Voltage Divider Calculator
Calculate output voltage, resistor values, and analyze load effects in voltage divider circuits.
Calculator Mode
Vout = Vin x R2 / (R1 + R2)
Vin (+)
|
[R1]
|
+--- Vout
|
[R2]
|
GND
Output Voltage
6.000 V
Ratio: 50.0%
Circuit Parameters
Divider Current0.6000 mA
R1 Power3.6000 mW
R2 Power3.6000 mW
What is a Voltage Divider?
A voltage divider is a simple circuit that creates a lower output voltage from a higher input voltage using two resistors in series. The output voltage is "tapped" from the junction between the resistors.
| Component | Symbol | Position | Role |
|---|---|---|---|
| Input Voltage | Vin | Top of circuit | Power source connection |
| Top Resistor | R1 | Between Vin and Vout | Drops portion of voltage |
| Output Voltage | Vout | Junction of R1 and R2 | Reduced voltage tap point |
| Bottom Resistor | R2 | Between Vout and GND | Determines output ratio |
| Ground | GND | Bottom of circuit | Reference (0V) |
Voltage Divider Formula
Vout = Vin × (R2 / (R1 + R2))
Where:
- Vout= Output voltage in Volts
- Vin= Input voltage in Volts
- R1= Top resistor (connected to Vin)
- R2= Bottom resistor (connected to GND)
Common Voltage Divider Ratios
Many applications require specific voltage conversions. Here are standard resistor combinations for common ratios.
| Vin → Vout | Ratio | R1 Value | R2 Value | Common Application |
|---|---|---|---|---|
| 5V → 3.3V | 0.66 | 10kΩ | 20kΩ | 5V to 3.3V logic level shifting |
| 12V → 5V | 0.417 | 14kΩ | 10kΩ | Automotive to logic conversion |
| 12V → 3.3V | 0.275 | 26.4kΩ | 10kΩ | 12V to MCU ADC input |
| 9V → 5V | 0.556 | 8kΩ | 10kΩ | 9V battery to logic |
| 24V → 5V | 0.208 | 38kΩ | 10kΩ | Industrial voltage sensing |
| Any → Half | 0.5 | 10kΩ | 10kΩ | General half-voltage reference |
Note: These values assume negligible load current. For actual implementations, verify with your specific load requirements.
Voltage Divider Design Formulas
Depending on what values you know, different formulas help you design the voltage divider.
| Find | Formula | When to Use |
|---|---|---|
| Output Voltage | Vout = Vin × R2 / (R1 + R2) | Know both resistors, find voltage |
| R2 (given R1) | R2 = (Vout × R1) / (Vin - Vout) | Fixed R1, calculate R2 |
| R1 (given R2) | R1 = R2 × (Vin - Vout) / Vout | Fixed R2, calculate R1 |
| Ratio | Ratio = R2 / (R1 + R2) | Determine divider factor |
| Current Draw | I = Vin / (R1 + R2) | Calculate power consumption |
| Power in R1 | P1 = I² × R1 | Resistor power rating |
| Power in R2 | P2 = I² × R2 | Resistor power rating |
Resistor Calculation Formulas
R2 = (Vout × R1) / (Vin - Vout)
R1 = R2 × ((Vin / Vout) - 1)
I_divider = Vin / (R1 + R2)
Where:
- R1, R2= Resistor values in Ohms
- I_divider= Current flowing through the divider
Understanding the Loading Effect
The loading effect occurs when current drawn by the load changes the effective resistance, reducing output voltage. This is the most common source of voltage divider problems.
| Scenario | Effective R2 | Effect on Vout | Solution |
|---|---|---|---|
| No load | R2 only | Calculated value | Ideal case |
| High-Z load (>10× R2) | ≈ R2 | Minimal drop (<10%) | Usually acceptable |
| Load = R2 | R2 ∥ RL = R2/2 | Significantly lower | Use buffer or lower R values |
Low-Z load (| Dominated by load | Much lower than calculated | Buffer required | |
Rule of thumb: Load impedance should be at least 10× R2 for less than 10% error. For critical applications, use 100× or add an op-amp buffer.
