LED Heatsink Calculator

This tool calculates the maximum allowable thermal resistance of the heatsink (R_th,sa) required to keep the LED junction temperature (T_j) safely below its maximum rated value. It derives this by first computing total dissipated power (P_diss = P_LED × (1 − Efficiency/100)), then applying the thermal resistance chain: R_th,sa = [(T_{j max} − T_ambient) / P_diss] − R_th,jc.

LEDs convert only a portion of electrical input power into visible light—the rest becomes waste heat. Efficiency determines how much of the input power must be dissipated thermally. For example, a 10 W LED at 35% efficiency dissipates 6.5 W as heat (10 × 0.65), not the full 10 W—so omitting efficiency leads to oversized (and costly) heatsinks.

R_th,jc (junction-to-case thermal resistance) quantifies how easily heat flows from the LED die to its package surface (e.g., MCPCB or metal-core base). It’s a datasheet parameter—typically 0.5–4.0 °C/W for high-power LEDs—and must be entered accurately, as errors here directly impact heatsink sizing.

Most high-power white LEDs specify T_{j max} = 125–150 °C; 125 °C is conservative and widely used. Ambient temperature (T_ambient) should reflect worst-case operating conditions—not room temperature. For enclosed fixtures, use 40–60 °C; for outdoor applications in hot climates, consider 50–70 °C.

R_th,sa is the *maximum allowable* thermal resistance from case to ambient air—including interface materials (thermal paste/pad), heatsink, and airflow effects. To select a real heatsink, choose one with a published R_th,sa ≤ your calculated value—preferably 10–20% lower to account for real-world derating and mounting variability.

A negative result means the thermal budget is impossible with the given inputs: even with zero heatsink resistance, junction temperature would exceed T_{j max}. Solutions include reducing drive power, improving efficiency (e.g., using higher-efficiency LEDs), lowering ambient temperature, or selecting an LED with lower R_th,jc or higher T_{j max}.

No—it calculates the *required* R_th,sa but does not model airflow. However, heatsink datasheets specify R_th,sa values for defined conditions (e.g., “natural convection, vertical orientation” or “2 m/s airflow”). Always match your chosen heatsink’s test condition to your actual application environment.

Yes—but treat the entire array as a single thermal system. Sum the total electrical input power, use the weighted average efficiency (or worst-case per LED), and apply the highest R_th,jc among the LEDs. Ensure mechanical layout allows uniform heat spreading; localized hot spots may require additional thermal analysis.

TIM resistance (R_th,interface) adds directly to the total thermal path: R_th,total = R_th,jc + R_th,cs + R_th,sa, where R_th,cs is case-to-sink resistance (dominated by TIM). The calculator assumes R_th,cs is included in—or negligible relative to—your R_th,sa target. Always select low-resistance TIM and apply it correctly.

While not explicitly modeled here, R_th,sa generally improves (decreases) with larger surface area, greater fin density, and higher thermal conductivity materials. As a rule of thumb, doubling heatsink volume typically reduces R_th,sa by ~20–40% under natural convection—but airflow, orientation, and material matter more than mass alone.