If you have a 5V power supply and need 3.3V to power a sensor, all it takes is two resistors to create a voltage divider. Our free online voltage divider calculator helps you instantly calculate the required resistor combination, or conversely, find the output voltage from known resistor values.

Typical Use Cases

Sensor Power Supply: Development boards like Arduino typically output 5V, while sensors like the DS18B20 or HC-SR04 require 3.3V. A two-resistor voltage divider provides a stable step-down voltage.
Signal Attenuation: When measuring a 0~10V analog signal, microcontrollers can often only accept 0~5V. A voltage divider circuit scales the signal down to a safe, readable range.
Reference Voltage: In comparator or ADC circuits that require a precise reference voltage, a precision resistor divider is commonly used.

Principles and Formulas

The core of a resistor voltage divider is the voltage division rule for series resistors. Current flows through two series resistors, R1 and R2 (with R1 connected to the positive power supply), and the output voltage (V_out) is taken across R2. The formula is simple:

V_out = V_in × R2 / (R1 + R2)

Why does this work? Because the current in a series circuit is constant, the voltage drop across a resistor is proportional to its resistance. The proportion of voltage across R2 is equal to R2's share of the total resistance. This formula applies to both DC and AC, provided the load current is negligible (e.g., connected to an op-amp input); otherwise, the load resistance must be factored in.

How to Use

Open our calculator, and you will see three input fields: "Input Voltage (V_in)", "Resistor R1 (Ω)", and "Resistor R2 (Ω)", along with an "Output Voltage (V_out)" result box.
First, enter the input voltage, for example, 5V.
Next, enter the resistance values for R1 and R2 in ohms (Ω). You can include units like kΩ (e.g., "10k"), and the calculator will automatically convert them.
Click the "Calculate" button (or rely on automatic real-time calculation), and the output voltage will instantly appear in the results area on the right.
If you want to calculate resistance in reverse, the tool also supports a "Known V_in and V_out, find R1/R2" mode. Just select this mode from the dropdown menu.

Complete Calculation Example

Goal: Get a 3.3V output from a 5V power supply.

Let's assume we have 10kΩ and 6.8kΩ resistors on hand and see what they output:

V_in = 5V
R1 = 10kΩ = 10000Ω
R2 = 6.8kΩ = 6800Ω
Applying the formula: V_out = 5 × 6800 / (10000 + 6800) = 5 × 6800 / 16800 ≈ 5 × 0.4048 = 2.024V

Result: The output is only 2.02V, which is far from 3.3V. This means we need to adjust the resistor ratio.

Reverse Design: We want a 3.3V output. Let's keep R1 = 10kΩ and calculate the required R2:

Rearranging the formula: R2 = R1 × V_out / (V_in - V_out) = 10k × 3.3 / (5 - 3.3) = 10k × 3.3 / 1.7 ≈ 19.41kΩ
Selecting a standard nominal value of 20kΩ and verifying: V_out = 5 × 20 / (10 + 20) = 5 × 20 / 30 = 3.33V, which is very close to 3.3V.

Entering these two resistors into the calculator (R1 = 10kΩ, R2 = 20kΩ) will display "V_out ≈ 3.33V" on the right. Perfect.

How to Interpret the Results

V_out is half of V_in: When R1 = R2, the output is exactly half of the input. For example, a 5V input yields 2.5V.
V_out is close to V_in: When R2 is much larger than R1, the output almost equals the input. For example, if R1 = 100Ω and R2 = 100kΩ, the output is ≈ 4.995V.
V_out is close to 0: When R2 is much smaller than R1, the output is almost 0. For example, if R1 = 100kΩ and R2 = 100Ω, the output is ≈ 0.005V.
Note: "Much larger" generally means over 100 times greater. In practical applications, voltage divider circuits have high output impedance and cannot drive loads directly, otherwise the voltage will drop.

Common Mistakes & Pitfalls

Ignoring load resistance: If a low-impedance load (like an LED) is connected to the divider's output, it acts in parallel with R2, significantly dropping the output voltage. The correct approach is to connect it to an op-amp (high input impedance) or add a buffer.
Mixing up resistor units: Some people enter "10" when they mean 10kΩ, resulting in severe output deviations. Our calculator supports unit inputs (like "10k") to prevent these errors.
Using a divider to directly drive high-current devices: Voltage dividers can only provide tiny currents (in the milliamp range) and cannot power motors, relays, or heavy loads.
Ignoring resistor tolerance: Standard resistors have a ±5% tolerance, while high-precision ones are ±1%. If precise voltage division is required (e.g., for an ADC reference), precision resistors must be used.

Important Notes

This calculator is based on the ideal voltage divider formula, assuming no current is drawn from the output (no-load condition). In real circuits, if the load resistance is less than 10 times R2, the calculated results will deviate. Simulation or actual measurement is recommended.
This calculator also applies to AC signal voltage division, but frequency characteristics must be considered (standard resistors work at low frequencies, but parasitic capacitance matters at high frequencies).
The resistor value input range in the tool is 0.01Ω to 1GΩ; values outside this require scientific notation.
Calculation results are for reference only. Actual circuits need adjustments based on nominal component values, temperature coefficients, power consumption, and other factors.

Frequently Asked Questions (FAQ)

How do I choose resistors for an exact 3.3V output?
Use the formula R2 = R1 × V_out / (V_in - V_out), then select the closest standard resistor (E24 series). This calculator can automatically recommend standard values.
Does a voltage divider circuit consume a lot of power?
It depends on the total resistance. For example, 5V across 10kΩ + 10kΩ draws 0.25mA, consuming about 1.25mW, which is very low. However, using 100Ω + 100Ω draws 25mA and consumes 125mW, causing the resistors to heat up.
My ADC has an input impedance of 10kΩ. Will this affect the voltage divider?
Yes. The actual output voltage will be lower than calculated. It's recommended to make the divider resistors much smaller than the ADC input impedance (e.g., R1 + R2 < 1/10 × 10k = 1kΩ), though this increases power consumption.
How do I use the calculator to find a resistor value from a known output voltage?
In our tool, select the "Find R2" mode, enter V_in, V_out, and R1 to instantly get the R2 value. You can also select the "Find R1" mode.
Can this be used to divide voltage to get a negative voltage?
A standard voltage divider can only produce voltages between the positive supply and ground; it cannot generate negative voltages. For negative voltages, you need a charge pump or a DC-DC converter.
What should I watch out for with AC voltage division?
Resistor dividers work for AC, but you must consider the resistor's voltage rating and power. Additionally, at very high frequencies (>1MHz), the parasitic capacitance of standard resistors affects the division ratio, so non-inductive resistors are recommended.

Now you can try your own numbers in the calculator above—enter your power supply voltage and the resistor values you have on hand to see the output, or reverse-engineer the voltage divider circuit you need.