Voltage Divider Calculator

What a voltage divider actually does

A voltage divider is simply two resistors in series between the voltage to be divided and ground. The output is taken at the common point between the two. In the formula Vout = Vin × R2 / (R1 + R2): Vin is the voltage applied at the top of the divider, R1 is the resistor on the Vin side (upper resistor), R2 is the one connected to ground (lower resistor), and Vout is the voltage taken across R2, relative to ground.

What matters is not the absolute value of the resistors but their ratio. 10 kΩ + 10 kΩ and 100 kΩ + 100 kΩ give the same half of Vin. The values do, however, set the current flowing in the loop, I = Vin / (R1 + R2), and therefore the circuit's power draw and its sensitivity to loading. To work out the power dissipated by each resistor, the Ohm's law calculator does the rest of the job.

Worked example: reading a 12 V battery with a 5 V input

Suppose you want to monitor a 12 V lead-acid battery with an analog input that only handles 0 to 5 V. Two common standard values are chosen: R1 = 10 kΩ (brown-black-orange) and R2 = 4.7 kΩ (yellow-violet-red).

• Total resistance: 10,000 + 4,700 = 14,700 Ω.
• Output at 12 V: Vout = 12 × 4,700 / 14,700 = 3.84 V.
• Battery on charge at 15 V: Vout = 15 × 4,700 / 14,700 = 4.80 V, still under 5 V.
• Current in the divider: I = 12 / 14,700 = 0.82 mA.
• Power in R1: P = I² × R1 = (0.000816)² × 10,000 ≈ 6.7 mW, and 3.1 mW in R2.

Two 1/4 W resistors are more than enough, with a huge safety margin. In code, you recover the real voltage by multiplying the reading by the factor 14,700 / 4,700 = 3.13. For a quick test setup, a breadboard and two wires are all you need; if you would rather adjust the ratio by hand, a potentiometer wired as a divider does the same job in an adjustable way.

Common pitfalls

A divider does not supply power. It produces a voltage, not current. As soon as a load is connected to it, that load appears in parallel with R2 and the output collapses. Rule of thumb: the load resistance should be at least 10 times R2. To power a 3.3 V module, you need a regulator, not a divider.

Units. Entering 4.7 instead of 4.7 kΩ throws everything off by a factor of 1,000. Check the Ω / kΩ / MΩ selector before reading the result.

Tolerance. Two ±5% resistors can shift the ratio by several percent; for a serious measurement, use ±1% parts or calibrate once with a multimeter. And do not expect just any value: resistors come in E12/E24 series (1; 1.2; 1.5; 1.8; 2.2; 2.7; 3.3; 3.9; 4.7; 5.6; 6.8; 8.2). If the calculation gives 3.45 kΩ, use 3.3 kΩ or combine two values with the series/parallel calculator.

Values that are too large. With R1 and R2 at 1 MΩ, the divider draws almost no current but becomes sensitive to noise and to the input impedance of the measuring circuit. For a microcontroller analog-to-digital converter, it is best to stay under 10 kΩ as seen from the output.