Voltage Divider Calculator: Calculate Output Voltage & Resistor Values

The best way to calculate a voltage divider is to multiply the input voltage by the lower resistor divided by the total resistance. A Voltage Divider Calculator uses the formula Vout = Vin × R2 / (R1 + R2) to calculate the output voltage.

Voltage dividers are commonly used to reduce or scale a voltage for electronic circuits, sensors, reference inputs and measurement systems. The basic calculation is simple but resistor selection and the resistance of the connected load can affect the actual output voltage.

Key Takeaways

• A voltage divider uses resistors to produce a fraction of an input voltage.
• The standard two resistor formula is Vout = Vin × R2 / (R1 + R2).
• Increasing R2 relative to R1 increases the output voltage.
• The same voltage division principle can be extended to three, four or five resistors.
• E24 resistor values can be used when selecting practical components.
• A connected load can change the calculated output voltage.

What Is a Voltage Divider?

A voltage divider is a resistor network that produces a lower voltage from a known input voltage. The simplest voltage divider uses two resistors connected in series between the input voltage and ground. A typical arrangement is:

Vin → R1 → Vout → R2 → Ground

The output voltage is measured across R2. Because the same current flows through R1 and R2 in the unloaded series circuit, the input voltage is divided according to the ratio of the resistor values.

A passive resistor voltage divider can only produce an output voltage equal to or lower than the input voltage. It cannot increase the input voltage.

Voltage Divider Formula

For a standard two resistor voltage divider, the output voltage is:

Vout = Vin × R2 / (R1 + R2)

Where:

The formula shows that the output depends on the proportion of R2 compared with the total resistance.

• Vout = output voltage
• Vin = input voltage
• R1 = upper resistor between Vin and Vout
• R2 = lower resistor between Vout and ground

Voltage Divider Example

Suppose:

Then:

Vout = 12 × 10 / (10 + 10)

Vout = 6 V

Therefore two equal resistors produce an output equal to half of the input voltage. For another example if Vin = 12 V, R1 = 4.7 kΩ and R2 = 10 kΩ:

Vout = 12 × 10 / (4.7 + 10)

Vout ≈ 8.16 V

The larger R2 is relative to R1 the higher the output voltage becomes.

• Vin = 12 V
• R1 = 10 kΩ
• R2 = 10 kΩ

How Does a Voltage Divider Calculator Work?

A Voltage Divider Calculator applies the standard voltage divider equation automatically. To calculate the output voltage:

For example entering 12 V, 10 kΩ and 10 kΩ gives an output of 6 V. If you need to determine an individual resistance from known voltage and current values a separate Resistance Calculator can be used.

• Enter the input voltage.
• Enter the value of R1.
• Enter the value of R2.
• Make sure the resistor units are compatible.
• Apply the voltage divider formula.
• Check the calculated Vout.

Voltage Divider Current

The current flowing through a simple unloaded two resistor divider can be calculated using:

I = Vin / (R1 + R2)

For example with:

The total resistance is:

Rtotal = 10 kΩ + 10 kΩ = 20 kΩ

Therefore:

I = 12 / 20,000

I = 0.0006 A

or:

I = 0.6 mA

This current is the divider current before considering any additional load connected to the output.

• Vin = 12 V
• R1 = 10 kΩ
• R2 = 10 kΩ

Voltage Divider Calculator With 3 Resistors

A voltage divider can use three resistors to create multiple voltage taps along a series chain. For a simple three resistor network:

Vin → R1 → R2 → R3 → Ground

The voltage at a selected point depends on the resistance between that point and ground compared with the total resistance. For example if the output is taken across R3:

Vout = Vin × R3 / (R1 + R2 + R3)

Suppose:

Then:

Vout = 12 × 3 / (1 + 2 + 3)

Vout = 6 V

The same series divider principle can provide different output voltages by taking the output from different resistor junctions.

• Vin = 12 V
• R1 = 1 kΩ
• R2 = 2 kΩ
• R3 = 3 kΩ

Voltage Divider Calculator With 4 or 5 Resistors

The voltage divider principle also works with four, five or more resistors connected in a series chain. The total resistance is:

Rtotal = R1 + R2 + R3 + ... + Rn

For a selected output resistor or section the voltage is proportional to the resistance between the output point and the reference point. For example a five resistor chain can provide several voltage taps.

The calculator needs to know the input voltage, resistor values and the point where the output voltage is measured. This makes a multi resistor voltage divider useful when a circuit requires more than one reference voltage from the same input supply.

Reverse Voltage Divider Calculator

A reverse voltage divider calculation is useful when the required output voltage is already known and you need to determine a suitable resistor value. If Vin, Vout and R1 are known, R2 can be calculated using:

R2 = (Vout × R1) / (Vin − Vout)

For example:

Then:

R2 = (5 × 10) / (12 − 5)

R2 ≈ 7.14 kΩ

The ideal calculated value is approximately 7.14 kΩ. In practical circuit design, you may need to select the nearest available standard resistor value and then recalculate the actual output voltage.

