Voltage Divider Calculator
Find the output voltage of a resistive voltage divider circuit. Enter input voltage and two resistor values to calculate the output voltage. Also shows current draw and power dissipation.
A voltage divider is one of the simplest and most useful circuits in electronics. Two resistors in series, tapped at the junction between them, produce an output voltage that is a fixed fraction of the input. The math is elegantly simple: V_out = V_in × R2 / (R1 + R2). Want half the input voltage? Use two equal resistors. Want one-third? Use R1 twice as large as R2. Want a tiny fraction? Make R1 much larger than R2.
Voltage dividers solve common problems: reducing a 12 V signal to 3.3 V to feed a microcontroller pin, biasing a transistor base, creating a reference voltage for an ADC, sensing battery level. They're the basis of potentiometers (variable voltage dividers), volume controls, and dimmer switches.
The catch: dividers are *terrible* power supplies. Their output voltage depends on the load. If a circuit drawing significant current is connected, the output voltage drops. They work well only when the load resistance is much higher than R2 — typically 10× or more. For actual power delivery, use regulators or buck converters.
Output impedance equals R1 in parallel with R2. For 10k + 10k divider: R_out = 5k. Any load less than 50k Ω will load down the divider noticeably. For loads under 10k Ω, redesign with smaller resistors (or different circuit).
Common applications: microcontroller voltage adaptation (5V → 3.3V), op-amp bias networks, sensor signal conditioning, audio attenuators, voltmeter and oscilloscope probes, battery monitoring, and any analog electronics needing a fixed voltage ratio.
Inputs
Results
Output Voltage
6.000 V
Ratio
50.0%
Current
0.600 mA
Voltage Divider Results
| Parameter | Value |
|---|---|
| Input Voltage (Vin) | 12.00 V |
| Output Voltage (Vout) | 6.0000 V |
| R1 (top) | 10.00 kΩ |
| R2 (bottom) | 10.00 kΩ |
| Division Ratio | 50.00% |
| Current Through Divider | 0.6000 mA |
| Power Dissipated (R1) | 3.6000 mW |
| Power Dissipated (R2) | 3.6000 mW |
| Total Power Dissipated | 7.2000 mW |
| Formula | Vout = Vin × R2 / (R1 + R2) |
Formula
How to use this calculator
- Enter input voltage in volts.
- Enter R1 (top resistor) and R2 (bottom resistor) in ohms.
- Calculator returns output voltage and current draw.
- For target V_out: solve R2/R1 = V_out/(V_in − V_out).
- Choose resistors so total R is large enough to minimize power waste.
- Ensure load resistance is at least 10× R2 to avoid loading effects.
Worked examples
Microcontroller logic level
**Scenario:** Connect 5V sensor signal to 3.3V Arduino input pin. R1 = 10 kΩ, R2 = 20 kΩ. **Calculation:** V_out = 5 × 20/(10+20) = 5 × 0.667 = 3.33 V. Current draw: 5/30k = 167 μA. **Result:** 3.33 V to Arduino, within 3.3 V limit. Current draw is tiny (0.17 mA). Total power ~0.83 mW — negligible. Common solution for level shifting one-way signals.
Battery voltage monitoring
**Scenario:** Monitor 12V battery on 3.3V microcontroller ADC. ADC max input = 3.3V. **Calculation:** Need V_out ≤ 3.3 V when V_in = 12 V. Use R1 = 33 kΩ, R2 = 10 kΩ. V_out = 12 × 10/43 = 2.79 V at 12V battery. At full charge (14V): V_out = 14 × 10/43 = 3.26 V — just under 3.3 V max. **Result:** Divider scales 12 V battery to safe ADC range. ADC reading × 4.3 = battery voltage. Total current: 12/43k = 0.28 mA. Use lower R values if loading effects are problematic.
Audio attenuator
**Scenario:** Reduce 1 V line-level audio signal to 0.1 V (-20 dB). Need low impedance for low-noise audio. **Calculation:** Want V_out/V_in = 0.1. Use R1 = 9 kΩ, R2 = 1 kΩ. Total 10 kΩ. V_out = 1 × 1/10 = 0.1 V. **Result:** 10:1 attenuation. Output impedance: 9k || 1k = 0.9 kΩ — low enough to drive most amplifier inputs without loss. Total power dissipated: ~0.1 mW. Useful for matching levels between equipment with different output capabilities.
