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Voltage Divider Calculator

Calculate the exact output voltage of a two-resistor voltage divider circuit to safely step down logic level signals — with current and power dissipation in Ω, kΩ, or MΩ.

Voltage Divider Calculator

Calculate the exact output voltage of a two-resistor voltage divider circuit to safely step down logic level signals.

Vin (12V)
│
┤ R1 (10kΩ)
│
├──── Vout (3.837V)
│
┤ R2 (4.7kΩ)
│
GND (0V)
01 — Circuit Parameters

= 10,000 Ω

= 4,700 Ω

Output Voltage — Vout
3.837 V
= 31.97% of Vin
Formula: Vin × R2 / (R1 + R2) = 12 × 4,700 / 14,700
02 — Full Circuit Analysis
Vout
3.837 V
Divider Ratio
31.97%
Current (I)
0.8163 mA
Power (P)
9.796 mW
Vin12 V
R110 kΩ (10,000 Ω)
R24.7 kΩ (4,700 Ω)
R1 + R2 (total)14,700 Ω
Vout = Vin × R2/(R1+R2)12 × 4,700 / 14,700 = 3.837 V
Current through divider816.327 µA (0.8163 mA)
Power dissipated9.796 mW (9.796 mW)
Summary: With a 12V input and a 10kΩ / 4.7kΩ resistor pair, your stepped-down output voltage is 3.837V (32.0% of Vin).
Practical Example

A Raspberry Pi GPIO pin operates at 3.3V logic, but a sensor outputs 5V. Directly connecting will damage the Pi. A voltage divider with R1 = 10kΩ and R2 = 6.8kΩ: Vout = 5 × 6800 / (10000 + 6800) = 2.02V — safely below the 3.3V logic high threshold. Alternatively, R1 = 10kΩ and R2 = 4.7kΩ gives: Vout = 5 × 4700 / 14700 = 1.60V. Rule of thumb: use the highest practical resistor values to minimize current draw (power waste), but keep values low enough that leakage currents don't affect the output (typically 1kΩ–100kΩ for logic level shifting).

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What is Voltage Divider Circuits: Resistor Ratio, Current, and Safe Level Shifting?

Calculate the exact output voltage of a two-resistor voltage divider and understand the trade-offs between resistor values, quiescent current draw, and load regulation.

Mathematical Foundation

Output Voltage
Vout=Vin×R2R1+R2V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}
R2R_2= The bottom resistor (tied to GND). Vout is taken at the junction of R1 and R2. The ratio R2/(R1+R2) always lies between 0 and 1, so Vout is always ≤ Vin.
R1R_1= The top resistor (tied to Vin). Higher R1 means less quiescent current but worse load regulation.
Quiescent Current & Power
I=VinR1+R2P=Vin×II = \frac{V_{in}}{R_1 + R_2} \qquad P = V_{in} \times I
II= Current continuously flowing from Vin to GND regardless of load. Minimized by using large resistor values (kΩ–MΩ range).
PP= Continuous power dissipated as heat. For battery-powered designs, this is wasted energy.

Laws & Principles

  • The voltage divider only maintains Vout = Vin×R2/(R1+R2) when the load impedance is much larger than R2 (typically ≥10×R2). If a load of similar impedance is attached, it forms a parallel combination with R2, pulling Vout down — this is called 'loading effect.' Add a voltage follower op-amp buffer to eliminate this problem.
  • Logic level conversion (5V → 3.3V): the most common use case. R1 = 10kΩ, R2 = 6.8kΩ gives Vout = 5 × 6800/16800 = 2.02V. R1 = 10kΩ, R2 = 22kΩ gives Vout = 5 × 22/32 = 3.44V. Different target voltages need different ratios.
  • E24 resistor series: real resistors come in standard E24 values (10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91). Always round to the nearest E24 value and verify Vout is within spec.
  • Do NOT use a voltage divider as a power supply or to run motors, LEDs, or MCU VDD lines with significant current draw. Use a voltage regulator (LM7805, AMS1117, etc.) instead. The divider loses regulation immediately under load.
  • Thevenin equivalent: any voltage divider can be replaced by a Thevenin source of Vth = Vout and Rth = R1||R2 (R1 in parallel with R2). This is critical for circuit analysis when a load is connected.

