Voltage Divider Calculator

The Voltage Divider: Electronics' Most Common Circuit

A voltage divider is two resistors in series that produce an output voltage that is a fraction of the input voltage. It is one of the most common circuits in electronics. Every time you need to reduce a voltage level, bias a transistor, set a reference voltage, or read a sensor, you are likely using a voltage divider. The circuit is simple: R1 connects from the input voltage to the output node, and R2 connects from the output node to ground. The output voltage appears across R2 and is proportional to R2's share of the total resistance.

The voltage divider has one major limitation that catches many beginners: it is not a voltage regulator. When you connect a load to the output, the load resistance appears in parallel with R2, which changes the effective resistance and therefore the output voltage. A divider designed to output 3.3 V from 5 V may drop to 2.8 V when a load is connected. This is why voltage dividers are used for signals and references, not for powering devices. For power applications, use a voltage regulator. For understanding the underlying current and resistance relationships, see our Ohm's Law Calculator.

What This Calculator Does

This tool has three modes. The first calculates the output voltage from the input voltage and two resistor values. The second finds the required R2 value when you know the input voltage, desired output voltage, and R1. The third calculates the loaded output voltage, showing how a load resistance affects the divider output. All three modes also show current draw and power dissipation.

• Output Voltage mode: Enter Vin, R1, and R2 to get Vout, current, and power dissipation
• Find Resistor mode: Enter Vin, desired Vout, and known R1 to calculate the required R2
• Loaded Divider mode: Enter Vin, R1, R2, and load resistance to see how the load affects Vout

How the Calculation Works

• Vin (Input Voltage): The source voltage applied across the full divider. This is the voltage you want to reduce
• R1 (Top Resistor): Connected between Vin and the output node. Increasing R1 decreases Vout. This resistor drops the voltage that R2 does not
• R2 (Bottom Resistor): Connected between the output node and ground. Vout appears across R2. Increasing R2 increases Vout. When R1 = R2, Vout = Vin / 2
• Vout (Output Voltage): The reduced voltage at the junction of R1 and R2. It is always less than Vin (unless R1 is zero). The ratio Vout / Vin equals R2 / (R1 + R2)
• Load Resistance: The resistance of whatever is connected to the output. It appears in parallel with R2, reducing the effective bottom resistance and therefore reducing Vout. A load resistance at least 10x R2 keeps the drop under 10%

How to Use the Calculator

1. Select the mode: Output Voltage, Find Resistor, or Loaded Divider
2. For Output Voltage: enter Vin, R1, and R2 to get Vout
3. For Find Resistor: enter Vin, desired Vout, and known R1 to get the required R2
4. For Loaded Divider: enter Vin, R1, R2, and load resistance to see the load effect
5. Review the results, current draw, and power dissipation

Example Calculations

Example 1: 5 V to 3.3 V for a Sensor

Maria, an embedded systems developer in Austin, needs to interface a 5 V Arduino with a 3.3 V sensor. She uses a voltage divider with R1 = 1.8 kOhm and R2 = 3.3 kOhm. Vout = 5 x 3300 / (1800 + 3300) = 5 x 3300 / 5100 = 3.24 V. This is close to 3.3 V but not exact. Using the Find Resistor mode with R1 = 1800: R2 = 1800 / (5 / 3.3 - 1) = 1800 / 0.515 = 3495 ohms. The nearest standard value is 3.3 kOhm (gives 3.24 V) or 3.6 kOhm (gives 3.38 V). She uses 3.6 kOhm for a slightly higher voltage, which is within the sensor's 3.0 to 3.6 V range. Current draw = 5 / (1800 + 3600) = 0.93 mA, which is acceptable for a battery-powered device.

Example 2: Battery Voltage Monitor

Kevin, a drone builder in Denver, wants to monitor his 12 V LiPo battery voltage with an ESP32 ADC that accepts 0 to 3.3 V. He needs to divide 12 V down to under 3.3 V. Using a 3:1 ratio (R1 = 20 kOhm, R2 = 10 kOhm): Vout = 12 x 10000 / (20000 + 10000) = 4 V. That is too high. He tries R1 = 30 kOhm, R2 = 10 kOhm: Vout = 12 x 10000 / 40000 = 3 V. This is safe for the ADC. At a full charge of 12.6 V: Vout = 12.6 x 10000 / 40000 = 3.15 V. At low battery (9 V): Vout = 9 x 10000 / 40000 = 2.25 V. The ADC can distinguish these levels easily. Current draw = 12 / 40000 = 0.3 mA, which is negligible compared to the motor current. For calculating the equivalent resistance of multiple resistors, use our Parallel / Series Resistance Calculator.

Example 3: Loaded Divider Problem

Jennifer, a hardware engineer in San Jose, designs a divider to produce 2.5 V from a 5 V supply using two 10 kOhm resistors (R1 = R2 = 10 kOhm). Unloaded, Vout = 5 x 10000 / 20000 = 2.5 V. She connects a 10 kOhm load (an op-amp input with bias resistors). Using the Loaded Divider mode: R2 effective = (10000 x 10000) / (10000 + 10000) = 5000 ohms. Vout = 5 x 5000 / (10000 + 5000) = 1.67 V. The output dropped from 2.5 V to 1.67 V, a 33% drop. This is because the load (10 kOhm) is equal to R2, so it halves the effective bottom resistance. She redesigns with 1 kOhm resistors and a 100 kOhm load: R2 effective = (1000 x 100000) / 101000 = 990 ohms. Vout = 5 x 990 / 1990 = 2.49 V, only a 0.4% drop. The lesson: the load resistance should be at least 10x R2 to keep the voltage drop under 10%.

