Resistor Voltage Drop Calculator

The resistor voltage drop calculator helps you accurately calculate the voltage drop in your circuit. In this tool, you will find three calculators that can help you determine the voltage drop across a resistor, the voltage drop in a two-resistor voltage divider, and the required drop resistance.

Keep reading our article, where we will approach the following subjects:

• What is the voltage drop through a resistor?
• How to calculate voltage drop across a resistor​.
• What is the resistor needed to drop 12V to 5V​?
• And much more.

So, let’s understand how to drop the voltage and avoid damaging your circuit.

When a load draws less voltage than the source provides, you can place one resistor — or an equivalent one from a string connected in series or parallel — to absorb the difference. The resistor drops the extra voltage across its terminals, so the load receives only the voltage it needs to work properly.

This is the role of a voltage drop​, which follows from the famous Ohm’s law. Check out our Ohm’s law calculator to learn more about this topic.

To compute the voltage drop, you just need to use Ohm’s law. In this case, you can consider a simple circuit consisting of a source connected in series with a single resistor. Therefore, the formula for the voltage drop across a resistor is:

where:

• $R$ — Resistance of your dropping resistor;
• $V$ — Voltage supplied by the source; and
• $I$ — Current.

You can also measure this voltage drop in the real world using a multimeter. To do that, set the multimeter to voltage mode and connect it in parallel with the resistor (across it) while the circuit is powered and running.

A two-resistor voltage divider is a circuit that takes an input voltage and produces a smaller output voltage, determined by the ratio of the two resistors in series.

This popular circuit is formed by two resistors $R_1$ and $R_2$, which are connected in series across the input voltage $V_\mathrm{in}$, and the output $V_\mathrm{out}$. You can see a diagram of this circuit below, and by selecting the corresponding option in the voltage drop across a resistor calculator.

The equation to compute the output voltage is given by:

This type of circuit is present in several devices, such as a potentiometer in an audio amplifier, radio, or guitar; multimeters and oscilloscope probes; thermostats and temperature sensors; joysticks and game controllers; and low-battery indicators.

The resistance of the dropping resistor depends on the voltage drop​ across it and the current flowing through the circuit. This can be computed using the equation:

where:

• $R$ — Resistance of your dropping resistor;
• $V_{\mathrm{supply}}$ — Voltage supplied by the source;
• $V_{\mathrm{load}}$ — Voltage needed by the load; and
• $I$ — Current.

Moreover, a resistor that drops voltage also turns that energy into heat, so it must be rated to handle the power without overheating. The dissipated power $P$ is computed using the following equation:

which gives you the resistor power​ rating. The power rating is defined as the maximum power your resistor can safely dissipate before it overheats and is damaged.

Unlike the resistance, the power rating is not color‑coded on the resistors, so if you do not have the product datasheet or the value printed on the resistor, you will need to infer it from the resistor’s physical size. In general, the power ratings for different resistors are:

• $1/8 \, – \, 1/4 \,\mathrm{W}$ for body lengths $3.2 \,– \,6.5 \,\mathrm{mm}$;
• $1/2 \, – \, 1 \,\mathrm{W}$ for body lengths $9.0 \,– \,12.0 \,\mathrm{mm}$; and
• Larger than $2 \,\mathrm{W}$ for body lengths $> 15 \,\mathrm{mm}$.

If you don’t have the right resistor, you can combine two or more of them to get the resistance you need. Use our series resistor calculator and parallel resistor calculator to quickly find the equivalent resistance.

Our resistor voltage drop calculator is easy and intuitive to use. All you need to do is:

1. Select your circuit configuration.
2. Fill in the inputs with your data for the voltages and the current.
3. Read the results of your calculations.

Let’s demonstrate this computation with an example. Suppose that you have a red LED with a $2\, \mathrm{V}$ forward voltage running at $20 \,\mathrm{mA}$ from a $9\,\mathrm{V}$ supply. To find the correct value of the resistor, select the option “Find the dropping resistor” in our resistance voltage drop calculator. By inputting these values in our tool, you will find:

$R = 350\,\mathrm{\Omega}$ and $P = 0.14\,\mathrm{W}$.

These are precisely the values of the resistance and power dissipation you need in your circuit to keep your LED working properly.

One important point to highlight about this question is that resistors in parallel share the same two nodes, so the voltage across each one is identical. This happens because the voltage doesn’t divide across parallel branches the way it does in series — it’s the current that divides.

However, you can determine the voltage drop by considering a circuit with resistors in parallel connected in series with another resistor, as shown in the figure below.

In such a case, you need to find the equivalent resistor for the circuit, and its resistance can be calculated using the following equation:

So, the formula for voltage drop across this resistor is: