Voltage Divider With a Load: Ideal vs Loaded Output

This page opens the voltage divider calculator with a 5V input, equal 1kΩ resistors for R1 and R2, and a 1kΩ load resistor already filled in — both the ideal and loaded output voltages below are computed live on load.

Most online divider explanations only cover the unloaded case, which quietly assumes whatever connects to the output draws no current — true for a op-amp input, false for almost anything else. This preset uses the tool's load field specifically to show the gap: with no load, equal 1kΩ resistors would give exactly 2.5V, but a 1kΩ load in parallel with R2 pulls the bottom leg down to 500Ω, dragging the real loaded output down to 1.67V — a 33% error that a naive calculation would miss entirely.

This is the single most useful thing this calculator does beyond a plain ratio formula: enter the actual input impedance of whatever you are driving — an ADC pin, a relay coil, another divider — and see the honest output voltage instead of the textbook one. If your load resistance is unknown or very high (a typical microcontroller GPIO), leave that field empty and the loaded and ideal figures converge.