Voltage Divider Calculator

What the Voltage Divider Calculator Does

A voltage divider is one of the simplest and most common circuits in electronics: two resistors in series that split an input voltage into a smaller output voltage. This calculator finds the output voltage (Vout) when you enter the input voltage (Vin) and the two resistor values (R1 and R2).

It is useful for hobbyists, students, and engineers who need to scale a voltage down to a safe level, set a reference voltage, bias a transistor, or read a higher voltage with a microcontroller's analog input. If you know three of the four values, you can rearrange the formula to solve for the missing one.

How It Works: The Voltage Divider Formula

The circuit places R1 between Vin and the output node, and R2 between the output node and ground. The output is measured across R2. Because the same current flows through both resistors, the output voltage is the input voltage scaled by the ratio of R2 to the total resistance:

Vout = Vin x R2 / (R1 + R2)

A few consequences fall straight out of this equation. If R1 equals R2, the output is exactly half of Vin. The larger R2 is relative to R1, the closer Vout gets to Vin; the smaller R2 is, the closer Vout gets to zero. The absolute resistor sizes do not change the ratio, only the current draw.

Worked Example with Real Numbers

Suppose you want to read a 12 V battery with a microcontroller pin that can only handle 5 V. You choose R1 = 10,000 ohms and R2 = 5,000 ohms, with Vin = 12 V.

Plug the values into the formula: Vout = 12 x 5,000 / (10,000 + 5,000) = 12 x 5,000 / 15,000 = 12 x 0.3333 = 4.0 V.

The 12 V source is safely reduced to 4 V, which sits comfortably under the 5 V limit. You can also check the current: total resistance is 15,000 ohms, so I = 12 / 15,000 = 0.0008 A (0.8 mA), and the power dissipated is small (about 9.6 mW total).

Practical Tips and Common Mistakes

The formula above assumes no load is connected to the output, or that the load draws negligible current. In practice, the biggest source of error is loading.

• Watch the load: connecting a device across R2 puts a third resistance in parallel with R2, lowering the effective R2 and pulling Vout below your calculated value. Keep the load resistance at least 10 times larger than R2 to limit this error.
• Mind the units: enter both resistors in the same units (both ohms, or both kilohms). Mixing them is the most frequent calculation error.
• Do not power high-current loads: a divider is a poor voltage regulator because output voltage sags under load and it wastes power as heat. Use it for sensing and references, not for supplying current.
• Use standard values: real resistors come in fixed series like E12 or E24, so pick the nearest standard value and recalculate. A 5,000 ohm part is uncommon; 4,700 ohms is standard and gives Vout = 3.82 V here.

Factors That Affect the Result

Resistor tolerance matters: a pair of 5% resistors can shift the output by several percent in either direction, which is fine for sensing but not for precision references. Use 1% (or better) parts where accuracy counts.

Temperature and resistor self-heating can change values slightly, and at higher frequencies stray capacitance can alter the division ratio. For steady DC signals these effects are usually negligible, but they explain why a measured Vout may differ a little from the calculator's ideal value.