Voltage Divider Calculator

Verify or size a two-resistor voltage divider with optional load resistance. Compare unloaded and loaded output, wasted current, and resistor dissipation before you wire it.

How It Works

Formula

V_{out}=V_{in}\cdot\frac{R_2}{R_1+R_2}

R_{eff}=R_2\parallel R_{load}=\frac{R_2\cdot R_{load}}{R_2+R_{load}}

V_{out,loaded}=V_{in}\cdot\frac{R_{eff}}{R_1+R_{eff}}

I=\frac{V_{in}}{R_1+R_{eff}}

R_1=R_{eff}\cdot\left(\frac{V_{in}}{V_{out}}-1\right)

Variables, symbols and units

$V_{in}$: Source voltage(V)
$V_{out}$: Output voltage(V)
$R_1$: Upper resistor(Ω)
$R_2$: Lower resistor(Ω)
$R_{load}$: Load resistance(Ω)
$R_{eff}$: Loaded lower-leg resistance(Ω)
$I$: Divider current(A)

Calculation method explained

Pick the job first. Verify mode checks a known R1/R2 pair and optionally shows how a finite load drags the output down. Sizing mode solves the missing resistor from the same divider relationship so you can aim at a target Vout without rearranging the algebra by hand.

For an unloaded divider, the output is $V_{out}=V_{in}\cdot\frac{R_2}{R_1+R_2}$ . When a real load is attached, the lower leg is no longer just $R_2$ ; it becomes the parallel combination $R_{eff}=R_2\parallel R_{load}$ , and the loaded output follows the same ratio with $R_{eff}$ in place of $R_2$ . The current row uses the loaded total resistance so the current and dissipation numbers stay tied to the same circuit state. In sizing mode, the calculator rearranges the same relationship to solve the missing resistor instead of asking you to do bench algebra yourself.

Frequently Asked Questions

Why does Rload change the divider output?

Because the input you are feeding is no longer an open circuit. It sits in parallel with R2, so the lower leg becomes smaller than R2 alone and the real output falls below the unloaded ratio.

Which current does this calculator show when a load is present?

It reports the loaded source current through R1 and the effective lower leg. That keeps the current and power rows consistent with the loaded divider you would actually have on the board.

When is a divider a poor choice for an ADC input?

Usually when the input resistance is low enough to pull Vout down noticeably, or when the divider must be made so low in resistance that it wastes more current than the design can spare. The loaded-vs-unloaded comparison exposes both problems quickly.

Does this prove a pin is safe?

No. This is user-entered circuit math only. Always compare the result against the input datasheet, including absolute maximum ratings, recommended operating range, leakage, and any sampling requirements.

What does this tool not model?

It does not model resistor tolerance, capacitor effects, transient sampling currents, temperature drift, PCB leakage, or protection diodes. Use it as a fast divider check, not a full circuit simulation.

Related Calculators

Ohm's Law Calculator

Solve for V, I, or R with Ohm's Law

Scientific Calculator

Evaluate math expressions with advanced functions

Percentage Calculator

Three-mode percentage calculator

Energy Converter

Convert one energy value between common energy units.

Voltage Divider Calculator

How It Works

Frequently Asked Questions

Related Calculators

All calculators

Ready to Calculate?