What is the voltage divider formula?

The basic formula is Vout = Vin × R2 ÷ (R1 + R2). R1 is the top resistor from Vin to the output node, and R2 is the bottom resistor from the output node to ground.

Can a voltage divider power a load?

A voltage divider is best for signal references and high-impedance inputs. It is not suitable for powering loads unless the load current is very small compared with divider current.

Why does Vout drop when I connect a load?

The load resistance is in parallel with R2. This lowers the effective bottom resistance, so the output voltage becomes lower than the unloaded calculation.

How do I choose R1 and R2 values?

Start with the required ratio, then choose standard resistor values. For ADC inputs, keep output impedance low enough for stable readings, or add a buffer capacitor/op-amp if needed.

Should I use 1% or 5% resistors?

Use 1% resistors for accurate divider ratios, ADC measurement, and references. 5% resistors are usually acceptable for simple logic-level or rough bias circuits.

🔀 Voltage Divider Calculator

Solve for Vout, R1, or R2 instantly. Includes circuit diagram, current, power dissipation, and load effect analysis.

Circuit Diagram

Vin — Input Voltage

R1 — Top Resistor

R2 — Bottom Resistor

⚠ Please enter valid positive values for all fields.

Vin — Input Voltage

Vout — Output Voltage

R2 — Bottom Resistor

⚠ Please enter valid positive values. Vout must be less than Vin.

Vin — Input Voltage

Vout — Output Voltage

R1 — Top Resistor

⚠ Please enter valid positive values. Vout must be less than Vin.

⚖️

Load Effect Analysis

Adding a load resistance (R_L) in parallel with R2 pulls Vout below its unloaded value. See the impact for different load values.

Load Resistance (R_L)

Calculate a voltage divider above first, then enter a load resistance here.

🧠 Engineering Notes

Output impedance mattersThe divider output impedance is R1 ∥ R2. If the connected load is not much higher than this value, Vout will drop noticeably.

ADC inputs need settling timeFor microcontroller ADC pins, very high resistor values can make readings unstable. Use lower values or add a small capacitor if the ADC source impedance requirement is strict.

Check resistor powerAt higher Vin, the divider may waste power and heat the resistors. Always check P = I²R and use a safe resistor wattage.

Use matched tolerance for accuracyThe ratio matters more than the absolute value. For measurement circuits, use 1% or better resistors with similar temperature coefficient.

How the Voltage Divider Formula Works

A voltage divider is one of the most fundamental circuits in electronics — two resistors in series that produce a fraction of the input voltage at the midpoint node. The output voltage is determined by the ratio of R2 to the total resistance:

Vout = Vin × R2 / (R1 + R2)

To find R1 given Vin, Vout, and R2: R1 = R2 × (Vin / Vout − 1). To find R2: R2 = R1 × Vout / (Vin − Vout).

Current Through the Divider

The current flowing through the series resistor chain (sometimes called the "bleeder current" or quiescent current) is:

I = Vin / (R1 + R2)

This is the no-load current. It flows from Vin through R1, through R2, and into ground. Keep this in mind when choosing resistor values — very high resistances save power but make the output more susceptible to loading effects.

Power Dissipation

Both resistors dissipate heat. Power in R1 is P₁ = I² × R1 and in R2 is P₂ = I² × R2. Total power is Ptotal = Vin × I = Vin² / (R1 + R2). Make sure the resistors are rated appropriately — 1/4 W carbon film resistors are fine for most low-power dividers, but for high-voltage or high-current circuits you may need 1/2 W or 1 W parts.

Load Effect and Output Impedance

When you connect a load (such as a microcontroller pin, ADC input, or transistor base) to Vout, it places a resistance in parallel with R2. This lowers the effective R2 and pulls Vout down. The loaded output voltage is:

Vout(loaded) = Vin × R2‖RL / (R1 + R2‖RL)

The output impedance of a voltage divider is R1 ∥ R2. A good rule of thumb: choose R1 and R2 at least 10× smaller than the expected load impedance to keep the loading error below ~10%.

Practical Design Tips

For a 3.3 V output from 5 V: use R1 = 5.6 kΩ and R2 = 10 kΩ (gives ~3.26 V). These are E24 standard values.
For ADC reference dividers feeding a microcontroller, keep total divider resistance under 10 kΩ for reliable readings.
Use 1% tolerance (E96) resistors when accuracy matters — 5% resistors can cause ±10% error in the output.
Temperature coefficient of the two resistors should match for a stable ratio over temperature.
Voltage dividers are passive: they cannot source current. For low-impedance loads, use a buffer (op-amp voltage follower) after the divider.

Common Applications

Level shifting — step down 5 V logic to 3.3 V for microcontrollers and sensors
ADC input scaling — bring a higher voltage within the ADC reference range
Bias networks — set transistor or op-amp bias points
Sensor interfaces — NTC/PTC thermistors and potentiometers form one half of a divider
Volume controls — a potentiometer is a tappable voltage divider

❓ Frequently Asked Questions

The main formula is Vout = Vin × R2 / (R1 + R2). R1 is connected from Vin to the output node, and R2 is connected from the output node to ground.

Usually no. A voltage divider is best for signal scaling, biasing and ADC inputs. For powering a load, use a voltage regulator, buck converter or buffer because the divider output changes when load current changes.

The load resistance is connected in parallel with R2. This reduces the effective bottom resistance and pulls the output voltage lower than the no-load calculation.

It depends on the ADC input impedance and sampling capacitor. As a practical starting point, keep the divider output impedance low enough for stable readings, or add a buffer capacitor / op-amp follower when using large resistor values.

For rough logic-level conversion, 5% may be acceptable. For measurement dividers, ADC scaling and references, use 1% or better resistors and verify the actual output with a multimeter.