Voltage Divider Calculator
Calculate output voltage, current, and power dissipation for resistor voltage divider circuits.
Enter the input voltage and two resistor values to find the output voltage, circuit current, and power dissipated in each resistor.
Voltage Divider Calculator
Calculate output voltage, current, and power dissipation for resistor voltage divider circuits.
About the Voltage Divider Calculator
A voltage divider is one of the most fundamental circuits in electronics. It consists of two resistors connected in series between a supply voltage and ground, with the output voltage taken from the junction between the two resistors. The circuit divides the input voltage into a smaller output voltage in direct proportion to the resistance values, following Ohm's Law and the rules for series resistors.
The core formula is Vout = Vin × R2 / (R1 + R2). This expression can be derived in two lines: the total current through the series combination is I = Vin / (R1 + R2) by Ohm's Law, and the voltage across R2 is I × R2. Substituting gives the divider formula. Because R2 appears in both the numerator and denominator, the output voltage is always less than the input voltage (for positive resistances). Equal resistors give exactly half the supply voltage, while different values shift the output proportionally.
Voltage dividers are used throughout electronics. In sensor interfacing, a 0–5 V sensor output is commonly scaled to 0–3.3 V for a microcontroller ADC by choosing R1 = 2.2 kΩ and R2 = 3.3 kΩ. In transistor biasing circuits, a voltage divider establishes the base voltage that sets the quiescent operating point of a bipolar junction transistor. In audio equipment, potentiometers are continuously adjustable voltage dividers that control volume. Digital logic level-shifting often uses a two-resistor divider to translate a 5 V logic output to the 3.3 V threshold of a receiving device.
Power dissipation is a critical design consideration. Each resistor dissipates P = I² × R watts, and the total dissipation equals Vin² / (R1 + R2). Choosing low-value resistors improves immunity to load currents but wastes more power. Always verify that the calculated dissipation is safely below the resistor's rated wattage — a common safety rule is to derate by at least 50 %, so a 250 mW resistor should not exceed 125 mW in continuous use.
The unloaded divider formula is exact for an ideal voltage source and ideal resistors. In practice, any load connected across R2 appears in parallel and lowers the effective resistance, pulling Vout below the unloaded value. For accurate loaded operation, ensure the load resistance is at least ten times larger than R2, or account for the parallel combination in your design.
Voltage divider examples
Three practical voltage divider circuits showing the formula in action.
| Vin / R1 / R2 | Vout | Note |
|---|---|---|
| 10 V, R1 = 1 kΩ, R2 = 1 kΩ | 5 V | Equal resistors always produce exactly half the input voltage. Current = 5 mA, total power = 50 mW. |
| 5 V, R1 = 2.2 kΩ, R2 = 3.3 kΩ | 3 V | Classic 5 V to 3 V level-shift used when connecting 5 V sensors to 3.3 V microcontroller ADC inputs. |
| 12 V, R1 = 10 kΩ, R2 = 5 kΩ | 4 V | R2 / (R1 + R2) = 5/15 = 1/3, so Vout = 12 × 1/3 = 4 V. Total power = 9.6 mW at 0.8 mA. |
How to use the voltage divider calculator
- Enter the supply voltage (Vin) — the voltage applied across the entire R1 + R2 series combination.
- Enter the value of R1, the upper resistor connected between the supply and the output node.
- Enter the value of R2, the lower resistor connected between the output node and ground.
- Click Calculate to see the output voltage, circuit current, and power dissipation for each resistor.
- Use the example buttons to load common divider configurations, or click Reset to start over.
Voltage divider calculator FAQ
What is the voltage divider formula?
The formula is Vout = Vin × R2 / (R1 + R2). It follows directly from Ohm's Law: the same current flows through both series resistors, so the voltage across each is proportional to its resistance. R2 in the numerator and the total (R1 + R2) in the denominator give the fraction of the supply that appears at the output.
Why does connecting a load change the output voltage?
Any load resistor Rload placed across R2 forms a parallel combination with R2, reducing the effective lower resistance below R2. This lowers the ratio R2_eff / (R1 + R2_eff), which reduces Vout. To minimise loading error, choose Rload ≥ 10 × R2, or use an op-amp buffer to present an infinite impedance to the divider output.
How do I choose R1 and R2 for a given output voltage?
Start from the ratio Vout / Vin = R2 / (R1 + R2). Pick R2 based on acceptable current draw, then solve R1 = R2 × (Vin / Vout − 1). For example, to get 3.3 V from 5 V, Vout/Vin = 0.66, so R1 = R2 × (1/0.66 − 1) ≈ 0.515 × R2. Using R2 = 10 kΩ gives R1 ≈ 5.15 kΩ — round to the nearest standard value.
What resistor power rating do I need?
Each resistor dissipates P = I² × R, where I = Vin / (R1 + R2). Select a resistor with a power rating at least twice the calculated dissipation. For example, if the calculation shows 80 mW, use a 1/4 W (250 mW) resistor for adequate safety margin.
Can a voltage divider supply significant current to a load?
Not without affecting accuracy. A voltage divider is designed for signalling and biasing, not power delivery. If your load draws current comparable to the divider's standing current (I = Vin / (R1 + R2)), the output voltage will sag. For moderate load currents, use a voltage regulator or op-amp buffer instead.