Electrical calculator

Heatsink Thermal Resistance Calculator — θJC, θCS, θSA & Junction Temperature

Size required heatsink θSA from Tj,max, or compute junction temperature from effective θJA or the θJC+θCS+θSA stack. Device dissipation only. Runs locally in your browser.

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Instant result

Result

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Enter values to calculate.

Inputs—

Mode—

Formula—

Trust summary CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance

Input interpretation: Enter values to calculate.
Result: —
Assurance: Engineering
Declared partition coverage: PASS · 4/4 declared partitions (ja, stack, required-sa, invalid-domain) · Matrix
Known limitations: Lumped steady-state Rθ; not Zth(t), CFD, or θCA+θSA (double-count)
Core CVP does not include live graph, viewport, or pointer interaction.
Model: Steady-state junction temperature from effective θJA or θJC+θCS+θSA, or required heatsink θSA,max for a Tj limit.
Scope: Steady-state lumped thermal resistance
Verification: Engine tested · Source checked · v1.1.0 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance· View Manifest · CVP overview · Specification
Versions: Calculation 1.1.0 · CVP protocol 1.0.0-proposed
CVP identity: 3/3 property · digest e84796291008
Legacy regression: 17/17 tests · Production surface contract 4/4
Trust layers: Verification VERIFIED · Production CURRENT · overall VERIFIED
Reference: O1 model · O2 expected_values · O2 numerical_behavior
Interfaces: PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session.
Supplemental domain review: Internal · Pass · electrical-engineer
Named expert review: Not performed
CVP suite: 4/4 golden · 8/8 CVP boundary · 9/9 invalid · 3/3 property · 3/3 metamorphic · 2/2 round-trip · 3/3 cross-interface · 1/1 cross-calculator · 4/4 CVP contract · Manifest
Sources: JEDEC JESD51 series — Integrated Circuits Thermal Measurement Method — Junction-to-ambient and junction-to-case thermal metrics on specified test boards
Analog Devices — Practical Design Techniques for Power and Thermal Management — Section 8, Fig. 8.50 — junction → case → case-to-sink → sink-to-ambient
Texas Instruments SPRA953 — Semiconductor and IC Package Thermal Metrics — RθJA, RθJC, and application dependence of junction-to-ambient metrics
Analog Devices — Thermal Characterization of IC Packages — θCA is case-to-ambient (all case-to-ambient paths); θJA = θJC + θCA
Sources
Evidence: 4 legacy golden · 5 legacy boundary · legacy regression suite · 4/4 oracle-backed golden · 9/9 invalid · Artifact integrity PASS · CalculatorX electrical review (EE-001)
Semantic contract: PASS

Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

Formulas

Core equations used by this calculator.

Effective θJATj = Ta + P · θJA

Heatsink stackTj = Ta + P · (θJC + θCS + θSA)

Required heatsinkθSA,max = (Tj,max − Ta)/P − θJC − θCS

iSteady-state lumped Rθ only. Not CFD or Zth(t). θJA and θJC+θCS+θSA are exclusive models. θCA is case-to-ambient (already includes the sink path) and must not be added to θSA. °C/W and K/W are identical for temperature differences.

How to use

Choose the thermal model

Effective θJA uses application junction-to-ambient resistance. Heatsink stack uses θJC+θCS+θSA. Required heatsink solves the maximum sink-to-ambient resistance for a Tj limit.

Enter ambient temperature and device dissipation

Ta and power dissipated as heat P (device loss converted to heat), not supply rating, load power, or output power.

Enter the resistances or Tj,max for the selected model

Effective θJA needs θJA. Stack needs θJC, θCS, and θSA. Required heatsink needs Tj,max, θJC, and θCS.

Read Tj, ΔT, or required θSA

Required mode reports θSA,max — the heatsink thermal-resistance target, not total θJA. Choose θSA ≤ this value under comparable cooling. θSA,max ≤ 0 means JC+CS already consume the thermal budget.

Example calculations

Common configurations with formula and result.

Effective θJA

Ta=25 °C · P=2 W · θJA=5 °C/W

Tj=25+2×5

Tj=35 °C · ΔT=10 °C

Heatsink stack

Ta=25 °C · P=2 W · θJC=1.5 · θCS=0.5 · θSA=3 °C/W

Tj=25+2×(1.5+0.5+3)

Tj=35 °C · θtotal=5 °C/W

Required heatsink θSA

Ta=25 °C · P=10 W · Tj,max=150 °C · θJC=1.5 · θCS=0.5 °C/W

θSA,max=(150−25)/10 − 1.5 − 0.5

θSA ≤ 10.5 °C/W

Symbols

Common values at a glance.

