Voltage Divider Calculator
Calculate the output voltage of a resistor voltage divider. Enter Vin, R1 and R2 to get Vout, divider current and power dissipation with the formula shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Voltage Divider Calculator Works
A voltage divider uses two resistors in series to produce an output voltage that is a fraction of the input. Enter the input voltage Vin, the top resistor R1 and the bottom resistor R2, and the calculator returns the output voltage Vout (measured across R2), the divider current and the power dissipated.
The Formula
The output is taken across R2 and depends on the ratio of R2 to the total resistance:
Vout = Vin × R2 ÷ (R1 + R2)
The series current is I = Vin ÷ (R1 + R2), and the total power dissipated as heat is P = Vin × I. This assumes an ideal divider with no load drawing current from the output.
Worked Example
For Vin = 12 V, R1 = 10,000 Ω and R2 = 10,000 Ω:
- Vout = 12 × 10,000 ÷ (10,000 + 10,000) = 12 × 0.5 = 6 V
- Current = 12 ÷ 20,000 = 0.6 mA
- Power = 12 × 0.0006 = 7.2 mW
With two equal resistors the output is exactly half the input, which is the most common starting point for a divider.
Common Divider Ratios
| R1 | R2 | Vout fraction | Vout from 12 V |
|---|---|---|---|
| 10k | 10k | 0.50 | 6.0 V |
| 10k | 5k | 0.333 | 4.0 V |
| 10k | 20k | 0.667 | 8.0 V |
| 1k | 3k | 0.75 | 9.0 V |
Design Tips
- The output is always lower than the input, never higher.
- Use larger resistor values to save power, or smaller values for a stiffer output that resists loading.
- If a real load is connected, treat R2 as R2 in parallel with the load resistance to estimate the true Vout.
- For powering circuits that draw current, choose a voltage regulator rather than a divider.
Frequently asked questions
What is the voltage divider formula?+
For two resistors in series, the output taken across R2 is Vout = Vin × R2 ÷ (R1 + R2). The output is a fraction of the input set by the ratio of R2 to the total resistance. The calculator applies this formula and also shows the current flowing through the divider.
How do I choose R1 and R2 for a voltage divider?+
Pick the ratio R2 ÷ (R1 + R2) to equal the fraction of Vin you want at the output. Then scale both resistors up or down together to set the current: larger resistors waste less power but are more sensitive to loading, while smaller resistors are stiffer but draw more current.
Does a load affect the voltage divider output?+
Yes. This calculator assumes no current is drawn at the output (an ideal, unloaded divider). Connecting a load in parallel with R2 lowers the effective resistance and pulls Vout down. Keep the load resistance much larger than R2 to keep the error small.
How much current flows through a voltage divider?+
The series current is I = Vin ÷ (R1 + R2). For example, 12 V across two 10k resistors gives 12 ÷ 20,000 = 0.6 mA. Lower resistor values raise this current and the power wasted as heat, so there is a trade-off between stiffness and efficiency.
Can a voltage divider supply power to a circuit?+
Not well. A resistive divider is fine for reference voltages, sensor signals and biasing, where little current is drawn. For powering a load that needs real current, use a voltage regulator instead, because a divider's output sags badly under load and wastes energy as heat.