Enter any two electrical values to calculate the other two. Works with voltage, current, resistance, and power.
Ohm's Law defines the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I x R. If you know any two values, you can calculate the third. This is the most fundamental equation in electrical engineering and electronics, applicable to everything from household wiring to circuit board design.
Enter any two of the four values (voltage in volts, current in amps, resistance in ohms, power in watts) and the calculator solves for the other two. The calculator also shows the relevant formula used and unit conversions (milliamps, kilohms, milliwatts, etc.). The Resistor Color Code Calculator helps identify resistance values from physical resistors.
LED circuits: To power an LED (typically 2V forward voltage, 20mA current) from a 5V supply, you need a resistor of R = (5-2)/0.02 = 150 ohms. Home electrical: A 120V circuit with a 15A breaker can handle 120 x 15 = 1,800 watts before tripping. Battery drain: A 12V battery powering a 4 ohm load draws 3 amps (I = 12/4).
Ohm's Law applies directly to everyday electrical work. When sizing wire for a circuit, you need to know the expected current draw to select appropriate wire gauge: 15-amp circuits use 14 AWG wire, 20-amp circuits use 12 AWG. LED strips typically draw 1 to 2 amps per meter at 12V, so a 5-meter run needs at least 10 amps of power supply capacity. For automotive work, calculating wire gauge prevents voltage drop and overheating over long runs. Speaker wire impedance matching (typically 4, 6, or 8 ohms) determines how much power an amplifier delivers. Understanding the power equation (P = V x I = I^2 x R = V^2 / R) lets you calculate heat dissipation, which is critical for component selection and thermal management.