What is thermal resistance (θ_JA)?

It is the resistance to heat flow from the component junction (hottest point) to the ambient air, in °C/W. A small IC might have θ_JA = 100 °C/W; a power transistor on a heatsink might be 5 °C/W.

How do I derate a component for safety?

Most manufacturers recommend derating to 50–70% of the maximum rated power at 25 °C. Check the derating curve in the datasheet for operation at elevated ambient temperatures.

Power Dissipation Calculator

Calculate power dissipated as heat in a resistor or component. Enter voltage, current and resistance (any two), plus thermal resistance (θ_JA) to estimate temperature rise.

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter voltage across component, current through component, resistance and thermal resistance (θ_ja) in the fields above.
Results update instantly as you type — or click Calculate.
Read your power dissipated and the full breakdown beneath it.

Power dissipated by a resistive element is P = V × I = I²R = V²/R (watts). This power becomes heat; the junction temperature rise is ΔT = P × θ_JA (°C), where θ_JA is the junction-to-ambient thermal resistance from the component datasheet. Excessive temperature rise causes reliability problems.

Formula

Power: P = V × I (primary), also P = I² × R and P = V² / R

Temperature rise: ΔT = P × θ_JA

where V is voltage (V), I is current (A), R is resistance (Ω), and θ_JA is thermal resistance (°C/W).

How it works

The calculator accepts any two of voltage, current, and resistance and derives the missing quantity using Ohm's law (V = IR) before computing power as P = VI; the junction temperature rise is then found by multiplying power by the component's thermal resistance θ_JA.

All three power forms (VI, I²R, V²/R) are calculated internally; the VI result is used as primary output. The model assumes steady-state DC conditions and a purely resistive load — reactive or switching components require more detailed thermal analysis.

Worked example

Worked example — 12 V, 0.5 A, θ_JA = 50 °C/W

Inputs: V = 12 V, I = 0.5 A, R not provided (set to 0).
Derived R = V / I = 12 / 0.5 = 24 Ω (internal use only).
Power P = V × I = 12 × 0.5 = 6.0 W.
Temperature rise ΔT = P × θ_JA = 6.0 × 50 = 300 °C.

Power dissipated = 6.0 W (6000 mW); junction temperature rise = 300 °C.

Key terms

Power dissipation: The rate at which electrical energy is converted to heat in a component, measured in watts (W); must not exceed the component's rated power.
Thermal resistance (θ_JA): The opposition to heat flow from a component's junction to ambient air, expressed in °C/W; a higher value means the component runs hotter for a given power level.
Junction temperature: The temperature at the active die or resistive element of a component; exceeding the maximum rated junction temperature causes degraded performance or failure.
Ohm's law: V = IR; relates voltage, current, and resistance in a linear resistive element, allowing any one quantity to be found from the other two.
Joule heating: Heat produced in a resistive element by current flow, equal to I²R watts; the fundamental mechanism of power dissipation in resistors and conductors.

Frequently asked questions

What is thermal resistance (θ_JA)?: It is the resistance to heat flow from the component junction (hottest point) to the ambient air, in °C/W. A small IC might have θ_JA = 100 °C/W; a power transistor on a heatsink might be 5 °C/W.
How do I derate a component for safety?: Most manufacturers recommend derating to 50–70% of the maximum rated power at 25 °C. Check the derating curve in the datasheet for operation at elevated ambient temperatures.