Voltage Divider

Finding Output Voltage From Two Series Resistors

A voltage divider uses two resistors in series across a supply voltage to produce a smaller, predictable output voltage at the point between them. Enter the input voltage and the two resistor values, and this calculator finds the output voltage tapped between R1 and R2, along with the voltage dropped across R1 and the current flowing through the circuit.

This is one of the most common building blocks in analog electronics: scaling a sensor’s raw output into a safe input range, creating a stable reference voltage, or shifting a signal between two different logic-level voltages — all with nothing more than two resistors.

The Formula

$$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$$

R1 sits between the input voltage and the output tap point; R2 sits between the tap point and ground. Because the two resistors are in series, the same current flows through both, and the two voltage drops always add back up to the full input voltage.

Worked Example

A 12V supply through a 1,000 Ω R1 and a 2,000 Ω R2:

1. Output voltage: $12 \times \frac{2,000}{1,000 + 2,000} = 8$ V.
2. Voltage across R1: $12 - 8 = 4$ V.
3. Current: $12 \div 3,000 = 0.004$ A (4 mA).

The two drops check out: 4V across R1 plus 8V across R2 sums back to the full 12V supply.

Key Factors to Consider

• A basic voltage divider’s output voltage sags noticeably once a real load draws current from the output tap. This calculator’s formula assumes an ideal, no-load (open-circuit) output — connecting a real load in parallel with R2 effectively changes R2’s value in the circuit, which pulls the actual output voltage below the calculated ideal value. A simple voltage divider is best suited for high-impedance loads that draw very little current.
• The absolute resistor values (not just their ratio) determine how much current the circuit continuously draws, and therefore how much power it wastes as heat. Two resistor pairs with the identical ratio produce the same output voltage, but a pair using much smaller resistance values draws proportionally more current and wastes more power — choosing resistor values involves balancing this power waste against the circuit’s sensitivity to a connected load.
• A voltage divider is a passive, purely resistive circuit — it can only step voltage DOWN, never up. Since the output tap always sits between the input voltage and ground, the output voltage can never exceed the input voltage — stepping voltage UP requires a fundamentally different circuit (like a boost converter), not a resistive divider.
• Voltage dividers are a common way to safely interface a sensor or logic signal between two different voltage levels. A classic real-world use is scaling a 5V microcontroller signal down to a safe 3.3V input for another chip, or vice versa — this is one of the most frequently encountered practical applications of the same simple two-resistor formula this calculator computes.

Common Mistakes

• Swapping R1 and R2. R1 always sits between the input voltage and the output tap, and R2 always sits between the tap and ground — reversing which resistor is which flips which fraction of the input voltage actually appears at the output.
• Assuming the calculated output voltage will hold once a real load is connected. This calculator’s formula assumes an ideal, no-load output — a real load draws current through R2 in parallel, pulling the actual output voltage below the calculated value unless the load’s impedance is much higher than R2.
• Expecting a voltage divider to step voltage up. A resistive divider can only reduce voltage, never increase it — stepping voltage up requires an active circuit like a boost converter, not two resistors.

Useful to Know

• Want to sanity-check the current and total resistance this circuit draws using Ohm’s Law directly? Ohm's Law Calculator solves for voltage, current, or resistance from the other two.
• Need the combined resistance of R1 and R2 together, or a circuit with more than two resistors? Resistors in Series and Parallel Calculator handles series and parallel combinations.