Loaded Voltage Divider
R2_effective = (R2 × RL) / (R2 + RL)
Vout_loaded = Vin × R2_eff / (R1 + R2_eff)
Where:
- RL= Load resistance
- R2_effective= R2 in parallel with load
Voltage Divider Applications
Voltage dividers are used throughout electronics for various practical applications.
| Application | Description | Typical R Values | Considerations |
|---|---|---|---|
| Level Shifting | Convert 5V signals to 3.3V | R1=1.8kΩ, R2=3.3kΩ | Speed limited by RC time constant |
| ADC Input Scaling | Scale high voltage for MCU ADC | 10kΩ - 100kΩ range | Match ADC input impedance |
| Voltage Sensing | Monitor battery/supply voltage | High values (100k+) | Minimize current drain |
| Biasing | Set transistor operating point | Application specific | Stability vs. efficiency trade-off |
| Reference Voltage | Create fixed reference point | Matched resistors | Temperature stability important |
| Sensor Interface | Create voltage from resistance change | Match sensor range | Thermistors, LDRs, potentiometers |
Resistor Selection Guidelines
Choosing appropriate resistor values involves balancing several factors.
| Resistor Range | Current Draw (12V) | Advantages | Disadvantages |
|---|---|---|---|
| 100Ω - 1kΩ (Low) | 12mA - 120mA | Less sensitive to loading, fast response | High power consumption, heat |
| 1kΩ - 10kΩ (Medium) | 1.2mA - 12mA | Good balance of power and loading | Moderate loading sensitivity |
| 10kΩ - 100kΩ (High) | 120µA - 1.2mA | Low power, battery-friendly | Sensitive to loading, noise pickup |
| 100kΩ - 1MΩ (Very High) | 12µA - 120µA | Minimal power consumption | Very load-sensitive, noise issues |
Best practice: Start with 10kΩ as R2, then calculate R1. This provides a good balance for most applications. Adjust based on specific power and loading requirements.
Precision and Tolerance
Resistor tolerance affects voltage divider accuracy. Here's how tolerance impacts output.
| Resistor Tolerance | Worst-Case Output Error | Cost | Application |
|---|---|---|---|
| ±20% (carbon composition) | Up to ±36% | Lowest | Non-critical applications only |
| ±10% | Up to ±18% | Low | General purpose |
| ±5% (standard) | Up to ±9% | Standard | Most applications |
| ±1% (metal film) | Up to ±2% | Moderate | Precision circuits |
| ±0.1% (precision) | Up to ±0.2% | High | Measurement, calibration |
Temperature coefficient: For precision applications, also consider TCR (temperature coefficient of resistance). Metal film resistors typically have 50-100 ppm/°C, while precision resistors achieve 10-25 ppm/°C.
Worked Examples
5V to 3.3V Level Conversion
Problem:
Design a voltage divider to convert a 5V signal to 3.3V for a 3.3V microcontroller input, using 10kΩ as R2.
Solution Steps:
- 1Identify values: Vin = 5V, Vout = 3.3V, R2 = 10kΩ
- 2Use formula: R1 = R2 × (Vin - Vout) / Vout
- 3Substitute: R1 = 10kΩ × (5V - 3.3V) / 3.3V
- 4Calculate: R1 = 10kΩ × 1.7V / 3.3V = 5.15kΩ
- 5Nearest standard value: Use 5.1kΩ (E24 series)
- 6Verify output: Vout = 5V × 10k / (5.1k + 10k) = 3.31V ✓
- 7Current draw: I = 5V / 15.1kΩ = 331µA
Result:
R1 = 5.1kΩ, R2 = 10kΩ gives Vout = 3.31V with 331µA draw
Battery Voltage Monitoring
Problem:
Design a voltage divider to monitor a 12V battery with a microcontroller ADC that has a 3.3V reference and 1MΩ input impedance.
Solution Steps:
- 1Target: Scale 12V max to 3.3V max (ratio = 0.275)
- 2Choose high-value resistors to minimize battery drain
- 3Set R2 = 10kΩ (well under 1MΩ ADC input)
- 4Calculate R1: R1 = R2 × (Vin - Vout) / Vout
- 5R1 = 10kΩ × (12V - 3.3V) / 3.3V = 26.4kΩ
- 6Use standard values: R1 = 27kΩ (gives 3.24V at 12V input)
- 7Current draw: I = 12V / 37kΩ = 324µA (acceptable for battery)
- 8With ADC load: R2_eff = 10k ∥ 1M ≈ 9.9kΩ (negligible effect)
Result:
R1 = 27kΩ, R2 = 10kΩ for 12V battery monitoring with 324µA drain
High-Precision Reference
Problem:
Create a precision 2.5V reference from a 5V supply for ADC calibration, with less than 0.5% error.
