Voltage Divider Calculator

Enter the input voltage and two resistor values to find the output voltage, current, and power dissipation of a resistive voltage divider — with an optional load resistor for real-world accuracy.

Quick Facts

Voltage divider formula
Vout = Vin × R2 / (R1 + R2)
R1 sits between Vin and the output node; R2 sits between the output node and ground.
Divider current
I = Vin / (R1 + R2)
The same current flows through both resistors when nothing else loads the output.
Loading rule of thumb
RL ≥ 10 × R2
Keeps the load's pull-down effect on Vout under about 1%.

Your Results

Calculated
Output Voltage (Vout)
-
Vout = Vin × R2 / (R1 + R2)
Divider Current
-
I = Vin / (R1 + R2), through R1
Power in R1
-
P = I² × R1
Power in R2
-
P = Vout² / R2

Ready

Enter Vin, R1, and R2 (and an optional load resistance), then press Calculate.

Formula and Method for the Voltage Divider

A voltage divider is two resistors, R1 and R2, connected in series across a voltage source Vin. The output, Vout, is tapped from the node between them, and it is always a fraction of Vin set by the resistor ratio: Vout = Vin × R2 / (R1 + R2). This calculator also reports the current through the divider and the power each resistor dissipates, and it can factor in a load resistor connected to the output.

Deriving the voltage divider formula

With no load connected, R1 and R2 form a single series loop, so the same current I flows through both: I = Vin / (R1 + R2), by Ohm's law applied to the total resistance. The output voltage is simply that current times R2: Vout = I × R2 = Vin × R2 / (R1 + R2). Notice Vout depends only on the ratio R2 / (R1 + R2), not on the absolute resistor sizes — 1 kΩ and 1 kΩ give the same 50% division as 100 kΩ and 100 kΩ, though the current and power differ enormously between the two.

Accounting for a load resistor

A real circuit connected to Vout draws its own current, which is electrically the same as placing a load resistor RL in parallel with R2. The pair combines to an effective resistance R2' = (R2 × RL) / (R2 + RL), which is always smaller than R2 alone, so the loaded output voltage Vin × R2' / (R1 + R2') is always lower than the unloaded value. This calculator's optional "Load Resistance" field applies that correction automatically — leave it blank to see the ideal, unloaded divider. As a design rule, keeping RL at least 10 times R2 limits the loading error to roughly 1% or less.

Choosing resistor values and power ratings

  • Lower resistances (hundreds of ohms to a few kΩ) draw more current and are more resistant to loading effects, but they dissipate more power as heat and draw more current from the source.
  • Higher resistances (tens of kΩ to MΩ) save power and current but make the divider more sensitive to loading and to any input bias current drawn by whatever reads Vout.
  • Power rating: each resistor must be rated above its calculated power dissipation (P = I² × R), typically with a safety margin of 2× or more for reliability.

Frequently Asked Questions

What is the voltage divider formula?
For two resistors in series across a source with no load, the output voltage is Vout = Vin × R2 / (R1 + R2), where R1 is the top (input-side) resistor and R2 is the bottom (ground-side) resistor from which the output is taken.
How does a load resistor change the output voltage?
A real load draws current from the R2 node, which is electrically equivalent to putting the load resistor in parallel with R2. The effective resistance drops to R2' = (R2 × RL) / (R2 + RL), which lowers Vout below the unloaded value. As a rule of thumb, keep the load resistance at least 10 times R2 to hold the loading error under about 1%.
How do I choose resistor values for a voltage divider?
Pick the R1:R2 ratio to get the voltage ratio you need, then scale both resistors up or down together. Smaller resistor values draw more current and waste more power as heat but resist loading effects better; larger values save power but make the divider more sensitive to the load and to input bias currents in whatever circuit reads Vout.
How much power does a voltage divider dissipate?
Each resistor dissipates P = I² × R (equivalently V² / R across that resistor), where I is the current flowing through it. Add the power in R1 and R2 for the divider's total dissipation, and choose resistor power ratings comfortably above the calculated values, especially at low resistances or high input voltages.

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