Thermal Resistance & Temperature Rise Calculator
Calculate generic steady-state thermal resistance, temperature rise, final temperature, thermal-limit power and temperature margin from a known heat load. THM-001 is intentionally generic: it models one equivalent thermal path instead of a specific MOSFET, LED, diode, BJT package or PCB trace.
Use dissipated heat power at the selected boundary. For example, a 100 W converter at 90% efficiency does not usually dissipate 100 W as heat; the first-pass heat load is closer to the 10 W loss if that loss enters the modeled thermal path.
Engineering tool
Thermal Resistance & Temperature Rise Calculator
Calculate steady-state temperature rise, equivalent thermal resistance, thermal-limit power, final temperature, allowable Rθ, and thermal margin from a known heat load.
Heat dissipated through the modeled thermal path, not necessarily electrical input power.
Equivalent steady-state thermal resistance for the known path.
Result console
- Temperature Rise (ΔT)
- 50°C / K rise
- Thermal Conductance
- 0.2W/K
This is a steady-state temperature rise through the entered equivalent thermal resistance.
Formula
Formula reference
Steady-state thermal resistance formulas
The model is steady-state and uses one equivalent thermal resistance. It does not model transient thermal impedance or multi-element thermal networks.
Rθ = ΔT / PΔT = P RθP = ΔT / RθTfinal = Tref + ΔTΔTallow = Tmax - TrefRθ,max = ΔTallow / PMargin = Tmax - TfinalVariable definitions
- Rθ
- equivalent thermal resistance
- ΔT
- temperature difference or temperature rise
- P
- heat dissipated through the modeled thermal path
- Tref
- absolute reference temperature
- Tmax
- absolute temperature limit
Thermal Resistance & Temperature Rise Formula Audit
| Thermal Resistance Definition | Rθ = ΔT / P. |
|---|---|
| Temperature Rise Formula | ΔT = P × Rθ. |
| Thermal Resistance Formula | Rθ = ΔT / P. |
| Thermal Power Formula | P = ΔT / Rθ. |
| Final Temperature Formula | Tfinal = Tref + ΔT. |
| Temperature Budget Formula | ΔTallow = Tmax - Tref. |
| Maximum Allowable Thermal Resistance Formula | Rθ,max = ΔTallow / P. |
| Thermal Margin Formula | Margin = Tmax - Tfinal. |
| Thermal Resistance Units | Thermal resistance is commonly expressed as °C/W or K/W. |
| °C/W / K/W Equivalence | The numeric value is the same because 1°C difference equals 1 K difference. |
| Absolute Temperature Convention | Absolute °C, K and °F values are converted with scale offsets. |
| Temperature Difference Convention | Δ°C and ΔK are numerically equal; Δ°F is scaled by 1.8. |
| °F Difference Scaling | 10°C rise = 10 K rise = 18°F rise. |
| Power / Heat Definition | P must be heat dissipated through the modeled thermal path. |
| Steady-State Boundary | Warm-up, cooldown and thermal capacitance are outside THM-001. |
| Device-Specific Boundary | THM-001 does not calculate package-specific RθJA. |
| RθJA Boundary | Datasheet RθJA depends on PCB, airflow, mounting and test setup. |
| Heat-Sink Boundary | Complete RθJC/RθCS/RθSA heat-sink sizing belongs to THM-002. |
| Thermal-Network Boundary | Series and parallel thermal resistance networks belong to THM-003. |
| Transient-Thermal Boundary | Thermal RC and transient impedance are not modeled here. |
| Zero-Power Policy | P = 0 is allowed for direct rise/final-temperature modes, but rejected for inverse division modes. |
| Zero-Rθ Policy | Zero thermal resistance is rejected as unrealistic for this calculator. |
| Negative-Margin Policy | Margin keeps its sign; negative values are not converted with abs(). |
Worked Examples
| Example | Calculation | Result |
|---|---|---|
| P = 10 W, Rθ = 5°C/W | ΔT = 10 × 5 | 50°C rise |
| P = 0 W, Rθ = 5°C/W | ΔT = 0 × 5 | 0°C rise |
| P = 2.5 W, Rθ = 8°C/W | ΔT = 2.5 × 8 | 20°C rise |
| ΔT = 50°C, P = 10 W | Rθ = 50 / 10 | 5°C/W |
| ΔT = 20 K, P = 2.5 W | Rθ = 20 / 2.5 | 8 K/W |
| ΔTmax = 75°C, Rθ = 15°C/W | Pmax = 75 / 15 | 5 W |
| Tref = 25°C, P = 10 W, Rθ = 5°C/W | Tfinal = 25 + 10 × 5 | 75°C |
| Tref = 298.15 K, P = 10 W, Rθ = 5 K/W | Tfinal = 298.15 + 50 | 348.15 K |
| 25°C absolute | TK = 25 + 273.15 | 298.15 K |
| 10°C temperature rise | ΔK = Δ°C | 10 K rise |
| 10°C temperature rise | Δ°F = 10 × 9/5 | 18°F rise |
| 25°C absolute | TF = 25 × 9/5 + 32 | 77°F |
| Tref = 25°C, Tmax = 125°C, P = 20 W | Rθ,max = (125 - 25) / 20 | 5°C/W |
| Tref = 25°C, P = 20 W, Rθ = 4°C/W, Tmax = 125°C | Tfinal = 105°C; margin = 20°C | Below limit |
| Tref = 25°C, P = 20 W, Rθ = 5°C/W, Tmax = 125°C | Tfinal = 125°C; margin = 0°C | At limit |
| Tref = 25°C, P = 20 W, Rθ = 6°C/W, Tmax = 125°C | Tfinal = 145°C; margin = -20°C | Exceeds limit |
| P = 1 W, Rθ = 1°C/W | ΔT = 1 × 1 | 1°C rise |
| P = 1 W, Rθ = 1 K/W | ΔT = 1 × 1 | 1 K rise |
| Rθ = -1°C/W | Negative thermal resistance | Rejected |
| P = -1 W | Negative heat dissipation | Rejected |
| Tref below 0 K | Absolute temperature below physical limit | Rejected |
| Tmax < Tref | Negative thermal budget | Rejected |
| ΔT = 50°C, P = 0 W | Rθ = ΔT / P | Rejected, not Infinity |
| Tref = 25°C, P = 0 W, Rθ = 5°C/W | Tfinal = 25 + 0 | 25°C |
Engineering Notes
Thermal Resistance
Thermal resistance relates a temperature difference to heat flow through a modeled path.
