Power Dissipation Calculator
Calculate heat in resistors, linear regulators, MOSFETs and electronic parts — with recommended wattage margin, junction temperature and thermal safety checks.
🔥 Key Insight: Power dissipation is the electrical power converted into heat. A part can be electrically correct but still fail if it cannot remove heat. Always check P = V × I, package wattage, ambient temperature and thermal resistance.
🌡️ Electrical Power Becomes Heat
Voltage Across Part
Current Through Part
Wattage Margin
Presets:5V 20mA LED12V 0.5A load3.3V 100mA
Known Mode
Voltage Across Resistor
Current
Resistance
Derating / Safety Margin
Presets:9V / 330Ω20mA / 220Ω24V / 4.7kΩ
Input Voltage
Output Voltage
Load Current
Ambient Temp
Thermal Resistance θJA
Max Junction
Presets:12V→5V 0.5A9V→3.3V 100mA24V→5V 200mA
Load / Drain Current
RDS(on)
Duty Cycle
Ambient Temp
θJA Optional
Switching Loss Optional
Presets:5A / 20mΩ10A / 8mΩPWM 2A
Power Dissipation
Ambient Temperature
Thermal Resistance θJA
Maximum Junction Temp
Target Margin
Result
Presets:1W θJA602W hot ambient0.5W SMD
📐 Formula Reference
Basic Power
P = V × I
P = I² × R
P = V² / R
P = I² × R
P = V² / R
Linear Regulator
PLOSS = (VIN - VOUT) × ILOAD
Efficiency ≈ VOUT / VIN × 100%
Efficiency ≈ VOUT / VIN × 100%
MOSFET Conduction
PCOND = I² × RDS(on) × Duty
PTOTAL ≈ PCOND + PSW
PTOTAL ≈ PCOND + PSW
Thermal
TJ = TA + P × θJA
Allowed P = (TJMAX - TA) / θJA
Allowed P = (TJMAX - TA) / θJA
📋 Quick Reference
🟠 Common Resistor Ratings
Tiny SMD1/16W
0805 SMD~1/8W
Through-hole1/4W
Power resistor1W+
🌡️ Typical θJA
Small SMD IC80–150°C/W
TO-220 no sink50–70°C/W
TO-220 heatsinklower
Datasheet valuebest
✅ Practical Margin
Resistors2× common
Hot box3× or more
Junction margin20°C+
Continuous loadderate
📚 Engineering Notes
Resistor wattage is not only about resistanceThe same resistor value can be safe or unsafe depending on voltage and current. Always calculate power and choose a resistor wattage with margin.
Linear regulators can get hot quicklyA 12V to 5V regulator at 0.5A burns about 3.5W as heat. That often needs a heatsink, better package, or a switching regulator.
MOSFET RDS(on) rises with temperatureDatasheet RDS(on) is usually specified at a certain gate voltage and temperature. Real hot operation may dissipate more power than the cold calculation.
Thermal resistance depends on PCB layoutCopper area, airflow, vias, enclosure temperature and mounting method can strongly change actual junction temperature.
What is a Power Dissipation Calculator?
A power dissipation calculator estimates how many watts a component converts into heat. It is useful for selecting resistor wattage, checking linear regulator heating, estimating MOSFET conduction loss and deciding whether a heatsink or better PCB thermal layout is needed.
Common checks users need
For simple parts, users usually look for P = V × I, P = I²R, P = V²/R, recommended resistor wattage and safe derating. For voltage regulators and power transistors, users often need junction temperature, thermal resistance and heatsink margin. For a complete workflow, pair this with the Ohm’s Law Calculator, LED Resistor Calculator, PCB Trace Width Calculator and Battery Life Calculator.
❓ Frequently Asked Questions
A practical rule is to choose at least 2× the calculated resistor power. Use more margin for hot enclosures, continuous operation or poor airflow.
A linear regulator drops voltage by converting the extra voltage into heat. Power loss is approximately (Vin - Vout) × current.
No. Use it for quick estimation. Final design should check package rating, datasheet thermal resistance, PCB copper area, airflow, enclosure temperature and testing.