Solution Steps:
- 1Need exactly half of input: Vout/Vin = 0.5
- 2For half-voltage: R1 = R2 (equal values)
- 30.5% error requires ±0.25% total tolerance
- 4Select 0.1% tolerance resistors (each contributes 0.1% error)
- 5Use matched pair: R1 = R2 = 10kΩ ±0.1%
- 6Worst case: Vout = 5V × 10.01k / (9.99k + 10.01k) = 2.5025V (0.1% error)
- 7Also consider TCR: Use ±25ppm/°C resistors
- 8Temperature drift: At 25ppm/°C, ±25°C range = ±0.125% additional
Result:
R1 = R2 = 10kΩ ±0.1%, 25ppm/°C for 2.500V ±0.5% reference
Tips & Best Practices
- ✓Start design with R2 = 10kΩ as a default—it works well for most applications and standard value math is easy.
- ✓Always calculate current draw: I = Vin / (R1 + R2). For battery circuits, keep this under 100µA if possible.
- ✓Use 1% tolerance metal film resistors for any application requiring better than ±10% accuracy.
- ✓For ADC inputs, ensure your divider's Thévenin impedance (R1 ∥ R2) is within the ADC's recommended source impedance.
- ✓Add a small capacitor (100nF) across R2 for noise filtering, but be aware this creates a low-pass filter affecting fast signals.
- ✓When scaling high voltages (>50V), consider safety spacing on PCB and voltage ratings of components.
- ✓For level shifting digital signals (5V to 3.3V), voltage dividers work for slow signals only—use proper level shifters for fast data.
Frequently Asked Questions
This is the loading effect. Your load acts as a resistor in parallel with R2, reducing the effective bottom resistance of the divider. For example, if R2 is 10kΩ and your load is also 10kΩ, the effective R2 becomes 5kΩ (half), significantly changing the voltage ratio. Solutions: (1) Use lower-value resistors in your divider so the load is negligible in comparison, (2) Add an op-amp voltage follower buffer between the divider and load, or (3) Ensure your load impedance is at least 10× higher than R2.
Generally no. A voltage divider is NOT a regulated power supply. It cannot maintain constant voltage under varying load—any current drawn by the load changes the output voltage (loading effect). The divider also wastes power through R1 even with no load. For power supply applications, use: (1) Linear regulators (LM7805, LM317) for simple applications, (2) Buck/boost DC-DC converters for efficiency, or (3) Zener diode regulation for light loads. Voltage dividers are appropriate for signal conditioning, ADC input scaling, and reference voltages where current draw is minimal.
Consider these trade-offs: LOW values (100Ω-1kΩ): Higher current draw but less sensitive to loading and noise—good for driving loads or fast signals. MEDIUM values (1kΩ-10kΩ): Best general-purpose range, balancing power consumption and noise immunity. HIGH values (10kΩ-100kΩ): Low power consumption ideal for battery circuits and high-impedance loads like ADC inputs. VERY HIGH values (>100kΩ): Minimum power but susceptible to noise and loading effects—only for very high-impedance applications. As a rule, use the lowest resistance values that meet your power budget.
Temperature affects resistor values through their Temperature Coefficient of Resistance (TCR). If R1 and R2 have different TCRs, the divider ratio drifts with temperature. For precision applications: (1) Use resistors with matched TCR (ideally from the same batch/reel), (2) Choose low-TCR resistors (metal film: 50-100ppm/°C; precision: 10-25ppm/°C), (3) Use a resistor network IC where both resistors are on the same die and track together, (4) Consider thin-film resistor networks for critical applications. For non-critical applications, standard metal film resistors are usually adequate.
Yes, but with caveats. For pure resistive dividers, AC and DC behave identically (the ratio works the same). However, at high frequencies, parasitic capacitance between resistor leads, PCB traces, and to ground affects the frequency response. The divider acts as a low-pass filter with cutoff frequency determined by these capacitances. For high-frequency work: (1) Use low-value resistors to minimize RC effects, (2) Keep leads short, (3) Consider compensated dividers with parallel capacitors, (4) Use 10:1 oscilloscope probes as a model—they include compensation capacitors for flat frequency response.
A potentiometer (pot) is essentially a variable voltage divider. It has three terminals: two fixed ends and one movable wiper. As you turn the pot, the wiper moves along the resistive element, changing R1 and R2 while keeping their sum constant. Advantages of pots: adjustable output, single component. Disadvantages: mechanical wear, contact noise, lower precision, limited power handling. Use fixed resistor dividers when: the ratio is known and constant, precision matters, high reliability is needed, or cost optimization is important. Use pots for: user-adjustable settings, prototyping, calibration adjustments.
Sources & References
Last updated: 2026-01-22