Junction Temperature Calculator
Steady-state Tj through the θjc + θcs + θsa chain, case temperature, headroom and the max power your cooling allows.
Datasheet θja values are measured on JEDEC standard boards — your layout differs; treat θja as comparative, not absolute. A 10 °C cooler junction roughly doubles electrolytic-cap-adjacent and semiconductor service life (Arrhenius). For pulsed loads use transient thermal impedance Zth(t) from the datasheet instead — short pulses tolerate far more than steady state.
Junction Temperature Calculator computes the silicon junction temperature through the thermal resistance chain — free, instant and private in your browser. Anyone validating that a transistor, regulator or LED stays inside its rating use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About Junction Temperature Calculator
Junction Temperature Calculator computes the silicon junction temperature through the thermal resistance chain using the standard engineering relation: Tj = Ta + P·(θjc + θcs + θsa). Worked live: 3 W through a 2 + 0.5 + 12 °C/W chain in 45 °C ambient puts the junction at ~88.5 °C. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use Junction Temperature Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use Junction Temperature Calculator?
- ✓Implements the real formula — Tj = Ta + P·(θjc + θcs + θsa) — with the substitution shown, not a black box
- ✓Built for anyone validating that a transistor, regulator or LED stays inside its rating
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate junction temperature?+
The silicon junction temperature through the thermal resistance chain follows Tj = Ta + P·(θjc + θcs + θsa). For example, 3 W through a 2 + 0.5 + 12 °C/W chain in 45 °C ambient puts the junction at ~88.5 °C. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
How hot is too hot for a semiconductor junction?+
Ratings run 125–175 °C, but lifetime roughly halves per 10 °C — design targets of 25–40 °C below maximum are normal. Electrolytic capacitors NEXT to a hot device are usually the first real casualty, not the silicon itself.
Can I just use the datasheet's θja number?+
Only as a rough comparison: θja is measured on a standardised JEDEC test board, not yours. Real copper pours can halve it; a tiny board can double it. For anything marginal, compute with θjc plus your actual cooling path, or measure case temperature and work back.
Is the Junction Temperature Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
Related Electronics tools
PCB Trace Width Calculator (IPC-2221)
Minimum trace width for your current and temperature rise — IPC-2221 formula with copper weight, layer choice and a step-by-step worked example.
● LiveMicrostrip Impedance Calculator
Z₀ of a surface microstrip from width, height and εr (IPC-2141A), plus effective dielectric constant and propagation delay.
● LiveStripline Impedance Calculator
Characteristic impedance of an embedded stripline trace from geometry and εr — the inner-layer companion to the microstrip tool.
● Live