An electrical circuit is connected as a series circuit or a parallel circuit. In a series circuit, the current is the same throughout every part of the circuit. In a parallel circuit, the voltage is the same for each individual branch. Ohm's Law gives a simple equation that can help you calculate the voltage, current and resistance of a parallel circuit.
Voltage

The voltage is the same for each resistor in a parallel circuit because there are only two electrically common points, and the voltage between common points is always the same. Voltage, or the electric potential difference, is the energy per unit charge; it is measured in volts on a voltmeter.
Current

Electric current can take multiple paths through a parallel circuit. The total current is equal to the sum of each branch's individual current. The circuit's total current can be calculated with the equation I = V/(1/R1 + 1/R2 + ... + 1/Rn), where I is the total current, V is the voltage and R1, R2 and so on are the resistance of each individual branch. Current is measured in amperes with an ammeter.
Resistance

The total resistance for the parallel circuit must be less than the resistance of any individual branch. Resistance is measured in ohms. Calculate the total resistance by dividing the total current from the voltage; this equation, I = V/R, is known as Ohm's Law. It can be rewritten as R = V/I. For example, a parallel circuit with 14 volts and 2 amps has a total resistance of 14/2, or 7 ohms. If you do not know the total voltage and current, calculate the total resistance from the individual branch's resistances with the equation 1/R = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn.
Circuit Durability

An individual branch opening stops only the current in that branch. The rest of the branches will continue to work because the current has multiple paths it can take across the circuit.