About the Voltage Divider Calculator
A voltage divider is two resistors in series that turn a higher voltage into a lower one. This calculator works in three ways: give it Vin, R1 and R2 to get the output voltage, or give it Vin, the voltage you want and one resistor to find the other. You can add a load resistance to see how whatever is connected to the output changes the result. It also shows the current through the divider, the power each resistor must handle and the closest real values from the E12, E24 and E96 series together with the voltage those parts would actually give.
Updated September 2026
How to use the Voltage Divider Calculator
- 1Choose what to find: the output voltage, R2 or R1.
- 2Type the input voltage. For a level shifter, this is the higher signal, for example 5 V.
- 3Type the resistors you know. Short forms like 4.7k, 4k7 and 2M2 work.
- 4If solving for a resistor, type the output voltage you want.
- 5Add the load resistance if the output feeds something that draws current.
- 6Read the result, then pick a real resistor from the E-series list.
The divider formula in plain words
With nothing connected to the output, the voltage at the middle is Vout = Vin × R2 ÷ (R1 + R2). R1 is the resistor between the supply and the output, R2 is the one between the output and ground. If both are equal, you get half the input. If R2 is twice R1, you get two thirds.
To find a resistor, rearrange the same formula: R2 = R1 × Vout ÷ (Vin − Vout), and R1 = R2 × (Vin − Vout) ÷ Vout. Only the ratio matters for Vout, so you can scale both resistors up or down together. That choice sets the current and, with it, the power wasted and how much a load disturbs the output.
Why the load matters
Anything connected to the output is in parallel with R2, which makes the lower leg smaller and pulls Vout down. A 10k and 10k divider on 5 V gives 2.5 V unloaded, but with a 10k load it drops to 1.67 V. The fix is to make the load much larger than R2, at least ten times as a rough rule, or to choose smaller resistors.
Smaller resistors mean more current. The calculator shows the divider current and the power in each resistor so you can balance accuracy against battery life. For a battery-powered device, dividers in the hundreds of kilohms are common. For a fast signal, low values keep the output from drooping.
Choosing real resistor values
- E12 has 12 values per decade and suits 10% and 5% parts, E24 has 24 and suits 5% parts, E96 has 96 and suits 1% parts.
- When you solve for a resistor, the list shows the closest standard value in each series and the voltage it gives, so you see the error before you buy.
- You can get closer by combining two resistors, or by changing the known resistor to a value that pairs well with a standard one.
- Tolerance adds error on top. Two 1% resistors can move the ratio by about 1%, so check tolerances when precision matters.
Where a divider is not the answer
A divider is fine for reading a voltage into an analogue input, setting a reference or lowering a logic signal a little. It is a poor way to power a device, because the output voltage sags as the load draws more. Use a voltage regulator or a converter for power. For a high voltage, also check that each resistor is rated for the voltage across it. Check each resistor with the Ohm's Law Calculator, read its stripes with the Resistor Color Code Calculator, size the power with the Watts, Amps and Volts Calculator, and see losses over a long cable with the Voltage Drop Calculator.
Features
- Solve Vout, R1 or R2
- Optional load in parallel with R2
- Current, power in each resistor and voltage drop
- Nearest E12, E24 and E96 value with the real Vout it gives
- Accepts k, M and 4k7 notation
- Clear messages when a target voltage is impossible
Frequently asked questions
How do I calculate a voltage divider?
Multiply the input voltage by R2 and divide by the sum of R1 and R2. With 12 V across a 10k and a 10k resistor the output is 6 V. Choose “Output voltage” and type the three values to see it.
How do I get 3.3 V from 5 V with resistors?
Pick “Find R2”, enter 5 V in, 3.3 V out and a convenient R1 such as 10k. R2 comes out at 19.4 kΩ, and the E96 value 19.6 kΩ gives 3.31 V. Check the tool for the E24 option too.
Why is my divider output lower than calculated?
The circuit you connect to the output is in parallel with R2 and pulls the voltage down. Enter its resistance as the load to see the real output, or use smaller divider resistors.
Can I use a voltage divider to power a device?
Not reliably. The output falls as the device draws current and the resistors waste power. A regulator is the right choice for power, a divider for signals and measurements.
What resistor values should I choose for R1 and R2?
Pick values large enough to keep the current and power low, yet small compared with the load. Between 1k and 100k works for many circuits. A load ten times larger than R2 disturbs the output by under 10%.
Why does it say the voltage is impossible?
A divider can only lower voltage, so the output must be above 0 and below the input. With a load connected there is another limit, because the load alone may already pull the output below your target.
What do E12, E24 and E96 mean?
They are preferred-value series for resistors, with 12, 24 and 96 values per decade. Higher series cost a little more and give finer steps with tighter tolerances.