How a voltage divider works
Two resistors in series between Vin and ground split the voltage in proportion to their values. The output is taken from the point between them:
Vout = Vin × R2 / (R1 + R2)
R1 is the resistor between Vin and the output, and R2 is the one between the output and ground. With R1 = 10 kΩ and R2 = 20 kΩ, a 5 V input gives 5 × 20 / 30 = 3.333 V.
The same current flows through both resistors: I = Vin / (R1 + R2). Each resistor turns I² × R into heat, which is the power shown in the results.
Finding the missing resistor
Rearranging the formula gives the other resistor directly:
R2 = R1 × Vout / (Vin − Vout)
R1 = R2 × (Vin − Vout) / Vout
The exact answer is rarely a value you can buy, so the calculator also shows the E24 values on either side of it and the output voltage each one really gives. E24 resistors are the common ±5% series, with 24 values per decade.
Common E24 pairs
| Vin | R1 | R2 | Vout |
|---|---|---|---|
| 5 V | 10 kΩ | 10 kΩ | 2.5 V |
| 5 V | 4.7 kΩ | 9.1 kΩ | 3.297 V |
| 3.3 V | 10 kΩ | 12 kΩ | 1.8 V |
| 9 V | 12 kΩ | 15 kΩ | 5 V |
| 12 V | 18 kΩ | 13 kΩ | 5.032 V |
| 12 V | 24 kΩ | 9.1 kΩ | 3.299 V |
| 24 V | 91 kΩ | 24 kΩ | 5.009 V |
Frequently asked questions
Can I power a circuit from a voltage divider?
Usually not. The formula assumes nothing is connected to Vout. A load draws current and acts like a resistor in parallel with R2, which pulls Vout down. Dividers are fine for feeding a high-impedance input such as an ADC pin, an op-amp or a comparator. To power something, use a voltage regulator.
What resistor values should I choose?
Only the ratio sets Vout, so the size is a trade-off. Low values waste current and heat; very high values are more easily pulled off by the input they feed and pick up noise. For reading a voltage with a microcontroller ADC, a total of 10 kΩ to 100 kΩ is a common starting point. Check the input impedance in your chip’s datasheet.
Can I use a voltage divider to connect a 5 V signal to a 3.3 V pin?
For slow signals, yes: 4.7 kΩ on top and 9.1 kΩ on the bottom gives about 3.3 V from 5 V. For fast signals such as SPI clocks, the resistors and pin capacitance slow the edges, so a level-shifter chip is more reliable.
How much power can the resistors handle?
Compare the power shown for each resistor with its rating. Common through-hole resistors are rated 1/4 W (250 mW), and small SMD parts often 1/10 W or less. Leave a margin, for example by keeping each resistor under half its rating.