How to Calculate Dropping Resistors

resistor image by Witold Krasowski from

Many students first learn to calculate the voltage drop across resistors in a beginning physics or electrical engineering class, when they learn about basic electrical circuits and Ohm's law. The law is put to practical use if you need to calculate the voltage drop across a resistor while working on an electrical circuit in a stereo, computer, or television. Conversely, you may have to calculate the value of the resistor you need to place in such a circuit to obtain the specific voltage or current required to operate a specific device. Such a resistor is sometimes termed a "dropping resistor."

Learn and understand Ohm's Law. It states that electrical current is equal to the applied voltage divided by the total resistance, or I=V/R. Current is the rate of charge flowing through a circuit, usually measured in amperes (amps). Voltage is the electric potential energy and is measured in volts. Resistance is the amount of resistance to current flow in a circuit. It is measured in ohms, and usually written as an omega symbol.

Calculate the total resistance of a group of resistors wired in series by summing their values. A series of resistors are those that are placed one after another on a circuit, without providing an alternative path for current flow. The total resistance of a series of resistors is equal to the sum of their values.

Calculate the total resistance of a group of resistors wired in parallel by summing the inverse of their values. A parallel circuit means that the current has two or more paths to flow. Think of it as a river that separates into two channels around an island, then rejoins with itself. The resistance of each channel in parallel is found by dividing each individual resistor into 1, then summing the value. So a 20-ohm resistor in parallel with a 15-ohm resistor would have a sum resistance of 1/20 + 1/15, or 8.57 ohms.

Plug the resistance values into Ohm's Law, together with the other known value (current and/or the voltage). Most problems will give you two of the three values, and ask you to solve for the missing one. If a problem gave you a 12-volt power supply with 12 amps of current flow, and asked you to find the voltage drop across all the resistors, you would plug it in like this: I=V/R R=V/I R=12 volts/12 amps = 1 ohm

Calculate the resistor value necessary to drop the current to a certain amperage by using Ohm's Law. If you have a 12-volt battery and want to power an LED (Light Emitting Diode) that requires a 0.5 amp current, you would calculate the value of the needed resistor by using the same equation:

I=V/R R=V/I R=12 volts/0.5 amps=24 ohm

You would need a 24-ohm resistor in this situation.

Conversely, if you are given two resistors in series with values of 100 ohms and 200 ohms, and know that a 12-volt power supply is attached to them, you can calculate the current flow through the circuit and how much voltage is dropped by each resistor.

I=V/R I=12volts/(100Ohms+200Ohms) I=0.04amps

Now that we know the current flow through the series circuit, we can solve for the voltage drop of each resistor.

V=IR V=0.04amps*100ohms V=4volts


V=0.04amps*200ohms V=8volts

Combined, the two resistors consume the entire 12 volts of the circuit - which is what you expect.