Ohm's Law Calculator
Enter any two electrical values to calculate the other two. Works with voltage, current, resistance, and power.
What Is Ohm's Law?
Ohm's Law is the foundation of electrical engineering. It describes how voltage, current, and resistance relate to each other in a circuit. Named after Georg Simon Ohm, who published the relationship in 1827, it is one of the first things you learn in any electronics class.
V = I x R | I = V / R | R = V / I
P = V x I | P = I² x R | P = V² / R
Understanding the Four Values
Voltage (V) is the electrical pressure that pushes current through a circuit, measured in volts. Think of it like water pressure in a pipe. Current (I) is the flow of electrons, measured in amps. Resistance (R) is the opposition to current flow, measured in ohms. Power (P) is how much energy the circuit uses per second, measured in watts.
Practical Examples
A standard US household outlet provides 120 volts. A 60-watt light bulb draws 0.5 amps (60W / 120V) and has a resistance of 240 ohms (120V / 0.5A). A 1,500-watt space heater draws 12.5 amps, which is why it needs its own circuit. Knowing these relationships helps you choose the right wire gauge, fuse size, and circuit breaker rating.
Why do I only need two values?
Ohm's Law and the power equation together give you enough information to calculate any two unknowns from any two knowns. The four values are mathematically linked, so two of them always determine the other two.
Does Ohm's Law work for AC circuits?
Ohm's Law works for DC and simple resistive AC circuits. For AC circuits with capacitors or inductors, you need to use impedance (Z) instead of resistance (R). The formula becomes V = I x Z, where Z includes both resistance and reactance.
What is the difference between watts and volt-amps?
In DC circuits, watts and volt-amps are the same. In AC circuits, watts measure real power (the power that does actual work), while volt-amps measure apparent power (which includes reactive power from capacitors and inductors). The ratio between them is called the power factor.
How do I know what wire gauge to use?
Wire gauge depends on the current (amps) flowing through it. Higher current needs thicker wire. For example, 15-amp circuits typically use 14 AWG wire, and 20-amp circuits use 12 AWG. Always check local electrical codes for your specific situation.