How a divider works
Two resistors in series across a supply split the voltage in proportion to their values. The output, taken from the middle, is Vout = Vin × R2 ÷ (R1 + R2). Dividers are used to scale a voltage down for a microcontroller's analogue input, to set the output of an adjustable regulator, or to bias a transistor.
Watch the load
A divider isn't a power supply. Anything connected to the output is in parallel with R2 and pulls the voltage down. Keep the load resistance at least 10 times the divider's output impedance, which is R1 and R2 in parallel, or buffer the output with an op-amp. To power something at a lower voltage, use a regulator instead.
Choosing values
Lower resistances make the output stiffer but waste more current. For a battery monitor, use high values, such as 100 kΩ and above, so the divider doesn't flatten the battery. The finder tries every pair in the series you choose and lists the closest, with the error and the current drawn. E96 1% resistors give the most accurate results. For general sums, use the Ohm's law calculator.
Questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), with R1 the top resistor and R2 the bottom one.
Which resistors give 3.3 V from 5 V?
R1 = 1.8 kΩ and R2 = 3.3 kΩ give 3.24 V, or 10 kΩ and 20 kΩ give 3.33 V.
Can I power a circuit from a voltage divider?
Not reliably. The output voltage drops as the load draws current. Use a voltage regulator instead.
Why does my divider read low?
Probably the load, or your meter, is pulling it down. Use lower resistor values or a buffer.