Voltage Divider Calculator

The voltage divider formula is: Vout = Vin × R2 ÷ (R1 + R2). A voltage divider circuit is two resistors in series across a voltage source. R1 connects to the input voltage, R2 connects to ground, and the output is taken from the junction between them. The output voltage is always a fraction of the input voltage, set by the ratio of R2 to the total resistance.

The formula comes straight from Ohm's Law. Because R1 and R2 are in series, the same current flows through both: I = Vin ÷ (R1 + R2). The output voltage is simply the voltage dropped across R2: Vout = I × R2. Substituting the first equation into the second gives Vout = Vin × R2 ÷ (R1 + R2).

Worked example: A 12V supply feeds a divider with R1 = 10 kΩ and R2 = 4.7 kΩ. The output voltage is Vout = 12 × 4700 ÷ (10000 + 4700) = 12 × 4700 ÷ 14700 = 3.84V. The current through the divider is 12 ÷ 14700 = 0.82 mA, and the resistors dissipate about 6.7 mW (R1) and 3.1 mW (R2) — well within the rating of standard 1/4-watt resistors.

Note that only the ratioof the resistors sets the output voltage: 1 kΩ/2 kΩ, 10 kΩ/20 kΩ, and 100 kΩ/200 kΩ all produce the same Vout. The absolute values determine how much current the divider draws and how sensitive it is to loading, which is covered in the sections below.

The voltage divider formula assumes nothing is connected to the output. The moment you attach a load — an ADC input, a transistor base, another circuit stage — its resistance R_L appears in parallel with R2. The effective lower resistance becomes (R2 × R_L) ÷ (R2 + R_L), which is always smaller than R2, so the real output voltage is always lower than the unloaded calculation.

For example, a 12V divider with R1 = R2 = 10 kΩ produces 6.00V unloaded. Connect a 10 kΩ load and R2 effectively becomes 5 kΩ, so the output falls to 12 × 5000 ÷ 15000 = 4.00V — a 33% error. The same load at 100 kΩ drops the output only to about 5.71V.

This is also why voltage dividers are not used for power delivery. A load that draws meaningful current has a low effective resistance, which collapses the output voltage, and the output changes every time the load current changes. On top of that, the divider itself burns power continuously in both resistors even when the load draws nothing. To supply power at a lower voltage, use a linear regulator or a switching DC-DC converter instead — dividers are for references and signals, not power.

Voltage divider circuits show up almost everywhere a signal needs to be scaled down or a reference voltage generated:

• Sensor scaling for ADC inputs.Microcontroller analog inputs typically accept 0–3.3V or 0–5V. A divider scales a higher sensor or battery voltage into that range — for example, monitoring a 12V automotive battery with a 10 kΩ/3.3 kΩ divider brings a 12V reading down to about 2.98V. Resistive sensors like thermistors and photoresistors are also read by placing them as one leg of a divider with a fixed resistor, converting their changing resistance into a changing voltage.
• Potentiometers. A potentiometer is an adjustable voltage divider: the wiper splits one resistive track into R1 and R2, so turning the shaft sweeps Vout continuously from 0V to Vin. Volume knobs, dimmer references, and position sensors all work this way.
• Logic level shifting (5V → 3.3V).To connect a 5V output to a 3.3V-only input, use R1 = 1 kΩ and R2 = 2 kΩ: Vout = 5 × 2000 ÷ 3000 = 3.33V. This works well for one-direction, moderate-speed signals like UART TX; for high-speed or bidirectional buses, use a dedicated level-shifter IC.
• Reference and bias voltages.Two equal resistors create a mid-rail reference for op-amp circuits, and unequal pairs set transistor bias points — usually followed by a buffer so the reference isn't disturbed by loading.