Voltage Divider Calculator
Calculate the output voltage of a resistive voltage divider circuit. Enter the input voltage and two resistor values to find the divided output voltage. See also our Ohm's Law Calculator and Series Resistor Calculator.
How to Calculate Voltage Divider Output
A voltage divider is one of the most fundamental circuits in electronics. It consists of two resistors connected in series between a voltage source and ground. The output voltage is taken from the junction between the two resistors. The output voltage is a fraction of the input voltage, determined by the ratio of the resistors.
The voltage divider works on the principle that the total voltage across the series combination equals the sum of individual voltage drops. Since the same current flows through both resistors (Kirchhoff's Current Law), the voltage across each resistor is proportional to its resistance value.
Voltage Divider Formula
Output Voltage:
Vout = Vin × R2 / (R1 + R2)
Division Ratio:
Ratio = R2 / (R1 + R2)
Current Through Divider:
I = Vin / (R1 + R2)
Power Dissipated:
P_total = Vin² / (R1 + R2)
P_R1 = I² × R1
P_R2 = I² × R2
Example Calculation
Design a voltage divider to convert 12V to approximately 5V for a microcontroller ADC reference:
Given: Vin = 12V, desired Vout ≈ 5V
Choose R1 = 10kΩ, R2 = 7.15kΩ (nearest standard: 6.8kΩ)
Vout = 12 × 6800 / (10000 + 6800) = 4.857V
Current = 12 / 16800 = 0.714 mA
Power = 12² / 16800 = 8.57 mW (negligible)
With equal resistors (R1 = R2 = 10kΩ), the output is exactly half the input: Vout = 12 × 10000 / 20000 = 6V. This is the simplest voltage divider configuration and is commonly used for level shifting between 5V and 3.3V logic systems.
Voltage Divider Reference Table
| Vin (V) | R1 | R2 | Vout (V) |
|---|---|---|---|
| 5 | 10kΩ | 10kΩ | 2.500 V |
| 5 | 20kΩ | 10kΩ | 1.667 V |
| 5 | 10kΩ | 20kΩ | 3.333 V |
| 9 | 10kΩ | 10kΩ | 4.500 V |
| 12 | 10kΩ | 10kΩ | 6.000 V |
| 12 | 10kΩ | 4.7kΩ | 3.837 V |
| 12 | 10kΩ | 2.2kΩ | 2.164 V |
| 24 | 47kΩ | 10kΩ | 4.211 V |
| 24 | 10kΩ | 10kΩ | 12.000 V |
| 120 | 100kΩ | 10kΩ | 10.909 V |
| 3.3 | 10kΩ | 10kΩ | 1.650 V |
| 3.3 | 10kΩ | 20kΩ | 2.200 V |
Frequently Asked Questions
What is a voltage divider used for?
Voltage dividers are used to reduce voltage levels, create reference voltages, bias transistors, read sensors (like thermistors and potentiometers), interface between different logic voltage levels (5V to 3.3V), and provide feedback in voltage regulators. They are one of the most common subcircuits in electronics.
Why does loading affect voltage divider output?
When you connect a load to the output, it acts as a parallel resistance with R2, reducing the effective R2 value and lowering the output voltage. To minimize loading effects, the load impedance should be at least 10× greater than R2. For low-impedance loads, use a buffer amplifier (op-amp voltage follower) after the divider.
How do I choose resistor values for a voltage divider?
First determine the desired ratio (Vout/Vin = R2/(R1+R2)). Then choose absolute values based on current requirements: higher values (100kΩ range) minimize power waste but are more susceptible to noise; lower values (1kΩ range) are more stable but waste more power. For most signal-level applications, 10kΩ total is a good starting point.
Can a voltage divider increase voltage?
No. A resistive voltage divider can only reduce voltage — the output is always less than or equal to the input. To increase voltage, you need an active circuit such as a boost converter, charge pump, or transformer. The maximum output of a voltage divider equals the input (when R1 = 0).
What is the difference between a voltage divider and a voltage regulator?
A voltage divider provides a fixed ratio of the input voltage but cannot maintain a constant output under varying loads — if the load changes, the output voltage changes. A voltage regulator (like an LM7805 or LDO) actively maintains a constant output voltage regardless of load current changes, making it suitable for powering circuits.
How accurate is a voltage divider with standard resistor values?
Accuracy depends on resistor tolerance. With 1% resistors, the output voltage can vary by up to ±2% from the calculated value. For precision applications, use 0.1% tolerance resistors or trim potentiometers. Temperature drift (typically 50-100 ppm/°C for standard resistors) also affects long-term accuracy.
Design Considerations
When designing voltage dividers for real-world applications, consider these important factors: First, the output impedance of the divider equals R1 parallel with R2 (R1×R2/(R1+R2)). Any load connected to the output must have significantly higher impedance to avoid affecting the division ratio. Second, the quiescent current (Vin/(R1+R2)) represents wasted power — in battery-powered applications, use high-value resistors (100kΩ+) to minimize drain. Third, for AC signals, parasitic capacitance across the resistors can create a frequency-dependent divider that attenuates high frequencies differently than low frequencies.
Capacitive Voltage Dividers
For AC circuits, capacitors can also form voltage dividers. The formula becomes Vout = Vin × (1/C2) / (1/C1 + 1/C2) = Vin × C1 / (C1 + C2). Note that the ratio is inverted compared to resistive dividers — the smaller capacitor gets the larger voltage drop. Capacitive dividers are used in high-voltage measurement, power supply feedback networks, and RF circuits where resistive losses are unacceptable.