LED Junction Temperature & Heatsink Calculator

For high-power LED and COB manufacturers — estimates junction temperature (Tj), heat dissipation, and the maximum heatsink thermal resistance that keeps the junction within its rated limit.

1 · Electrical input

Forward voltage Vf (V)

Forward current If (A)

…or enter electrical power directly (W)

Radiant efficiency η — fraction emitted as light (%)

Typical high-power white LED/COB: 30–50%. The rest becomes heat.

2 · Thermal path (junction → ambient)

Ambient temperature Ta (°C)

Max rated junction temp Tj,max (°C)

Rth junction→case, from datasheet (°C/W)

Rth case→sink (TIM total, °C/W)

Or compute below from TIM material.

Heatsink Rth sink→ambient, actual (°C/W)

Used for the Tj estimate. Leave blank to size the heatsink instead.

Optional · TIM (thermal interface material)

TIM conductivity k (W/m·K)

Bondline thickness t (mm)

Contact area A (mm²)

Rth_cs = t / (k · A) with t in m and A in m² (spreading resistance ignored).

Results

Electrical power

– W

Heat to dissipate

– W

Estimated Tj

– °C

Case temp Tc

– °C

Max heatsink Rth allowed

– °C/W

Thermal margin

– °C

P_elec = Vf × If · P_heat = P_elec × (1 − η) · Tj = Ta + P_heat × (Rth_jc + Rth_cs + Rth_sa) · Tc = Tj − P_heat × Rth_jc · Rth_sa,max = (Tj,max − Ta) / P_heat − Rth_jc − Rth_cs

Steady-state one-dimensional thermal-resistance model (series path junction → case → sink → ambient). Rule of thumb: every 10 °C reduction in Tj roughly doubles LED lifetime (Arrhenius). Always confirm against the LED datasheet derating curve.