How to Calculate for a Load Resistor

By Editorial Team

Updated September 09, 2022

The load resistance can be calculated using Ohm's Law.
i Neziha Kalı Ertuğrul/iStock/GettyImages

Load resistance, also referred to as load impedance, is the opposition that an alternating current experiences on the device being fed the signal. A speaker offers load impedance to incoming signals. Load resistance is employed in a number of ways. It can be used to determine matching impedances as well as bridging impedances, each of which has a different application in electronic circuits. In most cases, however, a high load resistance is preferred, as it results in maximum voltage transfer.

How to Calculate for a Load Resistor

Learning how to calculate current, resistance or voltage given any two of the quantities means learning to use Ohm's Law. You only need to use this law if you have a series circuit, but things get a bit more complicated for parallel circuits. A series circuit is one with only a single "track," and all of the components connected by the same length of wire. The resistance in this case is simply the sum of all the individual resistances.

A parallel circuit has multiple parallel "tracks," possibly with resistors or other components on each. This means that the current can take multiple different paths around the circuit, and this changes the calculation. In this case, you'll need to use an additional formula with a sum of reciprocal terms for the individual resistors, as described below.

Total Resistance Formula for Basic Circuits

Determine the total voltage in the circuit by adding all the voltage sources together.

Determine the total current in a circuit by adding each individual branch current in a parallel circuit. Current in a series circuit is the same, so only one current through one component is needed to determine the overall current in a circuit.

Use Ohm’s Law to determine the total resistance. The following formula is used to determine the total resistance in a circuit when both current and voltage are known:

R​ = ​V​ / ​I

where ​R​ = Resistance, ​V​ = Voltage, and ​I​ = Current.

How to Find Total Resistance for Parallel Circuits

Calculate parallel resistances using the following formula:

1 / ​R​ = 1/​R1 + 1/​R2 + 1/​R3 + 1/​Rn

Rn represents the total number of load resistances in parallel.

Work out the resistances of identical resistors in a parallel circuit by taking the resistance of one of the resistors and dividing it by the number of resistors.

Use Ohm’s Law to determine other values in the circuit using the total resistance that have been determined.

Add the resistances of both series and parallel circuits after computing the resistances of each circuit separately. Use the methods described in previous steps to determine the resistances of each network.


When calculating equivalent resistance in a parallel circuit, look for the one that has the smallest resistance. The total equivalent resistance value will always be anywhere between the smallest resistance value divided by the number of the resistors, and the smallest resistance value. For example, if you have 4 resistors in parallel with resistance values ranging from 5 to 10 ohms, with the smallest value being 5, the total equivalent resistance will lie between 1 and 5. This tip can be used to determine a rough estimate of the total resistance of a parallel circuit.

In a circuit with resistors in series, the current in each resistor is the same and in a circuit with resistors in parallel, the voltage is the same in each resistor.