Voltage Divider Calculator
Calculate a resistive divider output voltage, pick the resistor pair for a target voltage, and see the loading error a real input impedance introduces before it surprises you.
Last reviewed by the Radiatus Cloud team
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The simplest circuit with the most footguns
Two resistors in series across a supply produce an output at their junction proportional to the lower resistor. It is the first circuit anyone learns and it appears in every design, scaling a battery voltage down to an analogue input, setting a reference or biasing a transistor. The formula is trivial. The mistakes are not.
Loading is what breaks it
The clean ratio only holds when nothing draws current from the output. Connect a load and it appears in parallel with the lower resistor, dropping the output below the calculated value. A divider built from megohm resistors feeding a ten megohm ADC input reads several percent low, and the error moves with temperature and humidity. The usual rule is to make the divider current at least ten times the load current, which the calculator checks for you once you enter the load impedance.
Current draw versus accuracy
Lower resistances give a stiffer, more accurate divider but waste more current, which matters in a battery powered design where the divider runs continuously. Higher resistances save power but become sensitive to loading and to leakage. The tool shows the quiescent current and the total dissipation alongside the output, so the trade off is visible rather than guessed at, and it lists standard E24 pairs that land closest to a target ratio.
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Frequently Asked Questions
Why is my measured output lower than the calculation?
Loading. Whatever you connected to the output draws current, which effectively parallels the lower resistor. Enter the load impedance and the calculator shows the loaded output and the error.
What resistor values should I choose?
Aim for a divider current roughly ten times whatever the load draws. For a high impedance ADC input, values in the tens of kilohms are a good default: low enough to be stiff, high enough not to waste current.
Does tolerance matter?
Yes, and it compounds. Two five percent resistors can produce a ratio error of nearly ten percent in the worst case. For a measurement divider use one percent parts, or better, use the same value twice where the ratio allows since matched parts track each other.
Can a divider power something?
Almost never. It is a reference, not a supply. Any current the load draws changes the output, and the divider dissipates power continuously whether the load needs it or not. Use a regulator for a supply.
How do I scale a 24 V signal to a 3.3 V input?
You need a ratio of 3.3 to 24, so the lower resistor is about 0.1375 of the total. Enter 24 V in and 3.3 V out and the calculator suggests standard pairs, including the resulting real output for each.
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How to Use
Enter the supply and two resistors, or a target output voltage, to get the divider values and current draw.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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