Voltage Divider Calculator

What Is a Voltage Divider?

A voltage divider is one of the most fundamental circuits in electronics. It uses two resistors connected in series to produce an output voltage that is a fixed fraction of the input voltage. The output is taken from the junction between the two resistors, giving a lower voltage than the source without any active components.

Despite its simplicity, the voltage divider appears in almost every electronic circuit — from setting a reference voltage for a microcontroller to scaling down a sensor signal so it can be safely read by an analog input. Understanding how it works and how to calculate the correct resistor values is an essential skill for any electronics engineer, hobbyist, or student.

The Voltage Divider Formula

The output voltage of a resistor voltage divider is calculated using the following formula:

V out = V in × R2 / (R1 + R2)

Where V in is the input voltage applied to the top of the circuit, R1 is the top resistor connected between V in and the output node, R2 is the bottom resistor connected between the output node and ground, and V out is the resulting output voltage measured across R2.

The formula shows that V out is always a ratio of R2 to the total resistance. If R1 and R2 are equal, V out will be exactly half of V in. If R2 is larger than R1, V out will be more than half. If R2 is smaller than R1, V out will be less than half.

Deriving the Formula from Ohm's Law

The voltage divider formula comes directly from Ohm's Law. Because R1 and R2 are in series, the same current flows through both resistors. That current equals V in divided by the total resistance (R1 + R2). The voltage across R2 is then that current multiplied by R2, which simplifies to the formula above. This is why a voltage divider only works correctly as a reference when the load connected to V out draws very little current — any significant load current changes the effective value of R2 and shifts the output voltage away from the calculated value.

How to Use This Calculator

This calculator has three modes accessible from the tabs at the top. Each mode solves for a different unknown in the voltage divider circuit. Select the mode that matches what you need and fill in the known values.

Find V out — Calculate the Output Voltage

Use this mode when you have both resistors already chosen and want to know what output voltage they will produce. Enter V in, R1, and R2 along with their units, then click Calculate. The result shows the output voltage, the divider ratio as a percentage, the current flowing through the divider, and the individual voltage drop across each resistor.

Find R2 — Calculate the Bottom Resistor

Use this mode when you know your input voltage and have a target output voltage in mind, and you have already chosen R1. Enter V in, your desired V out, and your chosen R1 value. The calculator will find the exact R2 needed to achieve that output. You can then choose the nearest standard resistor value from the E12 or E24 series.

Find R1 — Calculate the Top Resistor

Use this mode when you know your input voltage, desired output voltage, and have already selected R2. Enter the three known values and the calculator returns the exact R1 required. This is useful when your R2 is fixed by another constraint — for example, when using a specific pull-down resistor or a fixed sensor impedance.

Selecting Units

Each input field has a unit dropdown. Voltages can be entered in Volts, Millivolts, or Microvolts. Resistances can be entered in Ohms, Kilohms, or Megaohms. You can mix units freely — for example, entering R1 in kilohms and R2 in ohms — and the calculator will convert everything to base units before computing the result.

Understanding the Circuit Diagram

The diagram shown above the calculator fields illustrates the standard voltage divider topology. The power source on the left supplies V in. Current flows up through R1 and then through R2 to ground. The output node sits between R1 and R2, and V out is the voltage measured from that node to ground.

This is an unloaded voltage divider — meaning no current is drawn from the V out node. In real circuits, any device connected to V out will draw some current and effectively act as a resistor in parallel with R2, which lowers the effective resistance and reduces the output voltage. For accurate results in loaded circuits, choose R1 and R2 values that are much smaller than the input impedance of the connected load.

Worked Examples

Example 1 — Reducing 12V to 5V for a Microcontroller

You need to connect a 12V sensor output to a microcontroller that accepts a maximum of 5V on its analog input pin. You choose R1 = 14 kΩ and want to find R2.

