Voltage Divider Resistor Calculator and Formula

A voltage divider resistor calculator finds the output voltage from a supply voltage and two series resistors, or calculates a resistor needed for a target output. Enter volts and ohms to see the divider’s output voltage and the resistor ratio; this basic model assumes the output is not significantly loaded.

Formula

For two resistors in series across a supply, with the output taken from their junction relative to ground:

Vout = Vin × R2 / (R1 + R2)

• Vin is the input or supply voltage, in volts (V).
• R1 is the resistor between the supply and the output junction, in ohms (Ω).
• R2 is the resistor between the output junction and ground, in ohms (Ω).
• Vout is the voltage at the junction relative to ground, in volts (V).

The divider current shown here is I = Vin / (R1 + R2), displayed in milliamps (mA). This is the current through the unloaded series pair. The calculator’s first result is Vout, the quantity asked for by a voltage-divider calculator.

Worked Example

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

1. Add the resistor values: R1 + R2 = 10,000 + 10,000 = 20,000 Ω.
2. Find the lower-resistor fraction: R2 / (R1 + R2) = 10,000 / 20,000 = 0.5.
3. Multiply by the input: Vout = 12 × 0.5 = 6.00 V.
4. Find divider current: 12 / 20,000 = 0.0006 A = 0.60 mA.

With equal resistors, the midpoint is half the supply voltage, provided the output is unloaded or the load’s effect is negligible.

How a Voltage Divider Works

Two resistors connected end to end share the same series current. The voltage across each resistor is proportional to its resistance, so the junction voltage depends on the share of total resistance below that junction. R2 is the lower resistor because it connects from the output node to ground; swapping R1 and R2 changes the result.

For example, if R1 is 20 kΩ and R2 is 10 kΩ, the output is one-third of Vin, not two-thirds: the lower resistor accounts for 10 kΩ of the 30 kΩ total. For a 12 V input, the ideal output is 4 V.

Choosing Resistor Values

First determine the desired output and the expected input range. The resistance ratio sets the ideal voltage; the total resistance sets the current and power use. Multiplying both resistor values by the same factor leaves the ideal ratio unchanged, but reduces current and increases sensitivity to loading and leakage.

• Check the ratio: Rearrange the formula as R1/R2 = Vin/Vout − 1. This helps select a pair for a target voltage.
• Check current: Divider current is Vin/(R1 + R2). A smaller total resistance draws more current from the source.
• Check power: For each resistor, calculate P = I²R, or use the voltage across it with P = V²/R. Choose resistors with adequate power rating and operating margin.
• Check tolerances: Real resistor values vary within their stated tolerance. That variation changes the output ratio, especially when the required output accuracy is tight.
• Check the load: Include whatever is connected to the output before deciding that the simple result is accurate.

Resistance may be entered in ohms, kilohms, or megohms, but convert both resistors to the same unit before using the formula. For example, 10 kΩ is 10,000 Ω. Voltage is measured in volts; the formula does not require a special US customary conversion because electrical voltage and resistance use the same units internationally.

Loading, Accuracy, and Common Mistakes

A connected load draws current from the divider’s midpoint. Electrically, a resistive load RL from output to ground sits in parallel with R2, so replace R2 in the formula with the equivalent resistance R2 × RL / (R2 + RL). The loaded output will generally be lower than the unloaded prediction. A common design guideline is to make the divider current several times larger than the load current, but the required margin depends on acceptable error and power consumption.

Inputs such as an analog-to-digital converter are not always simple resistors. Their input impedance, leakage, and sampling behavior can affect the reading. Consult the input’s specifications; a buffer amplifier may be more suitable if the divider cannot drive the load accurately.

• Do not use the wrong resistor as the numerator: the output is measured across R2, the resistor to ground.
• Do not assume that a correct ratio guarantees a stable voltage; input variation, resistor tolerance, temperature, and loading all matter.
• Do not round intermediate values too early. Keep adequate precision through calculations, then round the displayed result to match the needed accuracy.
• Do not treat a divider as a general-purpose power supply. Its output changes with load, and it wastes current continuously while powered.

When to Use a Divider—and When Not To

A resistor divider is useful for creating a reference or signal level when the load draws little current, such as scaling a measured voltage into an input range. It can also provide a bias voltage in a circuit, subject to the circuit’s impedance and stability requirements.

For powering a device, a divider is often a poor choice: a changing load changes the output, while lowering resistance to improve regulation increases wasted current. A voltage regulator or another appropriate power-conversion circuit is usually a better option when the output must remain controlled under varying load. A divider also cannot produce an output above its input, and its output is not isolated from the supply or ground.

Frequently Asked Questions

How do I calculate the output voltage of a resistor divider?
Multiply the input voltage by the lower resistor divided by the sum of both resistors: Vout = Vin × R2/(R1 + R2). This assumes the output is unloaded.

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

How do I choose resistors for a target voltage?
Use R1/R2 = Vin/Vout − 1 to find the required ratio, then choose practical values while checking divider current, power, tolerance, and load effects.

Why is my measured divider voltage lower than calculated?
A load may be drawing current from the output, or the supply and resistor values may differ from their nominal values. Input leakage, wiring, and measurement setup can also affect a high-resistance divider.

Can a voltage divider power a circuit?
Usually not when the circuit’s current varies or is substantial. The load changes the effective lower resistance and therefore the output voltage; use a suitable regulator when a stable supply is required.

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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