Voltage Divider Calculator
Enter the input voltage and two resistor values to calculate the output voltage, divider current and resistor power dissipation. Ideal for signal scaling, ADC reference voltages and sensor biasing.
Last updated: May 2026
Enter Vin, R1 and R2 to calculate.
Vout = Vin × R2 ÷ (R1 + R2)
Common presets
How a resistor voltage divider works
A voltage divider uses two resistors in series to create an output voltage that is a fixed fraction of the input. Because the same current flows through both resistors, the voltage is distributed proportionally to resistance. The output taps between R2 and ground.
The formula: Vout = Vin × R2 ÷ (R1 + R2). To halve a voltage, use equal resistors. To scale 5 V to 3.3 V, use R1=5.1 kΩ and R2=10 kΩ — giving exactly 3.33 V.
Typical use cases
- Scaling a 5 V sensor output down to 3.3 V for an ESP32 or STM32 ADC input
- Creating a reference voltage from a regulated supply
- Biasing a transistor base or FET gate
- Measuring higher voltages with a microcontroller ADC by dividing them down into the 0–3.3 V range
Frequently Asked Questions
Why does loading affect the output voltage?
Connecting a load in parallel with R2 reduces the effective resistance of the lower half, shifting Vout lower. A load impedance of 10× R2 or more is generally acceptable — it causes less than 10% deviation. If your load is low-impedance (e.g. a microcontroller GPIO input is typically >1 MΩ, but a motor driver can be a few hundred ohms), choose lower divider resistor values to make the load effect negligible.
How do I choose R1 and R2 to minimise power waste?
Higher resistance values (10 kΩ–100 kΩ range) draw less current and waste less power. A 10 kΩ + 10 kΩ divider on 5 V wastes only 1.25 mW total. A 100 Ω + 100 Ω divider on 5 V wastes 62.5 mW — significant in battery-powered designs. Use 100 kΩ or higher for always-on sensor biasing to minimise quiescent current.
Can I use a voltage divider to step down 12 V for a 3.3 V microcontroller?
Yes, but only for measurement or signal inputs — not to power the microcontroller or drive high-current loads. A common 12 V → 3.3 V divider: R1 = 8.2 kΩ, R2 = 3.3 kΩ (gives 3.30 V). For powering an MCU, use a voltage regulator (e.g. AMS1117 or LDO) instead — a divider cannot maintain stable voltage under varying load current.
What is the ratio formula for choosing resistor values?
Rearranging Vout = Vin × R2 ÷ (R1 + R2): R1/R2 = (Vin − Vout) ÷ Vout. For 5 V → 3.3 V: ratio = (5 − 3.3) ÷ 3.3 = 0.515. So R1 should be ≈ 0.515 × R2. Choosing R2 = 10 kΩ gives R1 ≈ 5.15 kΩ — use the nearest standard value of 5.1 kΩ.