Voltage Divider Calculator

Voltage Dividers reduce a voltage level using a pair of series resistors.

Formulas:

• Unloaded Output: Vout = Vin * (R2 / (R1 + R2))
• Loaded Output: Vout = Vin * (R2_eff / (R1 + R2_eff)) where R2_eff = R2 || Rload
• Thevenin Resistance: Rth = (R1 * R2) / (R1 + R2)

Usage: Input Vin, R1, and R2. Optionally enter Rload to evaluate the loading effect, which drops the output voltage.

When you need it: Scaling a voltage into an ADC range (reading a 12 V battery with a 3.3 V ADC), setting a reference or bias point, or level-shifting a sense line. The subtlety is loading: whatever you connect to the output forms a parallel path with R2 and pulls the voltage down.

Worked example: 12 V input, R1 = 10 kΩ, R2 = 3.3 kΩ → Vout = 12 × 3.3 / (10 + 3.3) ≈ 2.98 V, safely inside a 3.3 V ADC range. The output (Thevenin) resistance is 10k ∥ 3.3k ≈ 2.48 kΩ.

Tips & gotchas:

• Keep the divider's Thevenin resistance well below the load (ADC) input impedance, or the reading sags. For SAR ADCs, aim for a source impedance under ~10 kΩ (often much lower).
• Lower resistor values give a stiffer output but draw more standby current, I = Vin / (R1 + R2); higher values save power but load easily.
• Add a small capacitor across R2 to filter noise into an ADC — it forms an RC low-pass with the divider's output resistance.
• A divider is not a regulator: the output tracks the input and cannot supply real current to a load.