Voltage Divider Calculator
Enter any three of input voltage, the two resistors and output voltage to get the fourth, with the current, power and output resistance.
Answer
Vout = Vin × R2 ÷ (R1 + R2)
- Given (in base units): Vin = 12 V, R1 = 10,000 Ω, R2 = 4,700 Ω
- Vout = Vin × R2 ÷ (R1 + R2) = 3.83673 V
- Current through both
- 816.327 µA
- Power in R1 / R2
- 6.66389 mW / 3.13203 mW
- Output resistance (R1 ∥ R2)
- 3.19728 kΩ
The output stays near this value only while whatever you connect draws little current: give it a load of at least ten times 3.19728 kΩ, or buffer it.
Values
Fill in any 3; leave the one to work out empty.
How is this calculated?
Vout = Vin × R2 ÷ (R1 + R2), so R1 = R2 × (Vin − Vout) ÷ Vout and R2 = R1 × Vout ÷ (Vin − Vout).Runs in your browser — nothing you enter leaves this device.
About this tool
What it does
Voltage Divider Calculator works out the two-resistor circuit that turns a higher voltage into a lower one: the input voltage goes across R1 and R2 in series, and the output is taken across R2. Enter any three of the input voltage, R1, R2 and the output voltage, in the units you like, and the fourth is worked out with the formula and the working shown. It then adds what the formula alone does not tell you: the current flowing through the divider (which drains a battery all the time), the power each resistor has to dissipate, and the divider’s output resistance — the figure that tells you how much the output will sag when you connect something to it. Typical uses are reading a battery or a 12 V supply with a 3.3 V or 5 V microcontroller, setting a reference voltage, or biasing a transistor.
How to use it
- Fill in three of the four values — for example the input voltage and the output you want, and one resistor you have.
- Leave the fourth empty; it is worked out and highlighted.
- Pick standard values close to the answer (the Resistor Colour Code page lists E-series values), then put them back in to see the real output.
- Check the current, power and output resistance under the answer before building it.
Limits and your data
- The formula is for an unloaded divider. Anything connected to the output draws current and lowers it; the effect is small when the load’s resistance is at least ten times the divider’s output resistance, which the page shows.
- A divider is not a power supply. It wastes power as heat and its output changes with the load and the input, so do not use one to power a circuit; use a regulator.
- Resistor tolerance (±1% or ±5%) moves the output by up to about the same share. For an accurate reference, use 1% resistors or a voltage-reference chip.
- An analogue input on a microcontroller also has its own input resistance and needs the source to settle; for most, keep the divider’s output resistance below about 10 kΩ.
- The output must be lower than the input: two resistors can only divide a voltage down.
- The calculation runs in your browser. Nothing you enter is sent or saved.
Questions
What resistors turn 5 V into 3.3 V?
Enter Vin 5 V, Vout 3.3 V and R2 = 20 kΩ: R1 comes out at about 10.3 kΩ, so 10 kΩ and 20 kΩ give 3.33 V. For a fast signal line a proper level shifter is better than a divider.
How do I measure a 12 V battery with an Arduino?
Scale it below the 5 V (or 3.3 V) the analogue input accepts, with a margin for a charged battery: R1 = 30 kΩ and R2 = 10 kΩ give a quarter, so 14.4 V reads as 3.6 V. The divider then draws 0.3 mA all the time; use larger values if that matters.
Why is my output lower than calculated?
Something connected to the output is drawing current. Compare the load’s resistance with the output resistance shown: if it is not much larger, lower both resistors (keeping their ratio) or add a buffer amplifier.
Which resistor is R1?
R1 goes from the input to the output; R2 goes from the output to ground (0 V). Swapping them gives Vin − Vout instead.