What this calculator does
A voltage divider is two resistors in series with the output taken from the junction between them. It is the simplest way to scale a voltage down to something a sensor input or a microcontroller pin can read, and the output depends only on the ratio of the two resistors, not their absolute size.
That ratio is the useful part: a 10k and 10k pair gives exactly half the input, and so does a 100k and 100k pair. What changes with the absolute values is how much current the divider wastes, which is the trade-off this calculator makes visible.
The formula
Multiply the input voltage by R₂ divided by the sum of both resistors. The current through the divider is the input voltage divided by that same sum, and the power dissipated follows from voltage times current.
| Term | Meaning |
|---|---|
| Divider ratio | R₂ divided by (R₁ + R₂): the fraction of the input voltage that appears at the output. |
| Quiescent current | The current flowing continuously through both resistors, which is wasted as heat even with nothing connected to the output. |
The inputs explained
| Field | What to enter |
|---|---|
| Input voltage (V) | The voltage applied across both resistors together. |
| R₁ (top) (Ω) | The upper resistor, between the input voltage and the output point. |
| R₂ (bottom) (Ω) | The lower resistor, between the output point and ground. The output voltage appears across this one. |
When to use it
Scaling a voltage for a sensor input
Reading a 12 volt supply on a 3.3 volt input needs the voltage divided down, and the ratio of the two resistors sets how far.
Choosing between high and low resistances
The same ratio at higher resistances wastes less current but is more easily disturbed by whatever is connected to the output.
Setting a reference voltage
A divider is a quick way to produce a fixed fraction of a supply rail for comparison against another signal.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the output change as the lower resistor varies?
A fixed 10 kilohm upper resistor with a range of lower resistors.
Questions
Why does only the ratio matter?
The output is R₂ divided by the total, so scaling both resistors by the same factor leaves the fraction unchanged. A 1k/1k divider and a 1M/1M divider both halve the voltage; they differ only in current drawn.
Should I use large or small resistors?
Larger values waste less current, which matters on battery power. Smaller values hold the output steadier when something draws current from it. The usual compromise is the largest values that still keep the output stable under load.
Why does my real output not match the calculation?
Almost always because something is drawing current from the output, which effectively adds a third resistor in parallel with R₂ and pulls the voltage down. A divider is only accurate when the load draws very little compared with the divider current.
Can a divider power something?
Not reliably. It can supply a reference voltage to a high-impedance input, but any real current draw changes the output. Anything that needs to deliver current wants a regulator rather than a divider.
For the current and power behind these figures, see the Ohm's law calculator. For combining the resistors themselves, see the resistors in series and parallel calculator.