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Voltage Divider Calculator

Find the output voltage of a two-resistor voltage divider, or the R1 or R2 you need for a target voltage. You can include a load, too.

Level: Grades 11–12

Printable worksheet

Voltage Divider Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Solve for

In ohms. 10 kΩ is 10000 or 10e3.

Whatever is connected to the output. It sits side by side (in parallel) with R2. Leave blank for no load.

Result

Output voltage

3.8367 V

The divider gives 3.8367 V from 12 V.

R1 (top)
10,000 Ω
R2 (bottom)
4,700 Ω
Division ratio (Vout ÷ Vin)
0.319728
Current drawn from the source
0.8163 mA
Power in R1
6.6639 mW
Power in R2
3.132 mW

See the step-by-step solution ↓

Step-by-step solution

  1. Find the output voltage. It's the input times the bottom resistor's share of the total: Vout = Vin × R2 ÷ (R1 + R2)

    Vout=12×4,70010,000+4,700=3.8367346939 V

  2. Find the current drawn from the source: the input voltage ÷ the total resistance (Ohm's law)

    I=1210,000+4,700=0.0008163265 A

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find the voltage you'll get from two resistors, or the resistor you need to get the voltage you want. A voltage divider is two resistors in a row (in series) across a voltage source. You take the output from the point between them.

The same current flows through both resistors. So the input voltage splits between them in the same proportion as their resistances. That makes the output a fixed fraction of the input. Dividers scale a signal down for an analog input, which reads a smoothly varying voltage. They also set a reference or bias point, a steady voltage another part relies on. And they read sensors such as thermistors and light-dependent resistors.

This calculator solves the divider three ways. It finds the output voltage from both resistors, the top resistor R1 for a target output, or the bottom resistor R2. You can also enter the resistance of whatever is connected to the output. That load sits side by side (in parallel) with R2 and drags the output voltage down. The calculator also shows the current drawn and the power in each resistor, so you can check their ratings.

Where it's used

Why this math matters outside the classroom:

  • Light sensors. Hobbyists read a light sensor by pairing it with a fixed resistor so its changing resistance becomes a changing voltage.
  • Level shifting. Dividing 5 V with R1 = 1 kΩ and R2 = 2 kΩ gives 5 × 2 ÷ 3 ≈ 3.33 V for a 3.3 V input.
  • Battery monitors. Designers scale a 12 V battery down to a range a microcontroller can measure using two resistors.
  • Volume knobs. A potentiometer is an adjustable voltage divider, which is how many volume and brightness knobs work.

Grade level and standards: Grades 11–12; Common Core State Standards for Mathematics HSA.CED.A.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose what to solve for: the output voltage, R1, or R2.
  2. Enter the input voltage and the resistor you know. If you're solving for a resistor, also enter the output voltage you want.
  3. If something is connected to the output, enter its resistance as the load. Leave it blank for an unloaded divider.
  4. Read the result. Then pick the nearest standard resistor value and run the output calculation again. That shows the voltage you'll actually get.

What the inputs mean

Target output voltage (Vout)
The voltage you want at the output, measured from ground. It must be lower than the input, because a resistor divider can only reduce voltage.
R1, between Vin and the output
R1 sits between the input voltage and the output point. Enter ohms; 10 kΩ is 10000 or 10e3.
R2, between the output and ground
R2 sits between the output point and ground (0 V).
Load resistance (optional)
The resistance of the circuit the divider feeds, such as a meter, an amplifier input or a microcontroller pin. Inputs with very high resistance (megohms, or millions of ohms) barely change the result. Low-resistance loads change it a lot.

Formula and method

Vout = Vin × R2 ÷ (R1 + R2)
R1 = R2 × (Vin − Vout) ÷ Vout
R2 = R1 × Vout ÷ (Vin − Vout)
With a load:  R2 is replaced by R2‖RL = R2 × RL ÷ (R2 + RL)
Current:  I = Vin ÷ (R1 + R2‖RL)

Vin is the input voltage and Vout is the output voltage. R1 is the top resistor, R2 is the bottom one, RL is the load and I is the current. The symbol ‖ means 'in parallel with', so R2‖RL is the combined resistance of R2 and the load side by side.

With no load, the current is I = Vin ÷ (R1 + R2), and the output is that current times R2. Putting the two together gives the divider formula. The two resistor formulas are the same equation, rearranged to solve for R1 or R2.

A load across the output sits in parallel with R2. That lowers the bottom resistance, so the output voltage drops too. When you solve for R2 with a load, the calculator works in two steps. First it finds the bottom resistance the divider needs. Then it finds the R2 that, in parallel with the load, gives that value. If the load alone is already too small, no R2 can reach the target.

Worked example

A 12 V supply feeds R1 = 10 kΩ over R2 = 4.7 kΩ, with nothing connected to the output.

  1. Vout = 12 × 4700 ÷ (10,000 + 4700) = 3.8367 V.
  2. Current: I = 12 ÷ 14,700 = 0.000816 A, about 0.8163 mA.
  3. Now add a 10 kΩ load. R2‖RL = 4700 × 10,000 ÷ 14,700 ≈ 3197.28 Ω. The output falls to 12 × 3197.28 ÷ 13,197.28 ≈ 2.9072 V.

Result: The divider gives 3.8367 V with nothing connected, but only about 2.9072 V when a 10 kΩ load is connected.

Understanding the result

Here's a handy rule of thumb. Keep the load at least ten times larger than R2 if you want the output to stay within about 10% of the unloaded value. Lower divider resistances hold their voltage better under a load, but they waste more current.

A divider is not a power supply. Its output voltage changes as the load draws more or less current. So use a voltage regulator to power anything that draws a lot of current, or a changing amount.

Check the power figures against the resistors' ratings, especially with higher input voltages and low resistances.

Assumptions and limitations

  • Resistors are ideal, with exactly their marked values.
  • The input is an ideal voltage source with no internal resistance.
  • The load behaves like a fixed resistance.
  • Real resistors have tolerances, so the actual output may be off by a few percent. For precision, use parts rated 1% or better.
  • Some loads draw a current that changes or doesn't rise evenly with voltage, such as motors or LEDs. A single resistance can't represent them.
  • At high frequencies, stray capacitance at the output affects a divider. Stray capacitance is a small, unwanted ability of nearby wires to store charge. It isn't modeled here.

Frequently asked questions

How do I get 3.3 V from 5 V with a divider?

Pick R1 and solve for R2. With R1 = 1.8 kΩ, R2 = 1800 × 3.3 ÷ 1.7 ≈ 3494 Ω, so a standard 3.3 kΩ or 3.6 kΩ gets close. This works for signal levels, such as a logic input, but not for powering a 3.3 V circuit.

Why is my measured output lower than calculated?

Something connected to the output, even a multimeter, acts as a load in parallel with R2. Enter its resistance as the load to see the effect. Use larger values of R1 and R2 only if the load is far larger still.

Does the ratio depend on the actual resistor values?

With no load, only the ratio R2 ÷ (R1 + R2) matters. For example, 1 kΩ over 1 kΩ and 100 kΩ over 100 kΩ both halve the voltage. The actual values set how much current is drawn and how much a load changes the output.

How this calculator is checked

Method: Ohm's law applied to two series resistors (the voltage divider rule), with the load combined in parallel with R2. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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