Voltage Divider Calculator: Output Voltage, Load Effect & Resistor Power

Result

Formulas

\(R_{\text{effective}} = R_{\text{bottom}}\quad \text{when } R_{\text{load}} = 0\)
\(R_{\text{effective}} = \frac{R_{\text{bottom}} \times R_{\text{load}}}{R_{\text{bottom}} + R_{\text{load}}}\)
\(V_{\text{out}} = V_s \times \frac{R_{\text{effective}}}{R_{\text{top}} + R_{\text{effective}}}\)
\(R_{\text{total}} = R_{\text{top}} + R_{\text{effective}}\)
\(I_{\text{divider}} = \frac{V_s}{R_{\text{total}}}\)
\(P_{\text{top}} = I_{\text{divider}}^2 \times R_{\text{top}}\)
\(P_{\text{bottom path}} = \frac{V_{\text{out}}^2}{R_{\text{effective}}}\)

A voltage divider reduces a source voltage to a lower reference voltage using two resistors in series. The calculator determines the Output voltage at the junction of the resistors, then reports the divider current and power dissipated by each resistor path.

This calculation is commonly used for:

Analog-signal scaling into an ADC, comparator, control input, or measurement circuit
Battery and DC bus voltage sensing
Creating a low-current reference or bias voltage
Logic-level interfacing where a higher-voltage signal must be reduced
Preliminary resistor wattage selection for low-power control circuits

A voltage divider is not a regulated power supply. Its output changes when the connected circuit draws current unless the load resistance is large relative to the bottom resistance.

Divider Resistance and Load Effect

The circuit uses the following field inputs:

Input	Electrical meaning
Source voltage (V)	The voltage applied across the complete divider network
Top resistance (ohm)	The resistor connected between source positive and the output node
Bottom resistance (ohm)	The resistor connected between the output node and the circuit reference, usually negative or ground
Load resistance (ohm)	Optional resistance connected from the output node to reference, in parallel with the bottom resistor; enter 0 for an unloaded divider

With no load connected, current flows through the Top resistance and Bottom resistance only. The output node voltage is set by their resistance ratio.

When a load is connected across the output, it becomes a parallel path with the Bottom resistance. The calculator reports that combined value as Effective bottom resistance. Because the effective resistance below the output node decreases, the output voltage also decreases.

The calculator reports these electrical results:

Result	Electrical meaning
Output voltage	Voltage between the output node and reference
Effective bottom resistance	Bottom resistor in parallel with the entered Load resistance
Total divider resistance	Top resistance plus effective bottom resistance
Divider current	Current through the Top resistance and into the parallel bottom path
Top resistor power	Power dissipated in the Top resistance
Bottom path power	Total power dissipated by the effective bottom path

Voltage Divider Formula

For an unloaded divider, where Load resistance is 0:

\(\displaystyle V_{out}=V_s \times \frac{R_{bottom}}{R_{top}+R_{bottom}}\)

Where:

V_{out} = Output voltage
V_s = Source voltage
R_{top} = Top resistance
R_{bottom} = Bottom resistance

When a load is present, the calculator first finds the parallel equivalent of the Bottom resistance and Load resistance:

\(\displaystyle R_{effective}=\frac{R_{bottom}\times R_{load}}{R_{bottom}+R_{load}}\)

It then substitutes R_{effective} for the bottom resistor in the divider equation:

\(\displaystyle V_{out}=V_s \times \frac{R_{effective}}{R_{top}+R_{effective}}\)

Total divider resistance and divider current are:

\(\displaystyle R_{total}=R_{top}+R_{effective}\)

\(\displaystyle I_{divider}=\frac{V_s}{R_{total}}\)

Resistor power is determined from current and resistance:

\(\displaystyle P_{top}=I_{divider}^{2}\times R_{top}\)

The bottom path power is based on the output voltage across the effective bottom resistance:

\(\displaystyle P_{bottom\ path}=\frac{V_{out}^{2}}{R_{effective}}\)

Calculation Example

Enter the following values:

Field	Entered value
Source voltage	12 V
Top resistance	10,000 ohm
Bottom resistance	5,000 ohm
Load resistance	0 ohm

Because the Load resistance is 0, the divider is unloaded. The Effective bottom resistance remains 5,000 ohm.

\(\displaystyle R_{total}=10{,}000+5{,}000=15{,}000\ \text{ohm}\)

\(\displaystyle I_{divider}=\frac{12}{15{,}000}=0.0008\ \text{A}\)

\(\displaystyle V_{out}=12\times\frac{5{,}000}{15{,}000}=4\ \text{V}\)

The calculated results are:

Result	Value
Output voltage	4 V
Effective bottom resistance	5,000 ohm
Total divider resistance	15,000 ohm
Divider current	0.0008 A
Top resistor power	0.0064 W
Bottom path power	0.0032 W

The 10,000-ohm top resistor drops 8 V at 0.0008 A, dissipating 0.0064 W. The 5,000-ohm bottom resistor has 4 V across it and dissipates 0.0032 W.

For component selection, the calculated wattage is the electrical dissipation under the entered conditions. Resistor selection must also account for available part ratings, expected ambient temperature, enclosure heat, pulse conditions, surge exposure, and applicable derating.

Loaded Divider Behavior

A voltage divider is often accurate enough when it feeds a high-impedance input, such as a properly selected sensing input or measurement circuit. It can become unsuitable when the connected load has resistance comparable to the Bottom resistance.

For example, using the same 12 V source, 10,000-ohm Top resistance, and 5,000-ohm Bottom resistance, a 5,000-ohm Load resistance produces:

\(\displaystyle R_{effective}=5{,}000\parallel5{,}000=2{,}500\ \text{ohm}\)

\(\displaystyle V_{out}=12\times\frac{2{,}500}{10{,}000+2{,}500}=2.4\ \text{V}\)

The unloaded output was 4 V, but the loaded output falls to 2.4 V. The load has changed the resistance ratio rather than merely consuming a small amount of extra current.

For voltage-sensing applications, choose resistor values with the connected input impedance, leakage current, and expected measurement range in mind. If the downstream circuit needs a stable voltage while supplying meaningful current, use a buffer, regulator, reference source, or another active circuit instead of relying on a passive divider.

Field and Design Limits

The worksheet uses ideal two-resistor divider arithmetic. It treats an entered Load resistance as a fixed resistance in parallel with the Bottom resistance.

The calculation does not evaluate:

Resistor tolerance, temperature coefficient, or long-term drift
Source impedance, source-voltage variation, ripple, or load regulation
Nonresistive loads, including capacitive input behavior, switching current, inrush, or nonlinear electronic inputs
Heat rise, resistor voltage rating, pulse energy rating, spacing, insulation coordination, or enclosure conditions
Branch-circuit ampacity, conductor AWG or kcmil selection, terminal ratings, voltage-drop compliance, overcurrent protection, grounding, or raceway installation requirements
Equipment listing requirements, manufacturer instructions, or AHJ acceptance

Use the calculated output voltage and resistor dissipation as circuit-design values, then verify the actual connected load and component ratings under the expected operating conditions.

What does load resistance do to a voltage divider?

The load acts in parallel with the bottom resistor and usually lowers the output voltage compared with the unloaded divider.

Can this select resistor wattage?

No. It estimates power from ideal arithmetic. Choose real components using ratings, tolerance, temperature, manufacturer data, and a design review.