Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law.
Enter any two values. Leave the ones you want to calculate blank.
The Formula
I = V / R
R = V / I
P = V × I = I²R = V²/R
V = volts · I = amps · R = ohms · P = watts
Current I = V/R = 12/4 = 3 amps
Power P = V × I = 12 × 3 = 36 watts
Ohm's Law
Ohm's Law defines the relationship between voltage (V), current (I), and resistance (R): V = I × R. The extended power formula adds: P = V × I = I² × R = V²/R. Enter any two values and the calculator finds the other two.
The Electrical Units
Voltage (V): Measured in Volts — the electrical "pressure." Current (I): Measured in Amperes (amps) — the flow of electrons. Resistance (R): Measured in Ohms (Ω) — opposition to current flow. Power (P): Measured in Watts — energy per unit time.
Frequently Asked Questions
What is a simple way to remember Ohm's Law?
Use the "VIR triangle": cover the value you want to find, and the remaining two show whether to multiply or divide. Cover V to get I×R. Cover I to get V÷R. Cover R to get V÷I.
What voltage is household electricity?
In North America, standard household voltage is 120V AC (60 Hz) for most outlets, and 240V for large appliances like dryers and ranges. In Europe, it's 230V AC (50 Hz).