Voltage Divider Formula Calculator and Guide

A voltage divider formula calculator finds the output voltage across one resistor in a series pair. Enter the input voltage and both resistor values; it returns the ideal output voltage, plus the current through the divider and the voltage across the other resistor.

Formula

For two resistors connected in series across a voltage source, with the output taken from their junction relative to the lower end:

Vout = Vin × R2 / (R1 + R2)

• Vin is the input voltage across both resistors, in volts (V).
• R1 is the upper resistor, between the positive input and the output junction, in ohms (Ω).
• R2 is the lower resistor, between the output junction and the reference or ground, in ohms (Ω).
• Vout is the output voltage from the junction to ground, in volts (V).

The divider current, assuming no load is attached, is I = Vin / (R1 + R2), in amperes (A). The voltage across R1 is VR1 = Vin × R1 / (R1 + R2). The two resistor drops add to Vin. Resistor values may be entered in ohms, kilohms, or another unit so long as both use the same unit; the ratio in the output formula is unchanged.

Worked Example

Using the calculator’s default values, Vin = 12 V, R1 = 10,000 Ω, and R2 = 10,000 Ω:

1. Add the resistances: R1 + R2 = 10,000 + 10,000 = 20,000 Ω.
2. Calculate the output: Vout = 12 × 10,000 / 20,000 = 6.00 V.
3. Calculate the divider current: I = 12 / 20,000 = 0.0006 A = 0.60 mA.
4. Calculate the voltage across R1: VR1 = 12 × 10,000 / 20,000 = 6.00 V.

The equal resistors split the input voltage evenly: each has a 6.00 V drop, and the unloaded output is 6.00 V.

How a Voltage Divider Works

In a series circuit, the same current flows through R1 and R2. Their total resistance determines that current, while the voltage across each resistor depends on its share of the total resistance. The output at the junction is the drop across R2, so a larger R2 relative to R1 gives a higher output voltage.

For example, if R1 is 10 kΩ and R2 is 20 kΩ, the output is two-thirds of Vin. With a 12 V input, that is 8 V. If the resistors are equal, the output is half the input. The output cannot exceed the input or fall below ground in this simple, unloaded circuit with positive resistor values.

When to Use a Divider

A resistor divider is useful for creating a reference voltage or scaling a signal for an input that draws very little current. Common examples include reading a sensor signal or measuring a higher voltage with a suitably rated measurement input. It is not generally a substitute for a regulated power supply: a load can change the output, and the divider continuously draws current while powered.

To choose a target output, rearrange the formula:

R1 / R2 = (Vin / Vout) − 1

Pick a practical resistor pair with approximately that ratio, then check current, power, tolerances, and load effects. For instance, an output of 5 V from 12 V requires R1/R2 = 12/5 − 1 = 1.4. The ratio could be approximated with R1 = 14 kΩ and R2 = 10 kΩ, giving 5 V ideally.

Loading, Power, and Practical Limits

The calculator assumes that nothing else draws current from the output. If a load resistor RL is connected from the output to ground, it sits in parallel with R2. Use the effective lower resistance R2 × RL / (R2 + RL) in place of R2 in the divider formula. A load comparable to R2 can reduce Vout substantially. For better accuracy, the divider’s current is often chosen much larger than the load current, but that wastes power; a buffer or regulator may be a better choice.

Each resistor dissipates heat. For the unloaded divider, the current is Vin/(R1+R2), and resistor power can be calculated as P = I²R. Check that each resistor’s power rating and voltage rating are adequate, with reasonable margin. Also account for resistor tolerance: real component values vary, so the actual output will not be perfectly exact. A divider’s output impedance, seen at its junction, is approximately R1 in parallel with R2; a sensitive or low-impedance load can therefore affect it.

Common Mistakes and Alternatives

• Reversing R1 and R2: The output is measured across the lower resistor. Swapping the values changes the result.
• Mixing units: Convert values to the same unit before adding them. For example, 10 kΩ is 10,000 Ω.
• Ignoring the load: The ideal formula is not accurate when the connected circuit draws significant current.
• Rounding too early: Keep full precision through the calculation and round only the displayed result.
• Using it for substantial power: A divider wastes energy and its output varies with load. Use a regulator, converter, or buffer when a stable supply is needed.

For a high-impedance signal input, a divider may be simple and appropriate. For powering a device, a voltage regulator or switching converter is usually more suitable. A potentiometer can provide an adjustable divider, but its output is still sensitive to load and setting.

Frequently Asked Questions

How do I calculate voltage divider output?
Multiply the input voltage by the lower resistor, then divide by the sum of both resistors: Vout = Vin × R2/(R1+R2).

Which resistor is R1 and which is R2?
R1 is between the input supply and the output junction. R2 is between that junction and ground; the output is measured across R2.

Can a voltage divider reduce 12 V to 5 V?
Yes, ideally, if R1/R2 is 1.4. Real output depends on resistor tolerances and the load, so check the result in the actual circuit.

Does a voltage divider work with a load?
It can, but the load changes the effective lower resistance and usually reduces the output. Include the load in the calculation or use a buffer if the voltage must remain stable.

Can I use a voltage divider to power a circuit?
Only for very light, tolerant loads. A divider’s output changes with current demand and it wastes power, so a regulator is preferable for a dependable supply.

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in vin | Input voltage | V | 12 | 0
in r1 | Upper resistor R1 | ohm | 10000 | 0
in r2 | Lower resistor R2 | ohm | 10000 | 0
out vout | Output voltage | V | vin * r2 / (r1 + r2) | 2
out current | Divider current | mA | vin / (r1 + r2) * 1000 | 2
out vr1 | Voltage across R1 | V | vin * r1 / (r1 + r2) | 2