Enter Any Two Values
Formula
V = I × R
I = V ÷ R
R = V ÷ I
P = V × I
V is voltage, I is current, R is resistance, and P is power.
Examples
Example 1
V = 12 V, R = 6 Ω
I = V / R = 12 / 6 = 2 A
P = V × I = 12 × 2 = 24 W
Example 2
I = 25 mA, R = 220 Ω
V = I × R = 0.025 × 220 = 5.5 V
P = V × I = 5.5 × 0.025 = 0.1375 W
Example 3
V = 5 V, I = 20 mA
R = V / I = 5 / 0.02 = 250 Ω
P = V × I = 5 × 0.02 = 0.1 W
How to Use This Calculator
Enter two values
Provide any two of voltage, current, and resistance. Leave the value you want calculated blank.
Use consistent units
Enter volts, amperes, and ohms. Convert milliamps, kilohms, or other prefixes before entering a value.
Check power
The result includes watts, which helps when comparing the calculated dissipation with a component's power rating.
Assumptions and Limitations
Ohm's law directly describes an ideal linear resistance. Real components can change resistance with temperature, voltage, frequency, and operating conditions. The calculated power is steady-state dissipation and does not account for startup, pulse, surge, or transient ratings.
A resistor should not be selected at exactly the calculated wattage. Check its datasheet, temperature derating, tolerance, and required design margin.