About the Voltage Divider Calculator
A voltage divider is one of the simplest and most useful circuits in electronics: two resistors connected in series across a voltage, with the output taken from the point between them. It produces a fixed fraction of the input voltage, set by the ratio of the two resistors. Voltage dividers scale sensor signals, set reference levels, bias transistors and let a 5-volt signal talk safely to a 3.3-volt microcontroller.
This voltage divider calculator finds the output voltage from the input voltage and the two resistors, or works out the second resistor needed to get a target output. It also shows the current flowing through the divider, the voltage across each resistor, the power each one dissipates, and how heavily the output can be loaded before it sags.
How to Use the Voltage Divider Calculator
Choose Find Vout or Find R2.
Enter the input voltage and R1, the resistor connected to the input.
Enter R2, the resistor connected to ground — or, to find R2, the target output voltage.
Choose the resistor unit: Ω, kΩ or MΩ.
The Formulas
Vout = Vin × R2 ÷ (R1 + R2)
R2 = R1 × Vout ÷ (Vin − Vout)
R1 = R2 × (Vin − Vout) ÷ Vout
current through the divider I = Vin ÷ (R1 + R2)Step-by-Step Examples
12 V with R1 = 10 kΩ and R2 = 5 kΩ.
Vout = 12 × 5 ÷ (10 + 5) = 4 V
I = 12 ÷ 15,000 = 800 µA
voltage across R1 = 8 V, across R2 = 4 VTurning 5 V into 3.3 V with R1 = 10 kΩ.
R2 = 10 × 3.3 ÷ (5 − 3.3) = 19.412 kΩThe nearest standard value, 20 kΩ, gives 5 × 20 ÷ 30 = 3.333 V.
Two equal resistors always give half the input: 9 V becomes 4.5 V.
How a Voltage Divider Works
The same current flows through both resistors, because they are in series. By Ohm's law, the voltage across each is that current multiplied by its resistance, so the input voltage is shared between them in proportion to their resistances. A resistor twice as large drops twice as much voltage. The output, taken across R2, is therefore R2's share of the total resistance, times the input voltage. Only the ratio of the resistors sets the output; their absolute size sets the current and the power wasted.
Choosing Resistor Values
Many pairs of resistors give the same ratio, so how do you choose? Small values draw more current and waste more power, which matters for battery-powered projects, but they are less affected by any load. Large values waste very little power, but the output is easily pulled down by whatever it connects to, and it becomes more sensitive to noise. For signals feeding a microcontroller's input, values of about 10 kΩ to 100 kΩ are a common compromise.
The Loading Effect
The simple formula assumes nothing draws current from the output. In practice, whatever is connected — an input pin, a meter, another circuit — acts as a resistance in parallel with R2, lowering the effective bottom resistance and pulling Vout down. If the load resistance is ten times R2, the output drops by several percent; if it is similar to R2, the error is large. A useful rule of thumb is to keep the load at least ten times R2, or to follow the divider with a buffer amplifier.
Voltage Dividers Are Not Power Supplies
It can be tempting to use a voltage divider to run a lower-voltage device from a higher supply, but it rarely works well. The device draws current, which changes the output voltage; the resistors waste energy as heat; and any change in the device's current makes its voltage jump around. For powering circuits, a voltage regulator — linear or switching — holds the output steady whatever the load, and is almost always the right choice.
Sensors and Potentiometers
Many sensors are variable resistors: thermistors change resistance with temperature, light-dependent resistors with light and strain gauges with force. Placing one in a voltage divider turns its changing resistance into a changing voltage that a microcontroller can measure. A potentiometer is itself an adjustable voltage divider: its wiper slides along a resistive track, sharing the resistance between the two halves and giving any output from zero up to the full input, which is how many volume controls and adjustment knobs work.
Measuring Higher Voltages Safely
Microcontroller inputs usually accept only 0 to 3.3 V or 0 to 5 V. To measure a 12 V battery, a divider with R1 = 30 kΩ and R2 = 10 kΩ scales the voltage by a quarter, bringing 12 V down to 3 V, well within range. Software then multiplies the reading by four to recover the battery voltage. Choosing precise 1 percent resistors keeps the measurement accurate.
Understanding Your Result
The headline is the output voltage, or the R2 value needed.
The ratio line gives the output as a percentage and a fraction of the input.
The current line gives the current and the voltage across each resistor.
The power line gives the power each resistor dissipates.
The loading line gives the minimum load resistance for an accurate output.
When Should You Use This Calculator?
Use it to scale a voltage for a microcontroller or data logger input.
Use it to design sensor circuits with thermistors or light-dependent resistors.
Use it for electronics coursework on series circuits.
Common Mistakes
Swapping R1 and R2. The output is across R2, the resistor to ground.
Ignoring the load. A connected load lowers the output voltage.
Using a divider as a power supply. Use a regulator instead.
Choosing very small resistors. They waste power and may overheat.