heat Flux Density
Heat flux density (W/m²) measures the rate of heat energy passing through a unit area, independent of what's driving it, solar radiation, a furnace wall, or a heat exchanger surface. Peak solar irradiance on a clear day is roughly 1,000 W/m², which converts to about 317 BTU/(hr·ft²) using the factor 1 W/m² = 0.317 BTU/(hr·ft²) (from combining the watt-to-BTU/hr and square-metre-to-square-foot conversions). The CGS unit cal/(s·cm²) also appears in older heat transfer references. The Heat Flux Density Converter covers all three, useful for comparing solar panel or furnace specifications across different regional unit conventions.
Step 5: Convert Heat Transfer Coefficient
The heat transfer coefficient (h, in W/(m²·K)) quantifies how effectively heat moves between a surface and a surrounding fluid per degree of temperature difference, natural convection in still air typically gives values of 5–25 W/(m²·K), while forced convection with a fan or pump can reach 25–250 W/(m²·K), and boiling or condensing fluids can exceed 10,000 W/(m²·K). The US customary form, BTU/(hr·ft²·°F), relates by 1 W/(m²·K) ≈ 0.1761 BTU/(hr·ft²·°F). This coefficient is a core input to convective heat transfer calculations (Q = hAΔT), and the Heat Transfer Coefficient Converter converts between the SI and US customary forms along with the CGS cal/(s·cm²·°C).
Step 6: Convert Specific Heat Capacity
Specific heat capacity (c) describes how much energy is needed to raise one unit of mass by one degree, in J/(kg·K). Water's specific heat capacity, 4,184 J/(kg·K), equivalently exactly 1 cal/(g·°C) by the calorie's historical definition, is unusually high, which is why water is an effective coolant and thermal buffer, while most metals sit far lower (aluminum at about 897 J/(kg·K), copper at about 385 J/(kg·K)). The US customary unit BTU/(lb·°F) is numerically close to cal/(g·°C) since both trace back to similar historical definitions. The Specific Heat Capacity Converter converts between all of these directly.
Step 7: Convert Heat Density
Heat density (or volumetric energy density, J/m³) measures energy stored or released per unit volume rather than per unit mass, relevant for comparing fuel energy content, phase-change material capacity, or combustion energy release. Converting 1 cal/cm³ to SI gives 4,184,000 J/m³ (since 1 cal = 4.184 J and 1 cm³ is a millionth of a cubic metre), while 1 BTU/ft³ converts to approximately 37,259 J/m³. These figures matter in HVAC load calculations and material science when comparing energy storage capacity per unit volume across different measurement conventions. The Heat Density Converter handles J/m³, kJ/m³, BTU/ft³, and cal/cm³, making it straightforward to compare a fuel's energy density figure against a competing fuel or storage medium reported in a different unit system.
Key Terms
- Thermal Conductivity (k): a material property describing how readily heat conducts through its bulk, in W/(m·K)
- R-value: a measure of thermal resistance used in building insulation, inversely related to thermal conductivity per unit thickness
- Convection Coefficient: another name for the heat transfer coefficient, linking heat flux to a temperature difference at a surface
- Specific Heat Capacity: the energy needed to raise one unit of mass of a substance by one degree
- Coefficient of Thermal Expansion (CTE): the fractional change in a material's dimensions per degree of temperature change
- Joule (J): the SI unit of energy, used as the base unit for heat quantities like heat density
- BTU (British Thermal Unit): the US customary unit of heat energy, equal to approximately 1,055 joules