# How to Calculate Resistance in Ohms

By Kim Lewis
electronic parts image by sasha from Fotolia.com

Conductors are made from materials such as copper and aluminum. They are able to carry electric currents with ease. In circuits, they have a role as wires that connect components together so electricity can flow. Conversely, they have another role as resistors, where they are designed to limit the flow of current. Resistance is measured in units called ohms. It may be calculated using Ohm's Law, which states that the resistance R is equal to the Voltage V divided by the current I. It is also necessary to know how resistors are placed inside a circuit.

## Instructions

Apply Ohm's Law to resistors that are connected in series. Two or more resistors that are connected in series share the same current, but have different voltages. Together, they form a single path in a circuit. Their total resistance is equal to the sum of each resistor. The equation is Req = R1 + R2, for two resistors. It is Req = R1 + R2 + .. Rn for n resistors. This means that as more resistors are added, the total resistance increases.

Apply Ohm's Law to resistors that are connected parallel to each other. Two or more resistors are parallel when they share the same voltage, but have different currents. When connected in a circuit, they each form a separate path. The equation is 1/Req = 1/R1 +1/R2 for two resistors. It is 1/Req = 1/R1 + 1/R2 +.. 1/Rn for n resistors. This means that as more resistors are added, the total resistance decreases.

Calculate the total resistance of two 4 ohm series resistors that are parallel to a third 4 ohm resistor. First, find the equivalent resistance of the series resistors. The equation is Rs = 4 ohm + 4 ohms = 8 ohms. Next, plug this number into the equation for parallel resistance. This gives 1/Req = 1/8 ohms +1/4 ohms = 12/32 ohms. Rtotal is then 32/12 or 2.7 ohms.