Temperature Rise
Temperature rise is a difference, not an absolute temperature reading.
Heat Dissipation
Use the heat actually dissipated in the path. Electrical input power is not automatically equal to heat loss.
Ambient / Reference Temperature
The reference can be ambient air, board temperature, case temperature, or another defined thermal node.
Steady-State Thermal Model
The calculation assumes temperatures have reached equilibrium.
Thermal Path
The entered Rθ should represent the equivalent path from the heat source to the reference node.
RθJA
Junction-to-ambient resistance may be entered if already known, but this calculator does not derive package RθJA.
RθJC
Junction-to-case resistance is a component of a device thermal path, not a complete ambient path by itself.
Heat Sink Boundary
Detailed heatsink sizing and RθSA calculations belong to THM-002.
Thermal Network Boundary
Multiple series or parallel thermal resistances belong to THM-003.
Transient Thermal Boundary
Pulsed power, thermal capacitance, Zθ curves and warm-up time are outside THM-001.
Temperature Margin
A negative thermal margin is useful information and must not be hidden by absolute-value formatting.
Common Mistakes
- Confusing °C/W with absolute °C.
- Assuming 25°C equals 25 K.
- Using absolute temperature conversion for a temperature difference.
- Using temperature-difference conversion for an absolute temperature.
- Forgetting that ΔT = P × Rθ.
- Writing the Rθ formula backward.
- Using electrical input power directly as heat dissipation.
- Treating datasheet RθJA as a fixed material constant.
- Using a steady-state result as a transient warm-up prediction.
- Taking the absolute value of negative thermal margin.
- Calling Rθ,max a minimum thermal resistance.
- Letting P = 0 inverse calculations produce Infinity.
- Accepting negative thermal resistance.
- Treating THM-001 as a complete heatsink calculator.
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FAQ
Support reference
FAQ
What is thermal resistance?
Thermal resistance relates temperature difference to heat flow. In electronics cooling it is commonly written as Rθ = ΔT / P.
How do I calculate temperature rise from thermal resistance?
Use ΔT = P × Rθ, where P is dissipated heat power and Rθ is the equivalent thermal resistance.
How do I calculate thermal resistance from temperature rise?
Use Rθ = ΔT / P. The dissipated power must be greater than zero for this inverse calculation.
How do I calculate maximum power from thermal resistance?
Use P = ΔTmax / Rθ. This is a maximum steady-state thermal reference, not an absolute device power rating.
What does °C/W mean?
Degrees Celsius per watt means the modeled point rises by that many degrees Celsius for every watt of heat flowing through the thermal path.
Is °C/W the same as K/W?
Yes. For temperature differences, 1 °C difference equals 1 K difference, so °C/W and K/W have the same numeric value.
Is 1°C equal to 1 K?
Not as an absolute temperature. A 1°C temperature difference equals a 1 K temperature difference, but 1°C absolute equals 274.15 K.
What is the difference between absolute temperature and temperature difference?
Absolute temperature identifies a point on a temperature scale, such as 25°C or 298.15 K. Temperature difference is a rise or drop, such as 10°C rise or 10 K rise.
How do I calculate final temperature from ambient temperature?
First calculate ΔT = P × Rθ, then add it to the absolute reference temperature: Tfinal = Tref + ΔT.
How do I calculate the maximum allowable thermal resistance?
Use Rθ,max = (Tmax - Tref) / P. The actual equivalent thermal resistance should be less than or equal to this value.
What is thermal margin?
Thermal margin is Tmax - Tfinal. It keeps its sign so negative values clearly show an exceeded limit.
What does a negative thermal margin mean?
A negative margin means the modeled final temperature is above the specified temperature limit.
Can I use electrical input power as heat dissipation?
Only if the electrical input power actually becomes heat in the modeled path. For an efficient converter, the heat load is usually the loss, not the total input power.
What is RθJA?
RθJA is junction-to-ambient thermal resistance, but THM-001 treats it only as an already-known equivalent value when the user provides it.
Why does datasheet RθJA depend on PCB conditions?
RθJA depends on test board, copper area, airflow, mounting, orientation and environment, so it is not a universal material constant.
Does this calculator include transient heating?
No. THM-001 is a steady-state model. Thermal capacitance and warm-up/cool-down behavior belong to the Thermal RC Time Constant Calculator.
Can this calculator size a heat sink?
It can show a general equivalent thermal-resistance target, but complete RθJC, RθCS and RθSA heat-sink sizing belongs to THM-002.
What is the difference between THM-001 and the Thermal Resistance Network Calculator?
THM-001 uses one equivalent thermal resistance. THM-003 will calculate equivalent resistance from multiple series or parallel thermal paths.
Engineering Disclaimer
This calculator provides first-pass steady-state thermal estimates. Production electronics thermal design should verify datasheet test conditions, PCB copper, airflow, mounting, interface materials, transient behavior, temperature-dependent losses, electrical limits, SOA, reliability requirements and measured hardware temperature.