Engineering tool

Voltage Divider Calculator

Q: Why does loading affect a voltage divider's output?

A real load connected at Vout effectively parallels R2, lowering the actual output voltage below the ideal formula's prediction.

Enter any 3 of Vin, R1, R2, Vout — the calculator solves for the 4th using the voltage divider equation.

Vin

Vout

Enter exactly 3 of the 4 values to solve for the 4th.

Formula & edge cases

Vout = Vin × R2 / (R1 + R2)

Rearranged for each variable:
Vin = Vout × (R1+R2) / R2
R2 = Vout × R1 / (Vin−Vout)
R1 = R2 × (Vin−Vout) / Vout

1Worked example: Vin = 12V, R1 = 1kΩ, R2 = 2kΩ → Vout = 12 × 2000/(1000+2000) = 8V.

Edge cases: solving for R2 requires Vin ≠ Vout (otherwise R1 would need to be 0); solving for R1 requires Vout ≠ 0 (otherwise R2 would need to be 0). If the inputs would imply a negative resistance, the calculator reports this explicitly rather than showing an impossible answer. Reference: Kirchhoff's voltage law.

Common questions

What is a voltage divider used for?

Voltage dividers scale a higher voltage down to a lower one — commonly used to bring a sensor's output into a microcontroller's safe input range, or to create a reference voltage from a supply rail.

Why does loading affect a voltage divider's output?

The Vout = Vin × R2/(R1+R2) formula assumes nothing draws current from the Vout tap. If a real load is connected there, it effectively parallels R2, lowering the actual output voltage below what this calculator predicts — this is called "loading" the divider.

How is this different from Ohm's Law?

Ohm's Law (V=IR) plus the power equation (P=VI) form two independent equations across four variables, so any 2 knowns solve the other 2. A voltage divider is a single equation across four variables, so exactly 3 must be known to solve the 4th.