Voltage Divider Calculator
Calculate output voltage from two series resistors and an input voltage.
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A voltage divider uses two series resistors to produce an output voltage that is a fraction of the input. Enter all three values and the calculator computes the output instantly.
- Enter input voltage. This is the voltage applied across both resistors in series.
- Enter R1 and R2. R1 is the top resistor (between input and output), R2 is the bottom resistor (between output and ground). Both in ohms.
- Read the output. Vout is the voltage across R2. The calculator also shows the current through the divider and the division ratio R2/(R1+R2).
Use the Share button to save your circuit values or send them to a colleague.
About the Voltage Divider
The voltage divider formula is Vout = Vin × R2 / (R1 + R2). This is one of the most fundamental circuits in electronics, used for biasing transistors, setting reference voltages, creating analog-to-digital converter reference levels, and interfacing 5 V logic with 3.3 V microcontrollers.
Note that this calculation assumes no load is connected to the output. Any load in parallel with R2 will reduce Vout. For accuracy with a load, use a Thevenin equivalent: replace R1 and R2 with their parallel combination as the source resistance and Vout (unloaded) as the source voltage.
Frequently Asked Questions
What is the voltage divider formula?
The voltage divider formula is Vout = Vin × R2 / (R1 + R2). R1 and R2 are in series across the input voltage, and the output is taken across R2. To find R1 or R2 for a desired output, rearrange: R1 = R2 × (Vin/Vout - 1), or R2 = R1 × Vout / (Vin - Vout).
How do I use a voltage divider to step down 5V to 3.3V?
To divide 5 V to 3.3 V, you need a ratio of 3.3/5 = 0.66. Choose R2/(R1+R2) = 0.66. A common combination is R1 = 1.8 kΩ and R2 = 3.3 kΩ, giving Vout = 5 × 3300/(1800+3300) = 3.24 V, close enough for most 3.3 V logic inputs. For precision applications, use a dedicated level-shifter IC or a voltage regulator instead.
Why does my voltage divider output drop when I connect a load?
A load connected across R2 forms a parallel combination with R2, reducing the effective lower resistance. This shifts the divider ratio and lowers Vout. To minimize this effect, choose R1 and R2 values that are much smaller than the load resistance (typically 10x or less). For example, if your load is 10 kΩ, use R1 and R2 values of 1 kΩ or less. The tradeoff is higher quiescent current through the divider.
What resistor values should I use for a voltage divider?
The absolute values matter less than the ratio. However, too-low values waste current, and too-high values are sensitive to load. For battery-powered designs, use 100 kΩ to 1 MΩ to minimize standby current. For signal applications or ADC references, use 1 kΩ to 100 kΩ. For power circuits with significant load, keep both resistors below 1/10th of the load resistance.