Step-by-Step Example Walkthrough

" Raspberry Pi GPIO (3.3V) receiving signal from 5V Arduino sensor output. "

  1. Need Vout at or below 3.3V from Vin = 5V.
  2. Choose R1 = 10kΩ, R2 = 6.8kΩ (both E24 values).
  3. Vout = 5 × 6800 / (10000 + 6800) = 5 × 0.404 = 2.02V. ✓ Below 3.3V.
  4. Current draw: 5 / 16800 = 0.298 mA (negligible).
  5. Power: 5 × 0.000298 = 1.49 mW (very low — safe for 1/4W resistors).
  6. Load: Raspberry Pi GPIO input impedance is ~50kΩ, which is >>R2 (6.8kΩ). Loading effect is minimal.
Final Result: The 10kΩ/6.8kΩ divider safely converts 5V → 2.02V for GPIO input, consuming only 1.5mW continuously. Pin reads as GPIO HIGH (>= 1.8V threshold) reliably.

Quick Answer: How does a voltage divider work?

A voltage divider uses two resistors in series to produce an output voltage that is a fraction of the input. The output voltage equals Vin multiplied by R2 divided by the sum of R1 and R2. It is the most common way to step down logic levels (e.g. 5V to 3.3V) in electronics.

The Voltage Divider Formula

Vout = Vin * R2 / (R1 + R2)

The output voltage is always less than or equal to the input. The ratio R2/(R1+R2) sets the fraction. Equal resistors give exactly half the input voltage. Making R2 larger brings Vout closer to Vin; making R1 larger pushes Vout closer to ground.

Common Logic Level Conversions

Conversion R1 R2 Vout
5V to 3.3V10 kΩ20 kΩ3.33 V
12V to 5V14 kΩ10 kΩ5.0 V
9V to 3.3V17.28 kΩ10 kΩ3.3 V
24V to 5V38 kΩ10 kΩ5.0 V

Design Scenarios

Arduino to Raspberry Pi

Arduino outputs 5V logic. Raspberry Pi GPIO accepts max 3.3V. A 10k/20k divider drops 5V to 3.33V safely. Current draw is only 0.17 mA. The Pi GPIO input impedance (~50k) is far larger than R2, so loading is negligible.

Battery Voltage Monitoring

A 12V battery needs to be read by a 3.3V ADC. Using 27k/10k gives Vout = 12*10/37 = 3.24V. Use 1% tolerance resistors and account for battery voltage range (10V-14.4V) to ensure Vout never exceeds 3.3V at full charge.

Pro Tips

Do This

  • ✓Buffer with an op-amp for low-impedance loads. If the load resistance is comparable to R2, Vout will sag. Place a unity-gain voltage follower (op-amp buffer) between Vout and the load to eliminate loading.
  • ✓Use standard E24 resistor values. After calculating ideal R1 and R2, round to the nearest E24 value (e.g. 10k, 22k, 47k) and recalculate Vout to verify it still meets your tolerance.

Avoid This

  • ✗Do not use a voltage divider as a power supply. Unlike a voltage regulator, a divider cannot maintain stable voltage under changing load currents. Even 10 mA of load will cause significant voltage sag.
  • ✗Do not use very large resistor values for fast signals. R1 and R2 form an RC time constant with parasitic capacitance. Above 1 MHz, 100k resistors will round off signal edges. Use lower values (1k-10k) for high-speed digital signals.

Frequently Asked Questions

Why does the voltage drop when I connect a load?

The load resistor connects in parallel with R2, reducing the effective bottom resistance. A lower effective R2 means a lower R2/(R1+R2) ratio, which reduces Vout. To avoid this, ensure the load impedance is at least 10 times larger than R2, or buffer the output with an op-amp.

What resistor tolerance should I use?

For logic level conversion, 5% tolerance resistors are usually acceptable because typical GPIO HIGH thresholds have wide margins. For ADC reference voltages or precision measurement, use 1% or 0.1% metal film resistors. The Vout error is bounded by the sum of both resistor tolerances.

Can I use a potentiometer instead of two fixed resistors?

Yes. A potentiometer is essentially an adjustable voltage divider. The wiper position sets the ratio between the upper and lower portions of the resistance track. This is how volume knobs on audio equipment work. However, potentiometers have lower precision and can introduce noise from wiper contact.

What is the Thevenin equivalent of a voltage divider?

Any voltage divider can be replaced by a Thevenin equivalent source with Vth = Vout (the no-load output voltage) and Rth = R1 in parallel with R2 (R1*R2 / (R1+R2)). This equivalent circuit makes it trivial to calculate how much Vout drops when a load is connected: just treat it as Vth driving Rth in series with the load.

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