Thermal Resistance Calculator

Thermal resistance (K/W) and temperature rise for a flat material stack

This Thermal Resistance Calculator is built for one practical job: estimating how much temperature rise you should expect when heat is forced through one or more flat layers. If you have a heat source (a component, a plate, a heater, a device) and the heat must conduct through a pad, sheet, plate, or insulation layer, the key question is simple: “How many degrees will this add at my power level?” This calculator turns thickness, thermal conductivity, and contact area into a total thermal resistance in K/W, then converts that into a temperature rise for your heat load.

Use it when you are deciding whether a thermal pad is too resistive, whether an insulating layer will cause unacceptable temperature rise, or whether increasing contact area and reducing thickness will materially improve performance. The default inputs stay minimal: heat load (W), contact area (cm²), and one required layer (thickness in mm and thermal conductivity in W/m·K). If you want a more realistic stack, you can add up to two more layers and optionally add an “extra” thermal resistance term to represent contact resistance, interfaces, imperfect mating surfaces, or an unknown lumped contribution you want to account for.

The output is designed to be decision-oriented rather than academic. You get total thermal resistance (K/W) and the implied temperature rise across the stack (°C) at your heat load. You also get thermal conductance (W/K), heat flux (W/m²), and a simple layer contribution breakdown so you can see what is dominating. That makes it easier to pick the right lever: reduce thickness, increase area, pick a higher-k material, or stop optimizing the wrong layer.

Assumptions and how to use this calculator

• This calculator models one-dimensional conduction through flat layers: the standard R = L / (k·A) relationship, where heat flows straight through the stack.
• All layers are treated as being in series over the same contact area. If your real geometry has spreading resistance, edge losses, or parallel paths, results will be optimistic.
• Thermal conductivity is assumed constant. Many materials change k with temperature, moisture content, compression, or direction (anisotropy).
• Convection and radiation are excluded on purpose. This is for conduction through materials, not for full “junction-to-ambient” thermal modelling.
• The “Extra thermal resistance (K/W)” field is a lumped add-on. Use it for contact resistance, interface effects, or unknown contributions when you want a conservative estimate.

Common questions

What does thermal resistance (K/W) actually mean?

K/W means “degrees of temperature rise per watt of heat flow.” If your total thermal resistance is 2 K/W and your heat load is 10 W, the temperature rise across the stack is about 20 °C. This is why small increases in resistance can become a big problem at higher power.

Why does contact area matter so much?

For flat conduction, resistance is inversely proportional to area. Doubling the contact area halves the thermal resistance, all else equal. In real systems the effective area can be smaller than you think because of surface roughness, voids, uneven pressure, or non-uniform heat spreading. If your results look too good to be true, assume your effective area is smaller and re-run.

Do I have to fill in Layer 2 and Layer 3?

No. Leave them blank and the calculator ignores them. If you do use them, enter both thickness and thermal conductivity for each layer. Partial entries are treated as incomplete because the resistance calculation needs both values.

What should I put in “Extra thermal resistance”?

If you know you have additional interfaces (for example, a pad-to-heatsink contact, or a poorly mated joint) you can represent that uncertainty as a lumped K/W value. If you have no idea, leave it at 0 and treat the result as a best-case conduction-only estimate. If you are making a go/no-go decision, adding a conservative extra resistance is often more realistic than pretending interfaces are perfect.

When does this calculator not apply?

If the dominant path is convection to air, radiation, complex geometry (spreading resistance), fins, fluid cooling, or a known thermal network like junction-to-case-to-ambient with datasheet resistances, this tool is not the right model. It is intentionally focused on flat, series conduction through layers so you can make quick material and thickness decisions.