Voltage Divider Calculator
A voltage divider uses two resistors in series to produce an output voltage that is a fraction of the input. This calculator computes the output voltage, current through the circuit, and power dissipation in each resistor.
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Formula
Vout = Vin × R2 / (R1 + R2)
Output voltage Vout = Vin × R2/(R1+R2). This divides the input voltage proportionally to the resistor values. Current I = Vin/(R1+R2). Power in R1: P1 = I²×R1. Power in R2: P2 = I²×R2. Total power = Vin²/(R1+R2).
How to use the Voltage Divider Calculator
- 1
Enter your input voltage (vin)
Value should be in V.
- 2
Enter your resistor r1 (top)
Value should be in Ω.
- 3
Enter your resistor r2 (bottom)
Value should be in Ω.
- 4
Read your results instantly
Results update in real time as you type.
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How voltage dividers work
A voltage divider consists of two resistors in series connected across a voltage source. The output is taken from the junction between the two resistors. Because current is the same through both resistors (series circuit), voltage drops in proportion to resistance: larger R2 relative to R1 gives higher output voltage.
When R1 = R2, output is exactly half the input. When R2 >> R1, output approaches Vin. When R2 << R1, output approaches 0V. This makes voltage dividers ideal for creating reference voltages, scaling sensor outputs, and biasing transistor circuits.
Loading effect: why voltage dividers aren't ideal
The formula V_out = V_in × R2/(R1+R2) assumes no load — nothing connected to the output drawing current. In reality, connecting a load (another resistor, a microcontroller pin, an op-amp input) in parallel with R2 changes the effective R2 value and lowers the output voltage.
The rule of thumb: load resistance should be at least 10× the Thevenin resistance (R1||R2) of the divider to keep loading below ~9%. Op-amp inputs (very high impedance) barely affect the divider; digital inputs (50-100 kΩ) can load a divider significantly if R1 and R2 are large.
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Common applications
Voltage dividers appear everywhere in electronics. Potentiometers (volume knobs) are variable voltage dividers — turning the knob changes the ratio of R1 to R2, adjusting volume continuously. Analog-to-digital converters use them to create reference midpoints. Level shifters use them to drop 5V signals to 3.3V for microcontrollers.
Thermistors and photoresistors form one arm of a voltage divider — as temperature or light changes, the resistance changes, shifting Vout. A microcontroller reads this voltage and infers the physical condition. This is one of the simplest and most common sensor interfacing techniques.
Tips & Insights
Equal resistors give half voltage
When R1 = R2, Vout = Vin/2 exactly, regardless of the resistor values. 1kΩ/1kΩ or 100kΩ/100kΩ both give the same ratio.
Higher resistances = less power waste
Using 100kΩ resistors instead of 1kΩ draws 100× less current. For battery-powered designs, use the largest values that still work with your load.
Potentiometer as divider
A potentiometer is a mechanical voltage divider. The wiper sweeps between the two ends, giving continuously variable Vout from 0 to Vin.
Worked Examples
5V to 3.3V level shifter
Vout = 5 × 3300/(1800+3300) = 5 × 0.647 ≈ 3.24V. Close enough to 3.3V for most logic-level purposes.
12V to 6V reference
Vout = 12 × 0.5 = 6V. Current = 12/20,000 = 0.6 mA. Power = 12 × 0.6mA = 7.2 mW.
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Frequently Asked Questions
What is a voltage divider?
Two resistors in series across a voltage source, with output taken from their junction. Vout = Vin × R2/(R1+R2).
When should I use a voltage divider?
For creating reference voltages, scaling analog signals, reading resistive sensors, or stepping down signal voltages. Not for powering loads (use a voltage regulator instead).
What is the loading effect?
Connecting a load to the output draws current, effectively putting a resistor in parallel with R2 and lowering Vout. Use high-value resistors or a buffer amplifier to minimize loading.
Can a voltage divider power a circuit?
Generally no — it lacks current regulation. As load current changes, Vout changes. Use a linear or switching voltage regulator for powering circuits.
What is a potentiometer?
A potentiometer (pot) is a variable resistor acting as a voltage divider. Turning the knob varies the R1/R2 ratio, producing a continuously adjustable output voltage.
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