Voltage Division Calculator: Find Output Voltage

A voltage division calculator finds the voltage across a chosen resistor in a series divider. Enter the supply voltage and the two resistor values; the calculator returns the output voltage and the current through the divider.

Formula

For two resistors in series, with the output measured across the lower resistor, the voltage-divider formula is:

V_out = V_supply × R₂ / (R₁ + R₂)

• V_out is the output voltage across R₂, in volts (V).
• V_supply is the voltage across both series resistors, in volts (V).
• R₁ is the resistor between the supply and the output node, in ohms (Ω).
• R₂ is the resistor between the output node and ground, in ohms (Ω).

The divider current, assuming no other load draws current from the output, is I = V_supply / (R₁ + R₂). The calculator reports this in milliamps (mA); multiply amps by 1,000 to convert to milliamps. Resistor values can be expressed in ohms, kilohms, or another consistent unit, provided both values use the same unit.

Worked Example

Using the calculator’s default values—12 V supply, R₁ = 10,000 Ω, and R₂ = 10,000 Ω—the output is across R₂.

1. Add the resistances: 10,000 Ω + 10,000 Ω = 20,000 Ω.
2. Find the output: 12 V × 10,000 Ω / 20,000 Ω = 6.00 V.
3. Find the current: 12 V / 20,000 Ω = 0.0006 A = 0.60 mA.

Equal resistors split the supply voltage equally, so the output is half of 12 V. The current is the same through both resistors because they are in series.

How a Voltage Divider Works

In a series circuit, the same current flows through R₁ and R₂. Ohm’s law says the voltage across a resistor is its resistance multiplied by the current. Since R₂ is part of the total series resistance, its share of the supply voltage is R₂ divided by R₁ + R₂. That proportion gives the output formula.

The output node is the junction between the resistors. This calculator assumes the lower resistor connects from that node to ground. If you instead measure across R₁, use the complementary voltage: V_supply × R₁ / (R₁ + R₂). Reversing the resistor positions changes which fraction appears at the output.

A divider can reduce a voltage, but it does not create a regulated supply. Its output depends on the input voltage, resistor values, and any load connected to the output.

When to Use It and How to Choose Resistors

A resistor divider is useful for producing a reference voltage, setting a transistor’s bias, or scaling a signal for an input that draws very little current. It is often suitable for simple sensing and signal-level tasks, but may be unsuitable for powering a device or supplying a changing, substantial load.

• Choose a target output fraction: for example, equal resistors produce half the supply voltage.
• Choose practical resistor values while considering current and power. Lower resistance uses more current; higher resistance saves current but makes the output more sensitive to loading and interference.
• Check the resistor power: each resistor dissipates P = I²R. Select components with an appropriate power rating and operating margin.
• Check resistor tolerances. Real resistors differ slightly from their marked values, so the actual output may differ from the ideal calculation.

For example, a 12 V divider made from two 10 kΩ resistors draws 0.60 mA and produces an ideal 6 V output. If the output must remain accurate while driving a load, calculate the loaded divider or use a buffer or regulator instead.

Loading, Limits, and Common Mistakes

The basic formula assumes the output is unloaded: nothing else draws meaningful current from the junction. Connecting a load from the output to ground places it in parallel with R₂. The effective lower resistance becomes R₂ in parallel with the load, which is smaller than R₂; consequently, the output voltage falls. A common design check is to make the load resistance much larger than the divider’s effective output resistance, but the suitable margin depends on the required accuracy and load variation.

• Mixing units: Convert values before calculation if one resistor is entered in ohms and the other in kilohms. For example, 10 kΩ is 10,000 Ω.
• Using the wrong resistor in the numerator: For output across the lower resistor, use R₂, not R₁.
• Ignoring the load: The ideal result may not match a circuit whose input draws current.
• Rounding too early: Keep precision during intermediate calculations and round the displayed result at the end.
• Assuming a divider is a power supply: Its output can shift with load, supply changes, and resistor tolerances.

The calculator gives an ideal DC result for two resistors. It does not account for component tolerance, temperature effects, wiring resistance, AC frequency-dependent behavior, or protection requirements. For high-frequency signals, capacitor effects and circuit impedance may also matter.

Frequently Asked Questions

What is the voltage-divider rule?
The voltage-divider rule states that a series resistor receives a fraction of the total voltage proportional to its resistance. For output across R₂, V_out = V_supply × R₂ / (R₁ + R₂).

Does a voltage divider reduce current too?
The divider current is set by the supply voltage divided by the sum of the two resistor values. Adding a load at the output creates another current path and changes the effective circuit resistance and output voltage.

Can I use a voltage divider to power a device?
Only if the device’s current is small and sufficiently predictable, and the resulting voltage remains within its requirements. For a load that draws substantial or varying current, a regulator or suitable power supply is generally a better choice.

How do I calculate the output across the top resistor?
Use V_out = V_supply × R₁ / (R₁ + R₂) when measuring across R₁. The two resistor voltages add to the supply voltage in the ideal series circuit.

What happens if both resistors have the same value?
Each resistor gets half the supply voltage when the divider is unloaded. For a 12 V supply, equal resistors give an ideal 6 V across either resistor.

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in supply | Supply 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 | supply * r2 / (r1 + r2) | 2
out current | Divider current | mA | supply / (r1 + r2) * 1000 | 2