Voltage Divider Calculator
Calculate output voltage from a resistor voltage divider circuit using Vout = Vin × R2/(R1+R2).
Solve For
Circuit Diagram
Vout = Vin × R2 / (R1 + R2)
What Is the Voltage Divider Calculator?
A voltage divider is two resistors in series connected between a supply voltage and ground. The output is taken from the junction between R1 and R2. The formula is:
Vout = Vin × R2 / (R1 + R2)
The ratio depends only on the ratio of R1 to R2, not their absolute values, R1 = 1 kΩ, R2 = 1 kΩ and R1 = 10 kΩ, R2 = 10 kΩ both give 50%. However, larger resistors draw less current and waste less power, at the cost of higher output impedance (which sags more under load).
The loading effect, why your output drops under load
Formula
Voltage Divider Applications Reference
| Application | Vin | Typical R values | Purpose |
|---|---|---|---|
| 5 V → 3.3 V level shift | 5 V | R1=1 kΩ, R2=2 kΩ | Logic level conversion (MCU to sensor) |
| 12 V → 5 V reference | 12 V | R1=14 kΩ, R2=10 kΩ | ADC reference voltage |
| Battery voltage sensing | 4.2 V | R1=100 kΩ, R2=100 kΩ | Halve voltage for 0–2.1 V ADC input |
| Thermocouple / NTC bias | 3.3 V | R1=10 kΩ, R2=NTC | Temperature sensing with variable R2 |
| Audio volume control | Line level | 10 kΩ pot | Variable R2 as potentiometer |
| LED current limit | 5 V | R1=220 Ω, R2=LED | Current limiter for indicator LED |
| Wheatstone bridge | 5 V | R balanced | Precision sensor measurement (strain gauge) |
| Bias network BJT | Vcc | R1, R2 kΩ range | Set BJT base voltage for amplifier Q-point |
How to Use
- 1Select what you want to solve for: Vout (standard), R1, R2, or Vin (reverse calculation).
- 2Enter the known values. The unknown field hides automatically based on your selection.
- 3Optionally enter a Load Resistance (RL) to see how the output voltage changes when a circuit is connected.
- 4Click Calculate or press Enter. The primary result, full circuit parameters, and power dissipation appear.
- 5If you solved for R1 or R2, the Nearest E24 Standard Resistor panel shows the closest available resistor value and the resulting Vout deviation.
- 6The Step-by-Step Working panel shows the exact formula substitution for verification.
- 7Use Reset to clear all values and start fresh. Your last calculation is saved in localStorage automatically.
Example Calculation
Example: 5 V → 3.3 V for a logic level converter
Understanding Voltage Divider
The voltage divider is one of the most fundamental circuits in electronics, appearing in virtually every analog design. From biasing a transistor's base voltage to scaling a sensor output into an ADC's input range, understanding the divider formula and its limitations unlocks a wide class of practical circuit problems.
The loading effect is the most common source of unexpected behaviour with voltage dividers. In theory, Vout = Vin × R2/(R1+R2). In practice, any circuit connected to Vout draws current through R2, effectively placing a load resistor RL in parallel with R2. This reduces the effective R2 value and drops Vout below the design target. The calculator's loaded output panel quantifies this drop precisely, enter your load impedance to see the actual output voltage before you build the circuit.
The four solve-for modes (Vout, R1, R2, Vin) cover the complete range of design scenarios encountered in practice. The Vin solve-for mode is particularly useful for reverse-engineering an existing divider: if you measure Vout across R2 and know both resistor values, you can calculate the supply voltage that produced it.
Standard E24 resistor values are suggested whenever you solve for an unknown resistance. Real components come in a finite set of values, the E24 series has 24 values per decade spaced by approximately 10%. Using a nearby E24 value introduces a small error in Vout, which the calculator displays so you can decide whether it is acceptable for your tolerance budget.
Frequently Asked Questions
Can I use a voltage divider to power a circuit?
Voltage dividers are for signal conditioning, not power supply:
- ›Loading effect: any current drawn by the load reduces Vout below the design value.
- ›Efficiency: R1 always dissipates power, wasting energy even when load is disconnected.
- ›Regulation: Vout changes with load current, not stable for powering circuits.
- ›Correct use: ADC input scaling, logic level translation, sensor biasing (µA loads).
- ›For power: use LDO (e.g. AMS1117), buck converter, or switched-mode supply.
How do I choose R1 and R2 values for a voltage divider?
- ›Step 1: fix the ratio, R2/(R1+R2) = Vout/Vin.
- ›Step 2: choose scale based on allowed quiescent current. I = Vin/(R1+R2).
- ›Low impedance (kΩ range): better loaded regulation, higher quiescent power.
- ›High impedance (100 kΩ+): low quiescent current, sensitive to capacitive loads.
- ›Rule of thumb: RL ≥ 10 × R2 to keep loading error under ~9%.
- ›Use E24 standard values to find the nearest available resistor.
What is the loading effect and how do I minimize it?
- ›Cause: load RL in parallel with R2 reduces R2_eff = R2 × RL / (R2 + RL).
- ›10:1 rule: if RL ≥ 10 × R2, voltage drop is ≤ 9%.
- ›Voltage follower: op-amp buffer after the divider gives ~infinite input impedance.
- ›Enter RL in the calculator to see the exact loaded Vout and voltage drop percentage.
- ›MOSFET gate inputs (GΩ impedance) have negligible loading effect on dividers.
What are E24 standard resistor values and why should I use them?
- ›E24: 24 values per decade, 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0…
- ›Adjacent values differ by ~10%, matching standard 5% resistor tolerance.
- ›E96: 96 values per decade for 1% tolerance resistors (finer granularity).
- ›The calculator finds the nearest E24 value and shows the resulting Vout error.
- ›Off-the-shelf availability: E24 values are stocked by Mouser, DigiKey, and LCSC.
How do I use a voltage divider for analog-to-digital converter (ADC) input scaling?
- ›Determine the ratio: Vout_max / Vin_max = ADC_max / Vsensor_max.
- ›Example: scale 12 V sensor to 3.3 V ADC → ratio = 3.3/12 = 0.275.
- ›Pick R values: R2/(R1+R2) = 0.275 → R1 ≈ 3.64 × R2 → e.g. R1=36 kΩ, R2=10 kΩ.
- ›Add a 100 nF capacitor across R2 to filter high-frequency noise at the ADC input.
- ›Keep R2 ≤ 1/10 of ADC input impedance to avoid loading the internal sample capacitor.