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Voltage Divider Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Formula from standard circuit theory. Last verified 14 June 2026.

A voltage divider is one of the most fundamental passive circuits in electronics. It consists of two resistors (R1 and R2) connected in series between a supply voltage (Vin) and ground. The output voltage (Vout) is taken from the node between the two resistors. The formula is Vout = Vin * R2 / (R1 + R2), which shows that Vout is always a fraction of Vin determined by the ratio of R2 to the total resistance. Voltage dividers appear everywhere in electronics: setting the input bias of a transistor amplifier, scaling a sensor output down to microcontroller ADC range, creating a logic-level interface between a 5 V device and a 3.3 V device, and generating a stable reference voltage from a regulated supply. This calculator computes the output voltage, the current flowing through the divider, and the power dissipated in each resistor. Power dissipation is important for choosing the correct resistor wattage rating. Enter your known values and results update instantly. Note that the output voltage shown here is the unloaded (open-circuit) value. When a load is connected it will draw current through R1, reducing Vout.

Supply voltage in volts
Upper resistor in ohms
Lower resistor in ohms
Vout--
Current through divider (mA)--
Power in R1 (mW)--
Power in R2 (mW)--
Total power (mW)--

Voltage divider formula

Vout = Vin × R2 / (R1 + R2)
I = Vin / (R1 + R2)
P1 = I² × R1
P2 = I² × R2

Worked example

Vin = 12 V, R1 = 10,000 Ω, R2 = 10,000 Ω:

  1. Vout = 12 * 10,000 / 20,000 = 6.00 V
  2. Current = 12 / 20,000 = 0.0006 A = 0.60 mA
  3. P1 = P2 = (0.0006)² * 10,000 = 3.60 mW each

Frequently asked questions

What is a voltage divider?

A voltage divider is a simple circuit of two resistors in series connected across a voltage source. The output is taken from the junction between the two resistors. Because the resistors share the input voltage in proportion to their values, the output voltage is always lower than the input. Voltage dividers are used to set bias points, interface logic levels, and create reference voltages.

What is the voltage divider formula?

Vout = Vin * R2 / (R1 + R2). The output voltage equals the input voltage multiplied by the fraction of total resistance contributed by the lower resistor R2. If R1 equals R2, the output is exactly half of the input.

Why does loading affect the output voltage?

When a load is connected to Vout, it appears in parallel with R2, effectively reducing the lower resistance. This lowers the fraction of Vin seen at the output. For accurate results, the load resistance should be at least 10 times larger than R2. This calculator shows the unloaded output voltage.

How do I choose R1 and R2 values?

Start with the desired output ratio: R2 / (R1 + R2) = Vout / Vin. Then choose a total resistance (R1 + R2) that limits current to an acceptable level for the power budget. Smaller total resistance improves load regulation but wastes more power. Larger total resistance uses less power but is more sensitive to load effects.

What power rating should the resistors be?

Use the power values from this calculator to select resistor wattage. As a safety margin, choose resistors rated at least twice the calculated power dissipation. Standard resistors come in 1/8 W, 1/4 W, 1/2 W, 1 W, and 2 W ratings.

Sources

Reviewed by the CalculatorHub team, edited by James Graham. 14 June 2026.