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Free calculator

Use this calculator to

  • Solve any one of Vin, Vout, R1 and R2 from the other three
  • Design sense dividers that scale battery or rail voltages into an ADC input range
  • Set bias and reference points from a single supply rail
  • Check how far an output shifts when a resistor moves to the nearest standard value

Voltage Divider Calculator

Calculate any value in a 2-resistor voltage divider (R1, R2, Vin, Vout).

Enter any 3 values to find the 4th. Vout is the voltage across R2.

V
V
Ω
Ω
Result
—

Formulas

  • Vout = Vin × R2 / (R1 + R2)
  • R1 = R2 × (Vin / Vout − 1)
  • R2 = R1 / (Vin / Vout − 1)
  • Vin = Vout × (R1 + R2) / R2

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Voltage dividers are quietly everywhere in a modern commercial building. They scale a 0 to 24 V control signal to a 0 to 10 V BMS input, drop a battery monitor's raw rail to an ADC's safe range, set the burden across a current-transformer winding, and provide the reference rails that retrofit metering modules use to sample mains voltage. For an in-house controls or maintenance team, having the divider maths to hand turns a panel-side investigation from a guessing game into a five-minute check against expected output. The wrong divider value usually surfaces as a sensor that flatlines, drifts, or reports a value that fails sanity checks against neighbours.

For organisations rolling out sub-metering for SECR, ESOS or internal carbon reporting, divider accuracy is what underwrites the data. Sub-meters that drift 1% to 2% high or low over a year cumulatively rewrite the reporting baseline, and the corrections are visible to auditors. The cheap-and-cheerful end of the retrofit metering market is full of voltage-divider front ends with tolerance on the wrong side of the spec. Verifying the divider values during commissioning, and again during the annual maintenance walk, keeps the reported kWh figure within the tolerance that the reporting standard expects.

Where Purely Energy sees the downstream impact is in the verification of half-hourly data we feed into Purely Insights and into renewable contract reporting. If the underlying sub-meter is using a divider built from the wrong tolerance band of resistors, every monthly export of consumption data has the same systematic offset baked in. The fix is mechanical (replace the divider, recalibrate), but the audit conversation that follows can be expensive. Use this tool to sanity check divider outputs at install time; treat any unexpected reading as a flag worth investigating rather than calibrating around.

Common questions

How does a voltage divider work?

Two resistors in series across a supply split the voltage in proportion to their resistance. The output taken across R2 is Vout = Vin × R2 / (R1 + R2). With 24 V across two equal 10 kΩ resistors, the midpoint sits at 12 V. Enter any three of Vin, Vout, R1 and R2 into the calculator and it solves the fourth.

Can I use a voltage divider to power a 5 V device from 12 V?

Not for any real load. The divider's output voltage holds only while almost no current is drawn: as soon as the load takes current, it appears in parallel with R2 and the output sags. The divider also wastes power continuously through both resistors. Use a voltage regulator or DC-DC converter to supply a load, and keep dividers for references, biasing and measurement sensing.

How do I choose the actual resistor values, not just the ratio?

The ratio sets the output voltage but the absolute values set the trade-off. Low resistances of a few hundred ohms waste power and load the source, while very high resistances in the megohm range make the output sensitive to noise and to whatever reads it. A common rule of thumb is to let the divider pass roughly ten times the current the measuring input draws, which usually lands in the 10 kΩ to 100 kΩ region for ADC sensing.

What is the loading effect on a voltage divider?

Whatever you connect to the output sits in parallel with R2, lowering the effective bottom resistance and pulling Vout below the calculated figure. A 10 kΩ load across a 10 kΩ R2 halves it to 5 kΩ and visibly shifts the output. Keep the load resistance at least ten times R2, or buffer the divider with an op-amp follower when the reading must stay accurate.

How do I scale a high voltage down for an ADC or controller input?

Pick R2, then solve R1 so the maximum input voltage lands inside the ADC range with headroom. Sensing a 48 V battery bank with R1 = 100 kΩ and R2 = 10 kΩ gives 48 × 10 / 110, about 4.4 V, comfortably inside a 5 V input while allowing for charging voltages. Use ±1% resistors so the scaling factor is repeatable, and add a small filter capacitor across R2 if the source is noisy.

Related calculators

The conversions that procurement and engineering use alongside this one.

Voltage Divider Calculator | Purely Energy