Voltage Divider Calculator
Work out the output voltage of a two-resistor divider, find the resistor you need for a target voltage, and see how a load changes the result. Results update as you type.
What is a voltage divider?
A voltage divider is a pair of resistors connected in series across a supply. The supply voltage is split between the two resistors in proportion to their values, and the output is taken from the point where they join. It is the simplest way to turn a higher voltage into a lower one using only passive parts.
Designers use dividers to scale a battery voltage down to the range of an ADC pin, set the reference level for a comparator, bias a transistor or op-amp input, and read sensors such as thermistors and light-dependent resistors.
Voltage divider formulas
R2 = R1 × Vout / (Vin − Vout)
R1 = R2 × (Vin − Vout) / Vout
I = Vin / (R1 + R2)
P(R) = I² × R
With a load RL connected across the output, R2 and RL are in parallel, so replace R2 with R2 × RL / (R2 + RL) in the first formula. The output drops, and the lower RL is compared to R2, the bigger the drop.
How to use this calculator
Find Vout: enter Vin, R1 and R2. Add RL if something is connected to the output. Find R2 or Find R1: enter Vin, the output voltage you want, and the resistor you already have. The calculator also shows the nearest standard E24 value and the output voltage you would get with it.
Common divider ratios
| Vin | Target Vout | R1 | R2 | Typical use |
|---|---|---|---|---|
| 5 V | 3.3 V | 1.8 kΩ | 3.3 kΩ | 5 V logic to 3.3 V input (about 3.24 V) |
| 12 V | 3.3 V | 22 kΩ | 8.2 kΩ | 12 V rail sensing (about 3.26 V) |
| 12 V | 5 V | 6.8 kΩ | 4.7 kΩ | Signal scaling (about 4.9 V) |
| Any | Vin / 2 | R | R | Mid-supply bias for op-amps |
Voltage divider FAQ
How does a voltage divider work?
The same current flows through both series resistors. Because V = I × R, the larger resistor drops the larger share of the voltage. The output is the voltage across R2, which equals Vin multiplied by R2 divided by the total resistance.
Can I use a voltage divider to power a circuit?
Not reliably. A divider has high output impedance, so the voltage falls as soon as the load draws current. Use it for signals and references that draw little current. To power a device, use a linear regulator or a switching converter.
How does a load change the output voltage?
A load resistor sits in parallel with R2 and lowers the effective bottom resistance, so Vout drops. As a rule of thumb, keep RL at least 10 times larger than R2 so the error stays under about 10 percent. The calculator shows the loaded voltage when you enter RL.
What resistor values should I choose?
Choose the ratio first, then the scale. Values between 1 kΩ and 100 kΩ suit most designs. Lower values give a stiffer output but waste more current. Higher values save power on battery circuits but are more affected by load, ADC input leakage and noise.
How much current does the divider waste?
The divider draws Vin divided by (R1 + R2) all the time. A 12 V supply across 20 kΩ in total draws 0.6 mA. For battery-powered products, use larger resistors, or switch the divider on only when you take a measurement.
What power rating do the resistors need?
Calculate P = I² × R for each resistor and use a part rated for at least twice that value. The calculator shows the power in R1 and R2. Common 0603 and 0805 chip resistors are rated 0.1 W and 0.125 W, which is plenty for most signal dividers.
How do I read a battery voltage with a microcontroller ADC?
Pick a ratio that brings the maximum battery voltage below the ADC reference, for example 4.2 V down to 3.3 V or less. Use resistors with a total of 100 kΩ or less so the ADC input sees a low source impedance, and add a 100 nF capacitor from the output to ground to steady the reading.
Why do my measured results differ from the calculated ones?
Resistor tolerance is the main cause. Two 1 percent resistors can shift the ratio by up to about 1 to 2 percent. Meter loading, supply sag, and a load connected to the output also change the result. Use 0.1 percent resistors for precision references.
What is the E24 series and why does it matter?
E24 is the set of 24 standard values per decade (1.0, 1.1, 1.2, 1.3, 1.5 and so on) used for 5 percent resistors. Exact calculated values rarely exist, so pick the nearest standard value and check the resulting output. The E96 series gives finer steps for 1 percent parts.
Can a voltage divider convert logic levels?
Yes, for one-way signals from a higher voltage to a lower voltage, such as a 5 V output driving a 3.3 V input. It is slow with high resistor values because of input capacitance, so for fast or bidirectional lines such as I2C use a dedicated level shifter.
Does a voltage divider work with AC signals?
Yes, with pure resistors the ratio holds at all frequencies in theory. In practice stray capacitance and the input capacitance of the next stage form a low-pass filter with the divider, which limits bandwidth when the resistor values are large.
What is the difference between a voltage divider and a voltage regulator?
A divider only scales the voltage by a fixed ratio and changes with load and input. A regulator actively holds the output steady against changes in both. In regulator circuits, a divider is often used to set the output voltage through a feedback pin.