Voltage Divider Calculator

This calculator gives the unloaded output voltage of a two-resistor divider, the current flowing through the chain, the power dissipated in each resistor and the divider's output (Thevenin) impedance. Enter an optional load resistance and it also shows the loaded output voltage and how far it drops.

Dividers are used to scale a battery or sensor voltage down into a microcontroller's ADC range, to set feedback voltages on regulators, and to bias transistor inputs. They are a poor choice for powering anything that draws real current, because the output sags as soon as current is taken from it. For that, use a regulator.

The maths is Ohm's law applied to resistors in series. To pick standard part values, check them with the resistor color code calculator or combine two parts using the series/parallel calculator. Real resistors carry tolerances of 1% or 5%, so the actual output can differ by roughly that much.

This is the standard series-resistor result found in any circuit-analysis textbook, derived from Ohm's law and Kirchhoff's voltage law. It assumes ideal DC conditions. With AC signals, capacitance at the output (including an ADC's sampling capacitor) forms a low-pass filter with Rout, and for capacitive or inductive dividers you need impedances rather than resistances.

The ratio R2 ÷ (R1 + R2) sets the output voltage; the absolute size of the resistors sets everything else.

• Lower values (100 Ω – 10 kΩ): stiffer output that barely moves under load, but more current drawn and more heat. Avoid on battery devices that must sleep for months.
• Medium values (10 kΩ – 100 kΩ): the usual choice for feeding an ADC or logic input. Many microcontroller datasheets recommend a source impedance of about 10 kΩ or less for full ADC accuracy.
• High values (above 1 MΩ): almost no current, but leakage, noise pickup and input bias currents start to shift the reading. Add a small capacitor (for example 100 nF) from output to ground to steady an ADC reading if the signal changes slowly.

As a rule of thumb, keep the load at least 10 times R2 for less than about 10% error, and at least 100 times for around 1%.

These pairs use widely stocked resistor values. Results are unloaded and ignore tolerance.

• Powering a load from it: an LED, relay or motor needs current; the output will collapse. Use a regulator, or for an LED a series resistor sized with the LED resistor calculator.
• Swapping R1 and R2: R2 is always the resistor to ground. Swapping them gives Vin × R1 ÷ (R1 + R2) instead.
• Sizing for the nominal voltage: size the divider for the highest voltage the source can reach (a "12 V" lead-acid battery can exceed 14 V on charge), not its nominal voltage.
• Forgetting resistor power ratings: at high input voltages, check the mW figures against the resistor's rating (commonly 125 mW or 250 mW for small through-hole and SMD parts) and the resistor's maximum working voltage.