Voltage Divider Calculator

Calculate output voltage, R1, or R2 using Vout = Vin × R₂/(R₁+R₂).

🔌 Circuits📐 Vout = Vin·R₂/(R₁+R₂)⚡ Voltage Division
Input voltage (Vin) V
R₁ (upper resistor) Ω
R₂ (lower resistor) Ω
⚠️ Check values — Vout must be less than Vin.

What Is Voltage Divider?

A voltage divider uses two series resistors to produce an output voltage that is a fraction of the input voltage: Vout = Vin × R₂/(R₁+R₂). The output is taken across the lower resistor R₂. The ratio R₂/(R₁+R₂) determines what fraction of Vin appears at Vout — ranging from 0 (R₂ → 0) to 1 (R₁ → 0).

The voltage divider is the foundation of analog electronics. It is used to set bias points for transistors, scale sensor outputs to ADC input ranges, create reference voltages, and interface different voltage logic levels. A 5 V sensor output can be divided to 3.3 V (for a 3.3 V MCU) using R₁ = 1 kΩ, R₂ = 2 kΩ: Vout = 5 × 2/(1+2) = 3.33 V.

Important: the voltage divider formula assumes negligible load current. If the load (connected to Vout) draws significant current, it effectively adds a parallel resistance to R₂, reducing Vout. As a rule of thumb, the load resistance should be at least 10× R₂ to keep output error below ~10%.

Voltage dividers also appear as potentiometers (variable resistors with a sliding contact), Wheatstone bridges (four resistors measuring small resistance changes), attenuator pads (for audio signal levels), and thermistor/photoresistor sensing circuits where one 'resistor' changes with temperature or light.

Formula Reference Table

Solve ForFormulaNotes
Output voltageVout = Vin × R₂/(R₁+R₂)R₂ = lower (grounded) resistor
Upper resistorR₁ = R₂(Vin − Vout)/VoutGiven Vin, Vout, R₂
Lower resistorR₂ = R₁·Vout/(Vin−Vout)Given Vin, Vout, R₁
RatioVout/Vin = R₂/(R₁+R₂)The divider ratio
Current through dividerI = Vin/(R₁+R₂)Must be << load current for accuracy
Thévenin output impedanceR_th = R₁‖R₂ = R₁R₂/(R₁+R₂)Source impedance seen by load

3 Worked Examples

Example 1
Level Shifting — 5 V to 3.3 V

Shift a 5 V digital output to 3.3 V for a microcontroller ADC.

  • Target: Vout = 3.3 V from Vin = 5 V
  • Ratio = 3.3/5 = 0.66 = R₂/(R₁+R₂)
  • Choose R₂ = 2,200 Ω; R₁ = R₂(Vin−Vout)/Vout = 2,200×1.7/3.3 = 1,133 Ω → use 1,200 Ω
  • Vout check = 5 × 2,200/(1,200+2,200) = 5 × 0.647 = 3.24 V ≈ 3.3 V ✓
✓ R₁ = 1.2 kΩ, R₂ = 2.2 kΩ → Vout ≈ 3.24 V
Example 2
Bias a Transistor

Bias NPN transistor base to 1.5 V from 12 V supply.

  • Ratio = 1.5/12 = 0.125 = R₂/(R₁+R₂)
  • Choose R₂ = 1 kΩ; R₁ = R₂(12−1.5)/1.5 = 1,000 × 10.5/1.5 = 7,000 Ω → use 6.8 kΩ
  • Actual Vout = 12 × 1000/7800 = 1.54 V (close enough)
✓ R₁ = 6.8 kΩ, R₂ = 1 kΩ → Vout ≈ 1.54 V
Example 3
Thermistor Sensor

NTC thermistor (10 kΩ at 25°C) in divider with 10 kΩ fixed R from 3.3 V.