Using the Find R2 mode with V in = 12V, V out = 5V, and R1 = 14 kΩ:

R2 = V out × R1 / (V in − V out) = 5 × 14000 / (12 − 5) = 70000 / 7 = 10 kΩ

A standard 10 kΩ resistor is used for R2. The current through the divider is 12V / (14kΩ + 10kΩ) = 0.5 mA, which is low enough not to significantly load the sensor output.

Example 2 — Setting a Reference Voltage for an Op-Amp

An op-amp comparator circuit requires a reference voltage of 2.5V from a 9V supply. You choose R2 = 22 kΩ and want to find R1.

Using the Find R1 mode with V in = 9V, V out = 2.5V, and R2 = 22 kΩ:

R1 = R2 × (V in − V out) / V out = 22000 × (9 − 2.5) / 2.5 = 22000 × 2.6 = 57.2 kΩ

The nearest standard E24 value is 56 kΩ, which gives a V out of 2.53V — close enough for most reference applications.

Example 3 — Checking an Existing Circuit

You have a circuit with R1 = 4.7 kΩ and R2 = 2.2 kΩ powered from a 5V USB supply and want to verify the output voltage.

Using the Find V out mode with V in = 5V, R1 = 4.7 kΩ, R2 = 2.2 kΩ:

V out = 5 × 2200 / (4700 + 2200) = 5 × 2200 / 6900 = 1.594V

The current through the divider is 5V / 6.9 kΩ = 0.725 mA. Both resistors dissipate a total of about 3.6 mW, which is well within the power rating of standard 1/4W resistors.

Choosing the Right Resistor Values

The voltage divider formula gives you the exact ratio of R1 to R2 needed to achieve your target output. However, choosing the right actual resistor values involves a few additional considerations beyond the ratio alone.

Use Standard E-Series Resistor Values

Resistors are manufactured in standard values defined by the E-series (E12, E24, E48, E96). You cannot buy a 57.2 kΩ resistor off the shelf, but you can buy a 56 kΩ or 57.6 kΩ. After calculating the ideal value, round to the nearest available E-series value and re-run the calculator to check how much error that introduces into your output voltage.

Keep Total Resistance Much Lower Than Load Impedance

For the divider to work accurately, the total resistance (R1 + R2) should be at least 10 times lower than the input impedance of the load connected to V out. If your load has an input impedance of 100 kΩ, keep R1 + R2 at or below 10 kΩ. Ignoring this rule is the most common cause of a voltage divider producing a lower output than expected.

Avoid Unnecessarily Low Resistance Values

Lower resistances produce more accurate output voltages with varied loads, but they also waste more current and generate more heat. For battery-powered circuits, using very low resistances can drain the battery significantly even when the load is idle. Choose the highest resistance that still gives you accurate results with your specific load impedance.

Consider Resistor Tolerance

Standard resistors have tolerances of 1% or 5%. A 5% tolerance on both R1 and R2 can shift your output voltage by up to 10% in the worst case. For precision reference voltage applications, use 1% or better tolerance resistors. For general signal scaling where a few percent error is acceptable, standard 5% resistors work fine and cost less.

Common Applications of Voltage Dividers

Analog Sensor Signal Scaling

Many sensors output voltages higher than the input range of a microcontroller or ADC. A voltage divider scales the signal down to a safe and readable range. For example, connecting a 0–5V sensor to a 3.3V microcontroller analog input requires a divider that scales the signal to 0–3.3V maximum.

Battery Voltage Monitoring

Microcontrollers often need to monitor a battery voltage that is higher than their own supply voltage. A voltage divider brings the battery voltage down to the ADC input range, allowing the firmware to calculate the actual battery voltage by multiplying the measured value by the inverse of the divider ratio.

Pull-Down and Bias Resistors

In digital circuits, a resistor divider is often used to set a default logic level or bias point. A pull-down resistor connected from a signal line to ground forms the bottom half of a divider, ensuring the line reads LOW when no active signal is present. Pull-up resistors work similarly but pull toward the supply voltage.