Symbol	Meaning	Unit
Tj	Junction temperature	°C
Ta	Ambient temperature	°C
P	Device power dissipation (heat)	W
ΔT	Temperature rise P·Rθ	°C
θJA	Effective / application junction-to-ambient thermal resistance	°C/W
θJC	Junction-to-case thermal resistance	°C/W
θCS	Case-to-sink / TIM interface thermal resistance	°C/W
θSA	Sink-to-ambient thermal resistance (the heatsink)	°C/W
θSA,max	Largest sink-to-ambient θSA that still meets Tj,max under comparable cooling	°C/W
Margin	Tj,max − Tj when Tj,max is entered on a rise model	°C

i θCA is case-to-ambient and already includes interface + sink-to-ambient. Do not add θCA to θSA. °C/W ≡ K/W for temperature differences. Enter P as device loss converted to heat, not load or output power.

Heatsink Thermal Resistance Calculator — θJC, θCS, θSA & Junction Temperature specification

Version 1.1.0 · Engine tested · Supplemental domain review · Internal · 2026-09-14

Calculation status

Engine tested 17/17 tests · Production surface contract 4/4
Supplemental domain review Internal · Pass · electrical-engineer · 2026-09-14
Named expert review Not performed
Calculation version 1.1.0

Review policy · Evidence · Reviewed by CalculatorX electrical review (EE-001) (electrical-engineer)

Definition: Effective θJA: Tj = Ta + P·θJA. Explicit heatsink path: Tj = Ta + P·(θJC+θCS+θSA). Required heatsink: θSA,max = (Tj,max−Ta)/P − θJC − θCS. P is device power dissipated as heat. °C/W ≡ K/W for temperature differences.
What it calculates: Steady-state junction temperature from effective θJA or θJC+θCS+θSA, or required heatsink θSA,max for a Tj limit.
Inputs: mode junction_effective_ja | junction_heatsink_stack | required_heatsink_sa
Ta_C
P_W ≥ 0 (rise) or P_W > 0 (required)
Rth_JA or Rth_JC+Rth_CS+Rth_SA or Tj_max_C+Rth_JC+Rth_CS
Outputs: Tj_C?
dT_C
thermal_path
Rth_total?
Rth_SA_required?
margin_C?
Formula: Tj = Ta + P·θJA; Tj = Ta + P·(θJC+θCS+θSA); θSA,max = (Tj,max−Ta)/P − θJC − θCS
Assumptions: Steady-state lumped thermal resistance
Rθ constant (not Zth(t))
θJA is an effective / application value, not a JEDEC-board package constant
θJC+θCS+θSA is a single series path; parallel PCB/package paths are ignored
P is device dissipation converted to heat
Units: °C, W, °C/W ≡ K/W
Boundary conditions: P < 0 → VALUE_MUST_BE_NON_NEGATIVE
Required mode P ≤ 0 → VALUE_MUST_BE_POSITIVE
Tj_max ≤ Ta in required mode → VALUE_OUT_OF_RANGE
Rth_CA or Rth_HS → CONFLICTING_INPUTS
Mixed θJA and stack fields → CONFLICTING_INPUTS
θSA,max ≤ 0 → warning HEATSINK_INFEASIBLE (result still returned)
Example: Ta=25, P=10, Tj_max=150, θJC=1.5, θCS=0.5 → θSA,max=10.5
Validation cases: 5 published on this page · 17/17 tests · Production surface contract 4/4 · View evidence
mode=junction_effective_ja, Ta=25, P=2, Rth_JA=5 → Tj=35, dT=10, path=JA
mode=junction_heatsink_stack, Ta=25, P=2, JC=1.5, CS=0.5, SA=3 → Tj=35, Rth_total=5, path=JC+CS+SA
mode=required_heatsink_sa, Ta=25, P=10, Tj_max=150, JC=1.5, CS=0.5 → Rth_SA_required=10.5
mode=junction_effective_ja, Ta=25, P=0, Rth_JA=5 → Tj=25, dT=0
Rth_CA present → error CONFLICTING_INPUTS
Sources: JEDEC JESD51 series — Integrated Circuits Thermal Measurement Method — Junction-to-ambient and junction-to-case thermal metrics on specified test boards · accessed 2026-09-14
Supports: Tj = Ta + P·θJA as a thermal-metric estimate; θJA is test-condition specific
Analog Devices — Practical Design Techniques for Power and Thermal Management — Section 8, Fig. 8.50 — junction → case → case-to-sink → sink-to-ambient · accessed 2026-09-14
Supports: θJA = θJC + θCS + θSA; required θSA,max = (Tj,max−Ta)/P − θJC − θCS
Texas Instruments SPRA953 — Semiconductor and IC Package Thermal Metrics — RθJA, RθJC, and application dependence of junction-to-ambient metrics · accessed 2026-09-14
Supports: θJA is not a universal equivalent of the explicit case/heatsink stack
Analog Devices — Thermal Characterization of IC Packages — θCA is case-to-ambient (all case-to-ambient paths); θJA = θJC + θCA · accessed 2026-09-14
Supports: Do not add θCA to θSA — θCA already includes the ambient-side path
Last reviewed: 2026-09-14
Reviewed by: CalculatorX electrical review (EE-001) (electrical-engineer)
Calculation version: 1.1.0

