Voltage Divider Calculator

by

Enter the input voltage and two resistors to find the output voltage in the middle. You can also add an optional load and resistor tolerance to see how the result may change.

Formula (no load): Vout = Vin x R2 / (R1 + R2)
Use this if your circuit draws current from the output. The load acts in parallel with R2 and can pull Vout down.
Quick reminder:
R1 is the top resistor (from Vin to the midpoint). R2 is the bottom resistor (from the midpoint to ground).
Parallel Resistor Calculator RC Time Constant Calculator Ohm’s Law Calculator

How to use the Voltage Divider Calculator

Steps

  1. Enter your input voltage (Vin). Pick V or mV.
  2. Enter R1 (the top resistor from Vin to the midpoint). Pick ohm, kOhm, or MOhm.
  3. Enter R2 (the bottom resistor from the midpoint to ground). Pick ohm, kOhm, or MOhm.
  4. Optional: Open Advanced options and enter a load resistance (RL) if something is connected to Vout and draws current.
  5. Optional: Enter resistor tolerance (%) if you want a simple no-load Vout range estimate.
  6. Click Calculate. Read Vout first, then use the checks (divider current, output impedance, and power).

How this Voltage Divider Calculator works

No-load voltage divider equation

With two resistors in series, the midpoint voltage across R2 is calculated using the standard voltage divider equation: Vout = Vin x R2 / (R1 + R2). This is the basic “resistor divider” (also called a potential divider). See: Voltage divider (Wikipedia) and Voltage Dividers (SparkFun).

Loaded divider (when RL is set)

If you enter a load resistance (RL), the load is treated as being in parallel with R2. The effective bottom resistance becomes: Rbottom = (R2 x RL) / (R2 + RL). The calculator then uses Vout = Vin x Rbottom / (R1 + Rbottom). This models the common loading effect where Vout drops because the divider is supplying output current. See: Loading effect (Wikipedia).

Divider current, output impedance, and power checks

Divider current (no load) is calculated from Ohm’s law as I = Vin / (R1 + R2). The output impedance is approximated as the divider’s Thevenin resistance, Zout = R1 in parallel with R2. Power in each resistor (no load) is estimated using P = I^2R (Joule heating). These checks help you sanity-check resistor sizing and heating. See: Source impedance of a divider (Wikipedia) and Joule heating and P = I^2R (Britannica).

Resistor tolerance range (no load)

If you enter a tolerance (for example, 1% or 5%), the calculator estimates a simple worst-case Vout range for the no-load case. It assumes R1 and R2 can each be at their extremes (plus or minus the tolerance) at the same time. Vout is lowest when R1 is high and R2 is low, and highest when R1 is low and R2 is high. This worst-case method is conservative. See: Resistor tolerance definition (Vishay, PDF) and Worst-case tolerancing discussion (Texas Instruments, PDF).

Practical notes

This calculator assumes an ideal voltage source and resistor-only behaviour. Real circuits can shift Vout due to input leakage, bias currents, temperature effects (TCR), and resistor self-heating. If you are picking resistor values for a threshold input, divider current versus leakage can matter. See: Divider current vs leakage tradeoffs (Texas Instruments, PDF).

Sources

Did we solve your problem today?