Voltage Divider Calculator
Calculate output voltage from two resistors, or the R2 needed to hit a target voltage.
Output voltage
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Divider current
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For reference and prototyping. Verify any safety- or design-critical value against the component datasheet or a second method before relying on it. Full disclaimer.
About this tool
Two modes. Output voltage: enter Vin, R1, and R2 to get the voltage at the divider's midpoint. Required R2: enter Vin, R1, and a target output, and it solves for the R2 you need. It also shows the standing current the divider draws.
The formula. A voltage divider is two resistors in series across a supply, with the output taken from the point between them:
Vout = Vin × R2 ÷ (R1 + R2)
It's Ohm's law applied twice: the same current flows through both resistors, so the voltage splits in proportion to the resistances. Equal resistors halve the voltage. R2 ten times R1 gives about 91% of Vin.
Which resistor is which. R1 sits between Vin and the output tap; R2 sits between the tap and ground. The output is the voltage across R2. Swapping them inverts the ratio.
Divider current, and why it matters. Current through the chain is I = Vin ÷ (R1 + R2), flowing continuously whether or not anything uses the output. Low resistances waste power (a 12 V supply across two 100 Ω resistors burns 720 mW doing nothing). High resistances save power but make the output fragile — see loading below. For a reference voltage, a few tens of µA to a few mA of divider current is a typical compromise.
The big limitation: loading. This formula assumes nothing draws current from the output tap. Connect a real load, and it sits in parallel with R2, lowering the effective bottom resistance and pulling Vout down. The rule of thumb: make the divider current at least 10× the load current, so R1‖R2 are "stiff" relative to what they feed. A divider is fine for a high-impedance input (an op-amp, a comparator, most ADC inputs with a buffer), but not for powering anything.
Reading a sensor. A very common use is a resistive sensor (thermistor, LDR, potentiometer) as one half of the divider, turning its resistance change into a voltage a microcontroller ADC can read. Pick the fixed resistor near the sensor's mid-range resistance for the best sensitivity.
Use real values. The exact R2 the calculator gives usually isn't a stock value — pick the nearest E12/E24 resistor, or combine two, and re-check the output. Resistor tolerance (±1% to ±5%) also shifts the result.
The maths is shared with the Ohm's law calculator. For an LED's series resistor (a divider with the LED as the load), use the LED resistor calculator; to check a resistor's power, the resistor power calculator; to read resistor markings, the resistor color code.
Frequently asked questions
- Which resistor is R1 and which is R2?
- R1 is between Vin and the output tap; R2 is between the tap and ground. The output is measured across R2.
- Why does the divider current matter?
- It's the power the divider draws continuously (I = Vin ÷ (R1+R2)). Low resistances waste power; very high resistances make the output sag under any load.
- Does this account for a load on the output?
- No — it's the unloaded equation. A load in parallel with R2 pulls Vout below this value. Keep the divider current at least 10× the load current.
- Can I power a circuit from a voltage divider?
- Only a very-low-current, high-impedance one. For anything drawing real current, use a regulator — a divider's output collapses under load.
- The R2 value it gave me isn't a real resistor — what do I do?
- Pick the nearest standard (E12/E24) value or series two resistors, then recalculate the actual output. Account for tolerance too.
- How do I use this with a thermistor or LDR?
- Put the sensor as one half of the divider and a fixed resistor near its mid-range value as the other. The tap voltage then tracks the sensor's resistance for an ADC to read.