Calculator
Resistor power calculator
Estimate power dissipation in a resistor from electrical values.
Estimate dissipation
Use the voltage across the resistor, not the supply voltage, and its resistance to estimate steady-state DC current and dissipated power.
Worked example
With 5 V across 1 kΩ: I = 5 V / 1000 Ω = 0.005 A (5 mA). P = V² / R = 25 / 1000 = 0.025 W (25 mW).
How the estimate works
Basis: Ohm’s law and P = V × I = V² / R. Choose a resistor whose rated power exceeds calculated dissipation by a suitable margin after checking its datasheet, ambient conditions, enclosure, tolerance and derating. A 25 mW calculation does not predict temperature or prescribe a universal wattage rating; actual heating depends on real operating conditions. These idealized calculations do not check every real-world limit. Work with safe low voltages and verify your design independently.
For the circuit assumptions, common mistakes and practical checks, read the worked guide.