liminfoVoltage Divider Calculator
Free web tool: Voltage Divider Calculator
Vout
6 V
Current
0.6 mA
Power
7.2 mW
Ratio
0.5
About Voltage Divider Calculator
The Voltage Divider Calculator provides two complementary modes for designing and analysing resistor voltage divider circuits. In forward mode, you enter the input voltage (Vin), the top resistor (R1), and the bottom resistor (R2) to calculate the output voltage (Vout = Vin × R2 / (R1 + R2)), total current through the divider, total power dissipation, and the voltage ratio. An interactive SVG circuit diagram illustrates the topology so you can verify the configuration at a glance.
In reverse mode, you specify Vin, the desired output voltage, and the total resistance (R1 + R2), and the calculator derives the ideal R1 and R2 values. It then maps these ideal values to the nearest resistors in the E24 (5% tolerance) and E96 (1% tolerance) standard series, computing the actual output voltage for each series and the error in millivolts. This allows you to choose the best available standard resistor pair without manually scanning through resistor tables. E24 series contains 24 values per decade; E96 contains 96 values, offering much finer granularity and smaller errors.
Electronics engineers, students, and hobbyists use voltage dividers in countless applications: level shifting a 5 V signal to a 3.3 V ADC input, setting the reference voltage for a comparator, biasing the base of a transistor, creating a mid-rail reference for audio circuits, and dividing high voltages for ADC measurement. The reverse design mode with E-series matching is especially valuable during PCB layout when you need to select actual purchasable components. All calculations run entirely in your browser — no data is sent to any server.
Key Features
- Forward mode: Vin + R1 + R2 → Vout, divider current (mA), power dissipation (mW), and voltage ratio
- Reverse mode: Vin + desired Vout + total R → ideal R1/R2 values with actual purchasable component suggestions
- E24 series (24 values/decade, ~5% tolerance) resistor matching with actual Vout and mV error
- E96 series (96 values/decade, ~1% tolerance) resistor matching with actual Vout and mV error
- SVG circuit diagram visualises the R1-R2 divider topology with Vout tap point
- Supports resistor values from ohms to megaohms with auto-formatted display (kΩ, MΩ)
- Validates that desired Vout is strictly less than Vin (ratio must be between 0 and 1)
- 100% client-side processing — no account, no upload, and no data leaves your browser
Frequently Asked Questions
What is the voltage divider formula?
For a two-resistor voltage divider, the output voltage is Vout = Vin × R2 / (R1 + R2). R1 is the top resistor (between Vin and Vout), and R2 is the bottom resistor (between Vout and GND). The output is always less than the input, making it suitable for stepping down a voltage.
What is the E24 resistor series?
The E24 series contains 24 standard resistor values per decade (e.g., 1.0, 1.1, 1.2 … 9.1 × 10^n), corresponding to approximately ±5% tolerance. It covers all decades from 1 Ω to 10 MΩ in this calculator. E24 is the most commonly stocked series and is widely available in through-hole and SMD packages.
What is the E96 resistor series?
The E96 series contains 96 standard values per decade, corresponding to approximately ±1% tolerance. It provides much finer granularity than E24, resulting in smaller errors when mapping an ideal resistor value to a standard part. E96 parts are readily available in 0603 and 0402 SMD packages and are commonly used in precision analogue circuits.
Why does the reverse mode ask for a total resistance value?
The voltage ratio (Vout / Vin) alone only constrains the R2 / (R1 + R2) ratio — it does not uniquely determine both R1 and R2. By specifying the total resistance R1 + R2, you fix the divider impedance, which determines quiescent current draw and affects loading by downstream circuitry. Higher total resistance draws less current but is more susceptible to loading errors.
How do I choose the total resistance for a voltage divider?
A common rule of thumb is to make the divider current at least 10× the expected load current, which minimises loading error. For microcontroller ADC inputs (typically 1–10 µA load), a total resistance of 10–100 kΩ is usually appropriate. For power supply voltage sensing, 100 kΩ to 1 MΩ reduces quiescent power loss.
What is the voltage ratio shown in forward mode?
The ratio is R2 / (R1 + R2), which equals Vout / Vin. It tells you what fraction of the input voltage appears at the output. For example, a ratio of 0.275 means Vout is 27.5% of Vin. This ratio is useful when you need to reverse-calculate resistor values or document the divider design.
Can I use this for high-voltage measurement with an ADC?
Yes. For a 12 V signal measured by a 3.3 V ADC, you need a ratio of 3.3/12 = 0.275. Use reverse mode: enter Vin = 12, Vout = 3.3, and choose a suitable total resistance (e.g., 100 kΩ). The calculator will suggest E24 and E96 resistor pairs and show the actual Vout and error in millivolts.
What is the mismatch error shown for E24 and E96?
Mismatch error is the absolute difference between the desired Vout and the actual Vout produced by the nearest E-series resistor pair, expressed in millivolts. E96 typically produces a smaller error than E24 due to its finer value spacing. For precision applications, select E96 and verify the error is within your system's acceptable voltage tolerance.