Ohm's Law Calculator
Calculate voltage, current, resistance, or power from any two known values.
Ohm's Law Formulas
Understanding Ohm's Law
Ohm's Law is the fundamental relationship between voltage, current, and resistance in electrical circuits. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.
The Basic Formula
V = I × R
Where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
Power Relationship
P = V × I
Power in watts equals voltage times current. Combined with Ohm's Law, this gives us 12 useful formulas.
Practical Examples
LED Current Limiting
5V supply, LED with 2V forward voltage, need 20mA current:
R = (5V - 2V) / 0.02A = 150Ω
Battery Runtime
Device draws 200mA from 3.7V battery, what's the power consumption?
P = 3.7V × 0.2A = 0.74W
Heater Power
12V heater with 6Ω resistance, what current and power?
I = 12V / 6Ω = 2A
P = 2A × 12V = 24W
Wire Sizing
Motor draws 5A, max wire resistance to limit drop to 0.5V:
R = 0.5V / 5A = 0.1Ω max
Resistor Wattage
1kΩ resistor with 10V across it, what wattage needed?
P = 10² / 1000 = 0.1W
Use 1/4W or larger
Sensor Reading
Thermistor shows 10kΩ, powered with 3.3V, what current?
I = 3.3V / 10000Ω = 330µA
Important Considerations
Linear Devices Only
Ohm's Law applies to resistive (linear) devices. Diodes, transistors, and other non-linear components don't follow V=IR directly.
Temperature Effects
Resistance changes with temperature. Metals increase resistance when heated; semiconductors typically decrease.
AC vs DC
For AC circuits, impedance (Z) replaces resistance and includes inductive and capacitive reactance.
Power Rating
Always select components rated for at least 2× the calculated power dissipation for reliable operation.