Voltage divider.
Three of Vin, Vout, R1, R2 solves the fourth. Toggle the load resistor option to factor in R_L.
Voltage dividers, and why they sag
A voltage divider is two resistors in series across a supply; the point between them sits at a fixed fraction of the input voltage, set by the ratio of the resistors.
Vout = Vin × R2 / (R1 + R2). Anything connected to the output is in parallel
with R2, so with a load the effective bottom resistor becomes
R2′ = R2 × RL / (R2 + RL).
Example. 12 V in, R1 = 4.7 kΩ, R2 = 3.3 kΩ:
12 × 3.3 / 8.0 = 4.95 V. The divider draws 12 V / 8 kΩ =
1.5 mA and burns 18 mW, so any ¼ W resistors will do.
Gotcha: the load is part of the divider. Hang a 10 kΩ load on that output and R2′ drops to 2.48 kΩ, pulling Vout from 4.95 V down to 4.15 V. A 100 kΩ load only pulls it to 4.86 V. A common rule of thumb is to keep the load at least 10× R2. Dividers are for signals (ADC inputs, reference voltages, level-shifting a logic line), not for powering a board or a motor; use a regulator for that.
Questions people actually ask
Can I use a voltage divider to power a device?
Not reliably. The output voltage depends on the load, so as the device draws more current the voltage sags, and the divider wastes power all the time. Use a linear regulator or a buck converter to power things; use dividers for signals and references.
How do I get 3.3 V from 5 V with resistors?
R1 = 1 kΩ and R2 = 2 kΩ give 5 × 2 / 3 = 3.33 V. That works for stepping a 5 V logic signal down to a 3.3 V input, which draws almost no current. It is not a way to power a 3.3 V board.
How do I choose between 1 kΩ and 100 kΩ resistors for the same ratio?
Low values hold their voltage better under load but waste more current; high values save current but sag more and pick up noise. For a microcontroller ADC input, check the datasheet's recommended source impedance; the ATmega328P, for example, is optimised for 10 kΩ or less.
Why does my multimeter read a lower Vout than the calculator?
The meter itself is a load, typically about 10 MΩ. On a divider made of two 1 MΩ resistors that is enough to pull the output from 50% of Vin to about 47.6%. Resistor tolerance adds a few percent more.
What does the load resistor option do?
It models a load RL in parallel with R2. Tick the box and enter any four of Vin, Vout, R1, R2 and RL; the calculator solves the fifth and shows the current and power in each resistor.