Voltage Divider Calculator with Load and Reverse Solve

Real-world use cases

• Scale a sensor output into an ADC range

  A sensor swings up to 9 V but your microcontroller ADC only reads 0 to 3.3 V. Set Vin to 9 V, target Vout to 3.3 V, and pick a fixed R2; the tool solves R1 so the full-scale sensor reading lands just under the ADC ceiling. It also shows the current draw so you can keep the divider stiff against the ADC input impedance without wasting battery on a sense line that runs all the time.

• Drop a rail to a logic-level reference

  You need a 1.8 V reference tapped from a 5 V rail for a comparator threshold. Enter Vin = 5 V and target Vout = 1.8 V, fix one resistor, and read the other. Because the comparator input draws almost nothing, a high-value pair like 100 kΩ over a solved partner keeps current in the microamp range, and the power readout confirms the divider barely warms up.

• Size a battery voltage monitor

  To watch a 9 V battery on a 3.3 V ADC pin you want a divider that sips current so it does not flatten the cell over weeks. Use the forward mode with large resistors, say 100 kΩ / 10 kΩ, and the current readout shows roughly 80 µA. Tweak the pair until the output sits in range and the draw is acceptable for the battery life you need.

• Check how much a load ruins a divider

  Someone built a 12 V to 6 V divider with 1 kΩ resistors and it reads low under load. Enter the divider, then type the load resistance into the RL field. The tool shows the loaded output and the percent error, making it clear the load pulled 6 V down to 4 V, and the hint tells you to shrink R2 or add a buffer.

Common pitfalls

• Treating a divider as a regulator. The output tracks the input ratio, so if the supply moves the output moves with it. A divider holds nothing steady against supply or load changes — use an LDO or buck for an actual rail.

• Forgetting the load. The textbook formula assumes nothing draws current at the output. Hang a real load across Vout and it sits in parallel with R2, dragging the voltage down. Always check the loaded output before trusting the open-circuit number.

• Picking resistors too large for the load. High-value resistors save current but make the divider soft, so even an ADC input current shifts the reading. Keep R2 roughly ten times smaller than the load resistance for an accurate, stiff divider.

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