Voltage Divider Explained: The Two-Resistor Circuit Behind Every Sensor

The voltage divider is two resistors in series that produce a fraction of an input voltage — and it's everywhere: sensor circuits, reference voltages, level shifting, potentiometers. Understand this one circuit and a surprising amount of electronics stops being magic.

The formula

Vout = Vin × R2 ÷ (R1 + R2)

The output is the input scaled by the ratio of the bottom resistor to the total. Equal resistors → half the voltage. R2 = 2× R1 → two thirds.

Worked example

Vin = 10 V, R1 = 1 kΩ, R2 = 1 kΩ → Vout = 10 × 1000 ÷ 2000 = 5 V. The divider current is 10 ÷ 2000 = 5 mA — small, but real.

Designing a divider

Need 3.3 V from 5 V? Solve for R2:

R2 = R1 × Vout ÷ (Vin − Vout)

With R1 = 10 kΩ: R2 = 10000 × 3.3 ÷ 1.7 ≈ 19.4 kΩ — use a standard 18 kΩ or 20 kΩ and verify.

The catch: loading

A divider only delivers its calculated voltage when the load draws far less current than the divider itself. Attach a load that draws 1 mA to a 5 mA divider and Vout sags noticeably. Rule of thumb: divider current ≥ 10× load current — or buffer the output with an op-amp.

Common mistakes

• Choosing resistor values so low that the divider wastes power (a 1 kΩ divider across 12 V burns 144 mW continuously).
• Forgetting the load — measured Vout lower than calculated is almost always loading.
• Designing a target above Vin — a divider can only divide down.

Use the Voltage Divider Calculator in both modes: calculate Vout from real resistor values, or design the R2 you need for a target voltage.