Free Voltage Divider Calculator – Output Voltage from Two Resistors

How a Two-Resistor Voltage Divider Works

A voltage divider is one of the most common building blocks in electronics: two resistors wired in series that split an input voltage into a smaller, predictable output. This calculator applies the voltage divider rule, Vout = Vin × R2 ÷ (R1 + R2), where the output is measured across the lower resistor, R2. Alongside the output voltage it reports the current flowing through the pair and the total power the divider dissipates, giving you the complete picture rather than just one number. The variables are few and friendly. Vin is the input voltage applied across both resistors together. R1 is the upper resistor and R2 is the lower one, both in ohms, and Vout is the voltage that appears across R2. The rule works because the same current runs through both series resistors, so each takes a share of the input in proportion to its resistance; R2's share is R2 ÷ (R1 + R2) of the total. The calculator finds the series current as Vin ÷ (R1 + R2) and the power as Vin × that current, and it guards against dividing by zero if both resistances are left at zero. Here is a worked example. Feed 12 volts into a divider with R1 = 10,000 ohms and R2 = 20,000 ohms. The output is Vout = 12 × 20,000 ÷ (10,000 + 20,000) = 12 × 20,000 ÷ 30,000 = 8 volts. The current through the chain is 12 ÷ 30,000 = 0.4 milliamps, and the power dissipated is only about 4.8 milliwatts. Swap the resistors so R1 = 20,000 and R2 = 10,000 and the output flips to 4 volts, showing how the ratio, not the absolute values, sets the output. A second pass shows the loading problem in numbers: attach a 10,000-ohm load across that 20k lower resistor and the effective R2 becomes 20,000 × 10,000 ÷ 30,000 = 6,667 ohms, dragging the output from 8 volts down to 4.8 volts. Voltage dividers show up everywhere. They set reference voltages, scale a signal down so a microcontroller's analog input can read it safely, bias transistor circuits, and form half of resistive sensor arrangements such as thermistors, photoresistors, and potentiometers. If you are reading a 12 V signal with a 3.3 V microcontroller, a divider brings it into range. Choosing R1 and R2 with the right ratio gives you exactly the output fraction you need. Three real jobs make it concrete. An ESP32 project that must read a 12-volt car signal uses R1 = 10 kΩ with R2 = 3.9 kΩ to land at about 3.37 volts, just inside the board's 3.3-volt input range with a hair of margin to trim. A battery monitor scaling a 24-volt pack down for a microcontroller picks R1 = 100 kΩ and R2 = 10 kΩ to get 2.18 volts while drawing only a fifth of a milliamp, which matters when the monitor stays connected for months. A comparator circuit needing a mid-rail reference from 5 volts uses two equal 10 kΩ resistors for exactly 2.5 volts. One important caveat: the simple formula assumes the divider is unloaded, meaning nothing significant draws current from the output. When you connect a load, its resistance appears in parallel with R2 and pulls the output voltage down. For accurate results, keep the load resistance much larger than R2, or account for it explicitly. Also balance your resistor values: very small resistances waste power as continuous current, while very large ones make the output sensitive to loading and noise. The pitfall almost everyone meets once is trying to power something from a divider — an LED or a small module — and then wondering why the measured voltage collapses. A divider sets a voltage level; it does not supply current. Use a regulator for that. Resistor tolerance matters too: two 5 percent parts can shift your output by a few percent, so use 1 percent resistors where the reference has to be accurate. Any divider tapped off mains-derived voltage must follow local electrical code and be built by a qualified electrician. All calculations run locally in your browser, so your inputs stay private.

Voltage Divider Calculator Formula & Method

🔥 Trending Tools