Background

Interpretation and common distinctions.

Size a heatsink θSA or compute junction temperature from effective θJA or the θJC + θCS + θSA stack.

Default: Ta=25 °C · P=2 W · θJA=5 °C/W → Tj = 35 °C, ΔT = 10 °C.

Supported and not supported

Supported

Effective θJA: Tj = Ta + P · θJA
Heatsink stack: Tj = Ta + P · (θJC + θCS + θSA)
Required heatsink: θSA,max = (Tj,max − Ta)/P − θJC − θCS
Optional margin Tj,max − Tj on the two rise models
API / MCP via electrical.thermal.heatsink_rth

Not supported

Transient thermal impedance Zθ(t) / pulse heating
CFD or multi-heat-source coupling
Adding θCA to θSA (that double-counts the ambient-side path)
Safety certification from Tj < Tj,max

Agent / API notes

Capability id: electrical.thermal.heatsink_rth · tool id: heatsink-rth · pin calculation_version: 1.1.0.

v1.1.0 replaces the lumped “required total Rth” solve with required heatsink θSA, uses Input Schema oneOf for the three modes, and maps review metadata to L2 internally reviewed. Clients pinned to 1.0.0 receive VERSION_MISMATCH. Legacy mode=rise still aliases junction_effective_ja; mode=rth now aliases required_heatsink_sa and requires Rth_JC + Rth_CS. Rth_CA / Rth_HS are CONFLICTING_INPUTS.

Stable error codes: MISSING_REQUIRED_INPUT, CONFLICTING_INPUTS, VALUE_MUST_BE_NON_NEGATIVE, VALUE_MUST_BE_POSITIVE, VALUE_OUT_OF_RANGE, INVALID_NUMBER, INVALID_MODE. Warning: HEATSINK_INFEASIBLE when θSA,max ≤ 0.

Input schema oneOf: junction_effective_ja (Ta, P≥0, Rth_JA), junction_heatsink_stack (Ta, P≥0, Rth_JC, Rth_CS, Rth_SA), required_heatsink_sa (Ta, P>0, Tj_max_C, Rth_JC, Rth_CS). Pin calculation_version: 1.1.0.

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Frequently asked questions

Key distinctions behind the calculation.

Is required θSA the heatsink I should buy?

It is the heatsink thermal-resistance target θSA,max, not total θJA. Select a heatsink rated at θSA ≤ this value under comparable airflow and mounting. Datasheet θSA still depends on convection, orientation, enclosure, and altitude. Effective θJA mode does not size a heatsink.

Why not θJC + θCA + θHS?

θCA is case-to-ambient and already includes the heatsink path. Adding θHS (or θSA) to θCA double-counts ambient-side resistance. The explicit series path is θJC + θCS + θSA.

What power should I enter?

Device power dissipation — the heat the package must reject — not supply rating, load power, or converter output. The MOSFET Total Loss calculator can estimate P for a FET.

Can P be zero?

Yes on Effective θJA and Heatsink stack: P = 0 gives Tj = Ta and ΔT = 0. Required heatsink divides by P, so that mode requires P > 0.