Voltage Divider Calculator
Compute a resistor voltage divider's output, current and power from Vin, R1 and R2 — including the loading effect of a connected load resistor, with a schematic.
6 V
50.00% of V_in · current 600 µA · dissipates 7.2 mW
The divider formula
Two resistors in series across a source split the voltage in proportion to their resistances. The output is taken from the junction:
V_out = V_in · R2 / (R1 + R2)
R1 is the top (source-side) resistor, R2 the bottom (ground-side) one. Only the ratio sets the voltage — but the absolute values set the current I = V_in / (R1 + R2) and therefore the power wasted as heat.
Loading — the catch everyone hits
The clean formula assumes nothing draws current from the output. The moment you connect a load R_L, it sits in parallel with R2, lowering the effective bottom resistance and pulling V_out down. The rule of thumb: keep the divider resistances at least 10× smaller than the load (a “stiff” divider) so loading is negligible — at the cost of more wasted current.
Good and bad uses
Good for
Reference voltages for an ADC or comparator, biasing a transistor base, scaling a high voltage down to measure it, and setting a sensor's pull-up ratio.Bad for
Powering anything that draws real current — a divider is not a regulator. Its output sags with load and changes with input voltage; use an LDO or buck converter instead.Picking the resistor scale
- Too small (e.g. 100 Ω): stiff and immune to loading, but burns significant current and power continuously.
- Too large (e.g. 1 MΩ): almost no wasted current, but very sensitive to loading, leakage and noise pickup.
- Sweet spot: a few kΩ to tens of kΩ for most signal-level dividers — low enough to ignore stray effects, high enough to waste little power.