Thermal Resistance Calculator

Thermal resistance is a property that describes how much a material opposes the flow of heat. Just as electrical resistance quantifies opposition to current, thermal resistance quantifies the temperature difference required to drive a given rate of heat flow. A higher thermal resistance means the material is a better insulator — the same heat flux will produce a larger temperature drop across it. This concept is foundational in thermal circuit analysis, where resistances can be combined in series or parallel to model complex assemblies.

For a flat wall (slab geometry), the thermal resistance is given by R_th = L / (k × A), where L is the wall thickness (m), k is the material's thermal conductivity (W/m·K), and A is the surface area perpendicular to heat flow (m²). For a hollow cylinder — as found in insulated pipes — the geometry requires accounting for the increasing surface area with radius, giving R_th = ln(r_o / r_i) / (2π k L), where r_o is the outer radius, r_i is the inner radius, and L is the pipe length. Both forms return units of K/W (or equivalently °C/W), meaning a 1 W heat flow causes a 1 K temperature difference per unit of thermal resistance.

Thermal resistance is applied across a wide range of engineering disciplines. In building physics, it underpins the R-value used to rate insulation products. In electronics cooling, it governs how junction temperatures rise under load. In chemical process engineering, it determines heat exchanger sizing and piping heat loss. In aerospace, it is critical for thermal protection systems on re-entry vehicles and spacecraft. The thermal resistance model also enables straightforward estimation of heat loss rates when combined with a known temperature differential: Q = ΔT / R_th.

This calculator assumes steady-state, one-dimensional heat conduction with constant thermal conductivity. In practice, k can vary significantly with temperature — especially for polymers, insulation foams, and refractory materials at high temperatures — so using a mean temperature value for k improves accuracy. The flat wall formula assumes uniform cross-sectional area; tapered or irregular shapes require numerical methods or integration. For the cylindrical model, end effects are neglected, which is valid only when L is much larger than the radii. Composite layers (e.g., multiple insulation materials in series) can be handled by summing individual R_th values, but contact resistance at interfaces is ignored here and may be significant for pressed metal contacts or poorly bonded layers. Convective and radiative resistances at surfaces are also not included — full thermal circuit analysis should add surface film resistances for accurate end-to-end modelling. This tool is intended for first-principles estimation and educational use, not as a substitute for validated thermal simulation software in safety-critical applications.