If R2 is known and R1 needs to be calculated use:

R1 = R2 × (Vin − Vout) / Vout

These reverse calculations are useful when designing a divider for a specific target voltage.

• Vin = 12 V
• Vout = 5 V
• R1 = 10 kΩ

Choosing E24 Resistor Values

The mathematical resistor value calculated for a target output is not always an available standard component value. For example a reverse calculation may produce:

R2 = 7.14 kΩ

A practical design may need to select a nearby standard resistor value from the E24 resistor series. After selecting the nearest available value recalculate:

Vout = Vin × R2 / (R1 + R2)

This confirms the actual output voltage produced by the selected resistor combination. The same approach can be used when checking several possible resistor combinations. The goal is to choose practical resistor values while keeping the resulting output close to the required voltage.

Voltage Divider Loading Effect

The basic voltage divider formula assumes that the output is not significantly loaded by another circuit. When a load resistor is connected between Vout and ground, that load is effectively connected in parallel with R2. The effective lower resistance becomes:

R2(effective) = R2 || Rload

For two resistances in parallel:

R2(effective) = (R2 × Rload) / (R2 + Rload)

The loaded output can then be calculated as:

Vout = Vin × R2(effective) / (R1 + R2(effective))

Because the load normally reduces the effective lower resistance, the actual output voltage can be lower than the ideal unloaded calculation. This is especially important when a voltage divider is connected to an input that draws meaningful current. A high impedance input produces less loading while a lower resistance load can significantly change Vout.

Common Voltage Divider Calculation Mistakes

Using the Wrong Resistor in the Numerator

For the standard arrangement where R1 is the upper resistor and R2 is connected to ground, R2 belongs in the numerator: Vout = Vin × R2 / (R1 + R2). Using R1 in the numerator produces the complementary voltage rather than the voltage measured across R2.

Forgetting the Total Resistance

The denominator must contain the sum of the relevant series resistors: R1 + R2. For multi resistor dividers include all resistors in the series path when calculating the total resistance.

Mixing Resistance Units

Make sure resistor values use compatible units. For example: 10 kΩ = 10,000 Ω. Do not combine kΩ and Ω values without converting them correctly.

Ignoring the Load

The ideal formula may not match the measured voltage when a load is connected to the output. Include the load resistance when the connected circuit draws enough current to affect the divider.

Assuming the Divider Can Increase Voltage

A passive resistor divider cannot produce an output greater than its input voltage. It only reduces or scales the input voltage.

Using a Divider for a Large Power Load

A resistor divider is primarily useful for voltage scaling and reference applications. It is not generally a substitute for a voltage regulator when a circuit needs a stable supply capable of delivering significant load current.

When Is a Voltage Divider Calculator Useful?

A Voltage Divider Calculator is useful when you need to:

A voltage divider is different from cable voltage loss. If you need to calculate voltage loss along an electrical cable, use the Voltage Drop Calculator instead.

• Calculate the output voltage from known resistor values.
• Select resistor values for a target output voltage.
• Check two resistor divider calculations.
• Design three resistor or multi tap divider networks.
• Find suitable resistor combinations using standard E24 values.
• Estimate divider current.
• Check the effect of an output load.
• Verify a circuit calculation before selecting components.

Conclusion

A Voltage Divider Calculator makes it quick to determine the output voltage produced by a resistor network and to select resistor values for a target voltage. The standard two resistor divider uses the ratio of the lower resistor to the total resistance while longer resistor chains can provide multiple voltage taps.

For practical circuit design, the ideal calculation is only the starting point. Standard E24 resistor values, divider current and the resistance of the connected load can all affect the final output so these factors should be checked before using the selected values in a real circuit.

FAQs

What is the voltage divider formula?

The standard voltage divider formula is: Vout = Vin × R2 / (R1 + R2). Here R1 is the upper resistor and R2 is the lower resistor connected to ground.

How do you calculate a voltage divider with 3 resistors?

Add the series resistors to find the total resistance then use the resistance between the selected output point and ground as the numerator. For a three resistor chain with the output across R3: Vout = Vin × R3 / (R1 + R2 + R3)

Can a voltage divider use 4 or 5 resistors?

Yes. The same voltage division principle applies to four, five or more resistors connected in series. Different points in the resistor chain can provide different output voltages.

How do I calculate a resistor for a target voltage?

For a two resistor divider with Vin, Vout and R1 known: R2 = (Vout × R1) / (Vin − Vout). The calculated value can then be matched to a practical standard resistor value.

What is an E24 voltage divider?

An E24 voltage divider uses standard resistor values from the E24 preferred value series to create a practical voltage divider. After choosing the nearest available resistor values, recalculate Vout to determine the actual output.

Why is my voltage divider output lower than expected?

The connected circuit may be loading the divider. A load resistance connected to Vout appears in parallel with R2 reducing the effective lower resistance and potentially lowering the output voltage.

Can a voltage divider increase voltage?

No. A passive resistor voltage divider cannot increase the input voltage. It produces an output that is equal to or lower than the input.

What is the current through a voltage divider?

For an unloaded two resistor divider the divider current is: I = Vin / (R1 + R2). The current can change when a load is connected to the output.