When to use this calculator
**Use voltage dividers for:**
- **Logic level conversion**: 5V to 3.3V (or vice versa) for signals. - **ADC input scaling**: bringing sensor output into ADC range. - **Reference voltage generation**: precise low-current voltages. - **Transistor biasing**: setting base voltage for BJT or gate of FET. - **Audio attenuation**: line-level signal reduction. - **Battery monitoring**: scaling battery voltage for ADC. - **Volume/level controls**: potentiometers (variable dividers). - **Probe attenuation**: oscilloscope 10:1 probes.
**When NOT to use:**
- **Powering loads** (lights, motors, ICs that need actual current): use voltage regulators. - **Precision sensing**: temperature drift of resistors affects output. - **Battery-powered devices** where every μW matters: dividers waste power continuously.
**Better alternatives for these cases:**
- **Voltage regulator** (LM7805, LM317, LDO): much higher efficiency, regulated output. - **Buck/boost converters**: 85-95% efficient. - **Voltage reference IC** (LM4040, REF50xx): precision, temperature-stable. - **DAC**: programmable precision voltage.
**Output impedance considerations:**
A divider has finite output impedance = R1 || R2. Load draws current that drops V_out: - For 10k + 10k divider: R_out = 5 kΩ. - Load < 50 kΩ: > 10% voltage drop.
**Choose R1 || R2 small** for low impedance, but at cost of more current waste.
**Power dissipation budget:**
Total wasted power: P = V_in² / (R1 + R2). - 5V across 1k + 1k = 25 mW. - 12V across 10k + 10k = 7.2 mW. - 12V across 100k + 100k = 720 μW.
For battery applications, use higher R values to extend battery life.
**Common applications:**
- **Arduino sensor inputs**: dividing 5V analog sensors for 3.3V boards. - **Voltmeters**: very high-impedance internal divider. - **Op-amp circuits**: setting reference for comparators. - **Power MOSFET gate drive**: dividing gate voltage. - **RC timing networks**: voltage dividers in combination with capacitors.
**Voltage divider with capacitor (AC):**
For high-frequency stability: - Add small capacitor across R2 to filter noise. - Larger cap → lower cutoff frequency. - For audio: typically 100 nF to 10 μF range.
**Compensated probe (oscilloscope):**
To extend bandwidth, scope probes use compensated dividers: - 9 MΩ + 1 MΩ resistive divider (10:1 ratio). - Compensation capacitor in parallel with 9M for high-frequency response.
**Sensor signal conditioning:**
Many sensors have outputs that don't match ADC range. Voltage divider scales: - Thermistor (0-12V): divide for 0-3.3V ADC. - Pressure sensor (4-20 mA): convert via series R, then divide. - Voltage from car battery (12-14V): scale to ADC.
**Software:**
- **LTspice / Tinkercad**: simulation. - **Falstad**: web-based circuit simulator. - **Calculator apps**: many specialized voltage divider apps.
**Pitfalls:**
- **Loading effects**: connecting a load smaller than 10× R2 distorts V_out. - **Power consumption**: continuous current draw wastes battery. - **Temperature drift**: resistor tolerances and tempco affect precision. - **Component tolerances**: 5% resistors give ±5% V_out. - **Wrong taper for potentiometers**: linear sounds wrong for audio (use log). - **Reversed polarity**: ensure correct connection if V_in is signed. - **Forgetting input current of next stage**: high-impedance MCU pin is fine; low-impedance load isn't. - **Output > V_in**: impossible (divider can only reduce, not boost).
Common mistakes to avoid
- Using a voltage divider to power loads that draw significant current.
- Choosing resistors so small that power dissipation is excessive.
- Choosing resistors so large that the output impedance is too high for the load.
- Forgetting loading effects when a load is connected.
- Using linear potentiometers for audio applications (use log taper).
- Ignoring resistor tolerance and temperature drift in precision applications.
- Reversing R1 and R2 in the formula (R2 is the bottom, connected to ground).
- Trying to boost voltage with a divider (impossible — output always less than input).