Real-World Scenarios

Thermistor Temperature Reading

David, an IoT developer in Seattle, is reading a 10 kOhm NTC thermistor with an Arduino ADC. He builds a divider with a 10 kOhm fixed resistor as R1 and the thermistor as R2. At 25 degrees C, the thermistor is 10 kOhm, so Vout = 5 x 10000 / 20000 = 2.5 V. At 0 degrees C, the thermistor rises to about 32 kOhm: Vout = 5 x 32000 / 42000 = 3.81 V. At 50 degrees C, it drops to about 3.6 kOhm: Vout = 5 x 3600 / 13600 = 1.32 V. The Arduino reads these voltages through its 10-bit ADC (0 to 1023) and converts to temperature using the Steinhart-Hart equation. The divider is perfect for this application because the ADC input impedance (about 100 MOhm) is effectively infinite compared to the 10 kOhm thermistor, so loading is negligible.

Transistor Bias Network

Sarah, an analog circuit designer in Boston, is biasing a common-emitter amplifier. She needs 2.3 V at the base of an NPN transistor. The transistor base draws about 0.1 mA of bias current. She uses R1 = 27 kOhm and R2 = 15 kOhm. Unloaded: Vout = 12 x 15000 / 42000 = 4.29 V. That is wrong because the base current loads the divider. Using the loaded mode with an effective load of 2.3 V / 0.1 mA = 23 kOhm: R2 effective = (15000 x 23000) / 38000 = 9079 ohms. Vout = 12 x 9079 / 36079 = 3.02 V. Still not 2.3 V. She reduces both resistors by 10x (R1 = 2.7 kOhm, R2 = 1.5 kOhm) to make the divider stiffer. Now the load effect is much smaller: R2 effective = (1500 x 23000) / 24500 = 1408 ohms. Vout = 12 x 1408 / 4108 = 4.11 V. She needs to recalculate for the target. The point is that divider design for transistor biasing requires iteration between the divider ratio and the load effect.

Volume Control Potentiometer

Marcus, an audio hobbyist in Chicago, is building a guitar pedal with a 100 kOhm potentiometer as a volume control. The pot is a variable voltage divider: the wiper splits the resistance into R1 (above the wiper) and R2 (below the wiper). At the midpoint, R1 = R2 = 50 kOhm, and Vout = Vin / 2 (a 6 dB reduction). At full clockwise, R1 = 0 and R2 = 100 kOhm, so Vout = Vin (full volume). At full counterclockwise, R1 = 100 kOhm and R2 = 0, so Vout = 0 (silence). The audio signal sees the pot as a 100 kOhm load, which is fine for guitar pickups (typically 10 kOhm output impedance) but may load down some passive bass pickups. A 250 kOhm or 500 kOhm pot would be better for high-impedance sources.

Common Mistakes to Avoid

• Using a divider as a power supply: A voltage divider cannot deliver significant current without the output voltage dropping. If you need to power a device at 3.3 V from 5 V, use a voltage regulator (LDO or buck converter), not a divider. A divider with 10 kOhm resistors can deliver only about 0.25 mA before the voltage drops significantly. Regulators can deliver hundreds of milliamps or amps with stable output
• Ignoring the load effect: Any load connected to the divider output appears in parallel with R2 and reduces the output voltage. The load resistance should be at least 10x R2 for less than 10% voltage drop. If the load varies, the output voltage will vary too. Use the Loaded Divider mode to check
• Using resistor values that are too high: High-value resistors (1 MOhm and above) draw less current, which is good for battery life. But they are more susceptible to noise pickup and have worse load regulation. They also have more parasitic capacitance, which limits bandwidth. Use 1 kOhm to 100 kOhm for most applications
• Using resistor values that are too low: Low-value resistors (100 ohm and below) draw excessive current and waste power. A divider with 100 ohm resistors on a 12 V supply draws 60 mA and wastes 0.72 W continuously. This drains batteries and generates heat. Use higher values unless the application demands low impedance
• Not accounting for resistor tolerance: A divider with 5% resistors can have an output voltage that is off by 5% from the calculated value. For a 3.3 V target, that means 3.14 to 3.47 V. For precision applications, use 1% or 0.1% resistors. For critical references, use a precision voltage reference IC instead of a divider

Limitations of This Calculator

This calculator handles DC voltage dividers with two resistors and an optional load. It does not account for AC signal behavior, where the frequency response depends on parasitic capacitance and the source and load impedances. It does not handle multi-tap dividers (three or more resistors), adjustable dividers with potentiometers, or dividers with reactive components (capacitors or inductors). The loaded mode assumes a purely resistive load. For AC analysis of dividers with reactive components, use impedance calculations and a circuit simulator like SPICE. For voltage regulation, use a dedicated regulator IC, not a divider.

Authoritative Research and Resources

• NIST: SI Units for Electric Current - The National Institute of Standards and Technology maintains the official definitions of the volt, ohm, and ampere. These units underpin all voltage divider calculations. NIST also provides voltage reference standards that are used to calibrate precision measurement equipment.
• All About Circuits: Voltage Divider Circuits - A comprehensive textbook chapter covering voltage dividers, including loaded dividers, multi-tap dividers, and the effect of load resistance on output voltage. Widely used in university and vocational electrical engineering courses.
• Stanford University: Voltage Divider Tutorial - Stanford's electrical engineering department provides a concise tutorial on voltage divider design, including the trade-offs between resistor value selection, current consumption, and load regulation. Useful for understanding why divider design involves more than just picking a ratio.