Voltage Divider Calculator

A resistive voltage divider is one of the simplest and most common circuits in electronics. Two resistors connected in series across a supply create a tap point whose voltage is a fixed fraction of the input. Microcontrollers read higher sensor voltages through dividers, level shifters reference 3.3 V logic from 5 V rails, and multimeter front ends scale unknown voltages into ADC-safe ranges. Understanding the divider formula prevents fried inputs and wasted bench time.

The output voltage formula is:

V_out = V_in × R₂ / (R₁ + R₂)

Where V_in is the supply voltage applied across the series pair, R₁ is the resistor from the supply to the tap (top resistor), and R₂ is the resistor from the tap to ground (bottom resistor). The divider ratio R₂/(R₁+R₂) tells you what fraction of the input appears at the output when no load current is drawn.

Enter your supply voltage and both resistor values in ohms. The calculator returns V_out and the ratio. For design work, pick E12 standard values near your target — a 10 kΩ / 10 kΩ pair on 5 V yields exactly 2.5 V (50% ratio). To reach 3.3 V from 5 V you need ratio ≈ 0.66, so try 4.7 kΩ top and 10 kΩ bottom: V_out = 5 × 10 / 14.7 ≈ 3.40 V.

Worked example: A 12 V automotive sensor must interface to a 3.3 V ADC. Target V_out = 3.3 V gives ratio 3.3/12 = 0.275. Choosing R₂ = 10 kΩ, solve R₁ = R₂ × (V_in/V_out − 1) = 10 × (12/3.3 − 1) ≈ 26.4 kΩ. Nearest E12 value is 27 kΩ, giving V_out = 12 × 10 / 37 ≈ 3.24 V — safely below 3.3 V. Power in each resistor is I²R where I = V_in/(R₁+R₂); here about 3.2 mA and negligible dissipation.

Real dividers sag when loaded because the load resistance forms a parallel combination with R₂. For high-impedance ADC inputs (megohms) the error is usually small. For heavier loads, buffer with an op-amp follower or scale R values down while watching power budget. Pair this tool with the series/parallel resistor calculator when combining multiple divider branches or Thevenin equivalents.