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.
Common Mistakes
- Entering three values instead of leaving the unknown blank.
- Using milliamps, kilohms, or milliwatts without selecting the matching unit.
- Applying a DC resistance calculation to AC impedance without considering reactance.
- Selecting a resistor power rating with no temperature or safety margin.
Reference Notes
The calculator uses Ohm's law and the basic electric power relationship for linear resistive circuits. Practical component selection should also check tolerance, temperature coefficient, pulse rating, voltage rating, and datasheet derating.