Volume and Tone Controls in Audio

A potentiometer is simply an adjustable voltage divider. The wiper of the pot provides V out, which moves between 0V and V in as the knob is turned. This is how traditional analog volume controls and tone controls work in audio equipment.

Setting Threshold Voltages for Comparators

Op-amp comparators and dedicated voltage comparator ICs require a reference voltage to compare against an input signal. A voltage divider connected to the supply rail provides a stable, fixed reference that determines the switching threshold of the comparator.

Voltage Divider vs Voltage Regulator

A voltage divider and a voltage regulator both reduce voltage, but they work in fundamentally different ways and are suited to very different purposes. A voltage divider is a passive circuit — it uses only resistors and produces an output voltage that depends on both the resistor ratio and the load current. If the load current changes, the output voltage shifts. A voltage regulator is an active circuit that uses feedback to maintain a constant output voltage regardless of changes in load current or input voltage.

Use a voltage divider when you need a reference voltage for a high-impedance input, when the load draws negligible current, or when you are scaling a signal for measurement purposes. Use a voltage regulator when you need to power a circuit that draws variable or significant current and requires a stable, regulated supply voltage.

Limitations of a Resistor Voltage Divider

Output Voltage Drops Under Load

As mentioned earlier, connecting a load to V out reduces the effective resistance of R2 because the load appears in parallel with it. This lowers the divider ratio and reduces the output voltage. The heavier the load, the more the output voltage sags. This is why voltage dividers are only suitable for high-impedance loads or reference applications.

Power Is Continuously Wasted

Current flows through R1 and R2 at all times, even when the load is not drawing any power. This quiescent current is wasted as heat in the resistors. In battery-powered designs, this can significantly reduce battery life if the divider uses low resistance values. Always calculate the quiescent power dissipation and factor it into your battery life estimate.

Output Is Not Isolated from the Input

The output of a voltage divider is directly connected to the input through R1. Any noise, ripple, or transients on V in will appear at V out, scaled down but not eliminated. For noise-sensitive circuits, add a small bypass capacitor across R2 to filter high-frequency noise from the output.

Frequently Asked Questions

Can a voltage divider increase voltage?

No. A resistor voltage divider can only reduce voltage. V out is always less than V in because the formula V out = V in × R2 / (R1 + R2) always produces a value smaller than V in when R1 is greater than zero. To increase voltage, you need an active circuit such as a boost converter or a charge pump.

What happens if R1 is zero?

If R1 equals zero, V out equals V in — the full input voltage appears at the output. This is simply a direct connection with no voltage reduction. In practice, R1 should always have a non-zero value to form a functional divider.

What happens if R2 is zero?

If R2 equals zero, V out equals zero — the output node is shorted directly to ground. All of V in appears across R1 and no voltage is divided to the output. R2 must always be a non-zero value for the divider to produce a useful output.

Can I use a potentiometer as a voltage divider?

Yes. A potentiometer is an adjustable voltage divider. The two end terminals act as V in and ground, while the wiper terminal provides an adjustable V out that can be set anywhere between zero and V in by turning the knob. This is the basis of analog volume controls, position sensors, and manual voltage reference adjustments.

Does temperature affect the output voltage?

Yes, slightly. All resistors have a temperature coefficient, meaning their resistance changes with temperature. Standard carbon film resistors have a typical temperature coefficient of around 200 ppm/°C, meaning a 10°C temperature rise changes the resistance by 0.2%. Because both R1 and R2 are affected similarly, the ratio — and therefore the output voltage — remains relatively stable. For precision applications, use matched resistors with low temperature coefficients, or use a dedicated precision voltage reference IC instead.

How do I choose between E12 and E24 resistor series?

The E12 series has 12 values per decade and is the most widely available and least expensive option. The E24 series has 24 values per decade and gives you finer resolution for hitting a specific resistance target. For most voltage divider applications, E24 is the best balance of availability, cost, and accuracy. The E48 and E96 series offer even finer resolution but are typically only needed in precision measurement and instrumentation circuits.