Thermal Resistance Calculator

Thermal resistance (R) measures how strongly a material opposes the flow of heat. A higher R-value means the material is a better insulator. It is analogous to electrical resistance in circuits — instead of resisting current, it resists heat transfer.

For a flat plate, thermal resistance is R = L / (k × A), where L is the thickness in meters, k is thermal conductivity in W/(m·K), and A is the cross-sectional area in m². For hollow cylinders and spheres, the formula uses logarithmic or inverse-radius expressions due to curved geometry.

In the SI system, thermal resistance is measured in Kelvin per Watt (K/W). In the imperial system used in building insulation, R-values are expressed in ft²·°F·h/BTU. Our calculator provides both.

The critical radius of insulation is the outer radius at which adding more insulation to a pipe or cylinder starts decreasing heat loss rather than increasing it. Below this radius, adding insulation actually increases heat transfer due to the growing surface area. It equals k / h, where k is conductivity and h is the convective coefficient.

Conductive thermal resistance describes resistance to heat flow through a solid material. Convective thermal resistance describes resistance at a fluid–solid interface and equals 1 / (h × A), where h is the convective heat transfer coefficient. Both types can be combined in series or parallel in multi-layer systems.

The convective heat transfer coefficient (h) quantifies how efficiently heat is transferred between a solid surface and an adjacent fluid (air, water, etc.). It is measured in W/(m²·K). Typical values range from 5–25 W/(m²·K) for free air convection to over 10,000 W/(m²·K) for forced liquid cooling.

Thermal and electrical resistance are mathematically analogous. Heat flow (Q) is like electrical current, temperature difference (ΔT) is like voltage, and thermal resistance (R) is like electrical resistance. Ohm's law becomes Q = ΔT / R. This analogy allows engineers to use series and parallel circuit rules for multi-layer thermal systems.

Thermal conductivity (k) varies enormously between materials — diamond conducts heat about 100,000 times better than fiberglass insulation. Since R = L / (k × A), a low-conductivity material like wood or foam creates much higher resistance than a high-conductivity metal like copper for the same thickness.