Voltage Divider Calculator
Free Voltage divider Calculator for electrical projects. Enter dimensions to get material lists and cost estimates. Includes formulas and worked examples.
Formula
Vout = Vin ร R2 / (R1 + R2) | I = Vin / (R1 + R2)
The voltage divider formula calculates the output voltage as a fraction of the input voltage based on the ratio of R2 to the total series resistance (R1 + R2). The current through the divider is determined by the total resistance. Power dissipation in each resistor equals IยฒR.
Worked Examples
Example 1: 5V to 3.3V Level Shifting
Problem: Design a voltage divider to convert a 5V signal to 3.3V for a microcontroller input.
Solution: Required ratio: 3.3/5 = 0.66\nChoose R2 = 20kฮฉ\nR1 = R2 ร (Vin/Vout - 1) = 20k ร (5/3.3 - 1) = 20k ร 0.515 = 10.3kฮฉ\nNearest standard: R1 = 10kฮฉ\nActual Vout = 5 ร 20k/(10k + 20k) = 3.33V\nCurrent: I = 5V / 30kฮฉ = 0.167mA
Result: R1 = 10kฮฉ | R2 = 20kฮฉ | Vout = 3.33V | I = 0.167mA
Example 2: Battery Voltage Monitoring
Problem: Monitor a 12V battery with a microcontroller ADC that has a 3.3V maximum input.
Solution: Required ratio: 3.3/12 = 0.275\nChoose R2 = 10kฮฉ\nR1 = R2 ร (Vin/Vout - 1) = 10k ร (12/3.3 - 1) = 10k ร 2.636 = 26.36kฮฉ\nNearest standard: R1 = 27kฮฉ\nActual Vout = 12 ร 10k/(27k + 10k) = 3.24V\nCurrent: I = 12V / 37kฮฉ = 0.324mA
Result: R1 = 27kฮฉ | R2 = 10kฮฉ | Vout = 3.24V | I = 0.324mA
Frequently Asked Questions
What is a voltage divider?
A voltage divider is a simple circuit that uses two resistors connected in series to produce an output voltage that is a fraction of the input voltage. The output voltage is taken across the second resistor (R2). Voltage dividers are one of the most fundamental and commonly used circuits in electronics, found in sensor interfaces, reference voltage generation, level shifting, volume controls, and biasing transistor circuits. The key formula is Vout = Vin x R2 / (R1 + R2).
How do I calculate voltage divider output?
The voltage divider formula is Vout = Vin x (R2 / (R1 + R2)). R1 is the resistor connected between the input voltage and the output node, and R2 is the resistor connected between the output node and ground. For example, with Vin = 12V, R1 = 10kohm, and R2 = 10kohm, Vout = 12 x (10000 / 20000) = 6V. The ratio R2/(R1+R2) is called the divider ratio and always falls between 0 and 1, meaning the output voltage is always less than or equal to the input voltage.
Why does loading affect voltage divider output?
When you connect a load (such as a circuit or component) to the output of a voltage divider, the load resistance appears in parallel with R2, effectively reducing the R2 value and lowering the output voltage. For the divider to work accurately, the load resistance should be at least 10 times greater than R2 (the 10x rule). For critical applications, use a buffer amplifier (op-amp voltage follower) between the divider output and the load to prevent loading effects.
Can a voltage divider be used as a power supply?
Voltage dividers should generally NOT be used as power supplies. They are designed for signal-level applications, not for delivering significant power. Problems include: poor voltage regulation (output voltage changes with load), wasted power (current flows through R1 and R2 even with no load), and inability to supply meaningful current without large voltage drops. For power supply applications, use voltage regulators (like LM7805), DC-DC converters, or switching regulators instead.
How do I choose resistor values for a voltage divider?
When choosing resistor values: (1) Calculate the required ratio: ratio = Vout/Vin = R2/(R1+R2). (2) Choose R2 first, typically between 1kohm and 100kohm for general use. (3) Calculate R1 = R2 x (Vin/Vout - 1). (4) Select the nearest standard resistor values. (5) Verify the current draw is acceptable: I = Vin/(R1+R2). Lower resistor values give better load regulation but waste more power. Higher values save power but are more sensitive to loading. A common guideline is to keep divider current at least 10x the expected load current.
How accurate are the results from Voltage Divider Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.