Two resistors turn one voltage into a smaller one. Set the input voltage and the two resistors R1 and R2, and the calculator gives the tapped output from Vout = Vin · R2/(R1+R2) — the output is the same fraction of the input as R2 is of the total. Add a load and watch it pull the output down as R2 and the load combine in parallel, with the loaded and unloaded values shown side by side, plus the current the divider draws and the power it burns. A live schematic tracks it all, on your device.
You are in the Physics lab.
What the tap voltage, the load droop and the fraction bar tell you.
R1 sits on top, R2 below, and the output is tapped at the junction between them. The bar on the right shows Vout as a percentage of Vin — raise R2 relative to R1 and the bar climbs toward 100%, lower it and the bar drops. It is a direct picture of the divider ratio R2/(R1+R2).
Connect a load and a third resistor appears across R2. Because it sits in parallel with R2, it lowers the effective bottom resistance and pulls the output down. The readout shows both the loaded and the unloaded value so you can see exactly how much the load costs you — a load much larger than R2 barely matters; one comparable to R2 halves the effect.
The divider draws a source current of Vin/(R1+R2) even with nothing connected, and burns it as heat — that is the total power readout. Big resistor values sip almost nothing but are easily disturbed by a load; small values hold steady but waste current. The art of a divider is picking values that are stiff enough for the load yet frugal enough to ignore.
Series resistors, a shared current, and the loading rule.
R1 and R2 in series carry one current, I = Vin/(R1+R2). The output across R2 is that current times R2, which simplifies to Vin·R2/(R1+R2). The output is therefore the same fraction of the input as R2 is of the total resistance — pure proportion, straight from Ohm's law.
Scale both resistors by the same factor and the ratio — and the output — do not change. What does change is the current: doubling both halves the current and the wasted power, but also makes the output twice as sensitive to a load. That trade-off is the whole design decision behind choosing divider values.
Any load connected across the output sits in parallel with R2, combining to R2' = R2·RL/(R2+RL), which is smaller than either — so the output falls to Vin·R2'/(R1+R2'). Keep the load resistance well above R2 (ten times or more) and the droop is negligible; let it approach R2 and the divider sags badly. Our Ohm's law calculator and circuit simulator explore the same resistors further.
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