  • At 25°C: Vout = 3.3 × 10,000/(10,000+10,000) = 1.65 V
  • At 50°C (thermistor ≈ 4.5 kΩ): Vout = 3.3 × 10,000/14,500 = 2.28 V
  • ADC reads different voltage → MCU converts to temperature via lookup table
✓ Temperature sensing: Vout varies from 0 to 3.3 V with temperature

Real-World Applications

🌡️
Sensor Interfaces
Thermistors, LDRs, and potentiometers form one 'resistor' of a voltage divider. The output voltage changes with the sensor's resistance, creating an analog signal that microcontrollers can read via ADC inputs.
📱
Logic Level Shifting
When interfacing 5 V devices to 3.3 V microcontrollers, a voltage divider safely scales digital signals. R₁=2 kΩ, R₂=3.3 kΩ gives 3.3 V output from 5 V input.
🔊
Audio Attenuation
Passive audio attenuators use voltage divider networks to reduce signal levels by precise dB amounts. L-pads and H-pads are voltage divider networks designed to maintain specific impedance while attenuating.
🔭
Battery Monitoring
Microcontrollers measure battery voltage through a resistive divider. A 12 V battery divided by 4:1 (R₁=30 kΩ, R₂=10 kΩ) gives 0–3 V output, within ADC range.
📡
RF Attenuators
Pi and T attenuator pads use voltage dividers to reduce RF power levels by controlled dB amounts while maintaining 50 Ω impedance — essential for test and measurement equipment.

Common Mistakes to Avoid

⚠️
Forgetting load effect

Vout = Vin×R₂/(R₁+R₂) only holds when load resistance >> R₂. If load draws significant current, Vout drops. For precision: R_load ≥ 10×R₂, or use a buffer amplifier after the divider.

⚠️
Using equal resistors for unequal voltages

Equal R₁=R₂ always gives Vout = Vin/2. To get other ratios, resistors must be proportional to their desired voltage drops.

⚠️
Vout cannot exceed Vin

The voltage divider can only step DOWN voltage, never up. If Vout > Vin in your calculation, the formula is being misapplied.

⚠️
Not using standard resistor values

Calculated R₁ = 7,342 Ω doesn't exist. Choose nearest standard value (E24 series: ...6.8k, 7.5k, 8.2k...) and recalculate actual Vout. Always verify with the modified values.

⚠️
Ignoring the power dissipation

P = Vin²/(R₁+R₂). With low-value resistors on high voltage, power dissipation can be significant. Use higher resistance values to minimize quiescent current and heat.

Frequently Asked Questions

What is the voltage divider rule? ▾
Vout/Vin = R₂/(R₁+R₂). The output voltage across any element in a series circuit is that element's fraction of the total resistance times the total voltage. This is KVL + Ohm's Law combined.
How does loading affect a voltage divider? ▾
When load R_L connects across R₂, the effective lower resistance becomes R₂‖R_L = R₂·R_L/(R₂+R_L) < R₂. The lower effective resistance gives lower Vout. To minimize loading: keep R_L ≥ 10R₂. Alternatively, buffer the output with an op-amp voltage follower.
What is a potentiometer? ▾
A potentiometer is a variable voltage divider: a resistive element with a sliding contact. Moving the wiper from one end to the other varies Vout continuously from 0 to Vin. Used for volume controls, position sensors, and adjustable reference voltages.
How do I design a divider for sensor reading? ▾
1. Determine Vin (sensor or supply). 2. Determine Vout needed (ADC full-scale). 3. Choose divider ratio = Vout/Vin. 4. Select R₂, then R₁ = R₂(Vin/Vout−1). 5. Keep R_total high (10–100 kΩ) to minimize current. 6. Verify R_load ≥ 10R₂.
What is a Wheatstone bridge? ▾
Four resistors in a bridge configuration: R₁/R₂ = R₃/R₄ at balance (output = 0 V). When one resistor changes (strain gauge, thermistor), the bridge unbalances, producing a small differential output voltage. This amplifies small resistance changes into measurable voltages — the principle behind load cells and strain gauges.
Why do voltage dividers waste power? ▾
Current I = Vin/(R₁+R₂) flows continuously even with no load. This quiescent current generates P = I²(R₁+R₂) = Vin²/(R₁+R₂) of heat. With 12 V and 1 kΩ total: P = 144 mW wasted. Higher-value resistors reduce waste but increase sensitivity to load current.
Can I use a voltage divider for power supply regulation? ▾
No — voltage dividers are not regulated. Output voltage changes with load current, temperature, and supply variation. For regulated voltages, use linear regulators (LDO) or switching regulators (buck/boost). Voltage dividers are only suitable for high-impedance inputs like ADC pins.
What is the Thévenin equivalent of a voltage divider? ▾
V_th = Vin × R₂/(R₁+R₂) and R_th = R₁‖R₂. From the load's perspective, the entire divider looks like a voltage source V_th in series with resistance R_th. This allows quick analysis of how a load affects Vout without redesigning the full circuit.

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