Voltage Divider Calculator

How to Use This Voltage Divider Calculator

This voltage divider calculator solves for output voltage or resistor value — select your solve mode at the top: Output Voltage (V_out) to find what voltage your divider produces, or Resistor R2 to find the resistor value needed to hit a target output. Enter the input voltage (V_in) and R1, then add R2 or your target V_out depending on mode. Optionally enter a load resistance (R_L) in V_out mode to see how connecting a circuit to the output affects the voltage. Results include V_out, divider current in milliamps, and power dissipation in each resistor.

The Voltage Divider Formula

The voltage divider formula gives the output voltage at the junction of two series resistors:

V_out = V_in × R2 / (R1 + R2)

This formula holds whenever the output is unloaded (no current drawn from the V_out node). The output voltage is purely a function of the resistor ratio — not the absolute values. A 1 kΩ / 1 kΩ divider produces exactly the same output ratio as a 1 MΩ / 1 MΩ divider; the difference is the quiescent current and power consumption.

Solving for R2

To find the R2 value needed for a desired output, rearrange the formula: R2 = R1 × V_out / (V_in − V_out). For example, to get 3.3 V from a 5 V supply with R1 = 10 kΩ: R2 = 10 × 3.3 / (5 − 3.3) = 33 / 1.7 ≈ 19.41 kΩ. In practice, you would use the nearest standard resistor value (20 kΩ) and accept a small error, or use a trimmer potentiometer for precise adjustment.

The Effect of Load Resistance

Connecting a load to the V_out node places a resistance in parallel with R2. This parallel combination is always less than R2 alone, which shifts the divider ratio and lowers the output voltage. The loaded output is:

R2_eff = R2 × R_L / (R2 + R_L), then V_out_loaded = V_in × R2_eff / (R1 + R2_eff)

As an example, a 10 kΩ / 10 kΩ divider from 12 V gives 6 V unloaded. Connect a 10 kΩ load (R_L = 10 kΩ): R2_eff = 10 × 10 / (10 + 10) = 5 kΩ, and V_out_loaded = 12 × 5 / (10 + 5) = 4 V — a full 2 V below the unloaded value. Entering R_L in this calculator shows this drop instantly.

Voltage Dividers vs Regulators

A voltage divider is the simplest way to reduce a voltage, but it is only suitable when the load is known, fixed, and light. If the load current varies — or if you need a stable reference voltage regardless of load — use a voltage regulator instead. Linear regulators such as the LM7805 or LM317 maintain a fixed output voltage regardless of load current (within their rated range), and they include internal protection against overcurrent and overtemperature. Switching regulators are more efficient for large voltage differences or high current.

For digital signal level shifting (3.3 V to 5 V interfaces), a simple two-resistor divider is often acceptable because CMOS logic inputs draw only microamps. For powering sensors, op-amp circuits, or anything that draws milliamps or more, a linear regulator or a dedicated reference IC is the right choice. Use our watts to amps calculator to estimate load current before committing to a divider design.

Choosing R1 and R2 Values

Once you know the desired output ratio (V_out / V_in), you can pick any R1 and set R2 = R1 × V_out / (V_in − V_out). The trade-off is between stability and power consumption:

• Lower resistance (100 Ω – 10 kΩ) — high quiescent current, low output impedance, output is stable even with moderate loads. Use in precision references or when driving low-impedance loads.
• Higher resistance (100 kΩ – 1 MΩ) — very low quiescent current, suitable for battery-powered designs. Output impedance is high, so even a small load causes significant voltage droop. Only use with very high-impedance loads such as op-amp inputs or microcontroller ADC pins.

The standard design rule: for less than 10% loading error, the load resistance should be at least 10× the total divider resistance (R1 + R2). For less than 1% error, target 100× or more. For ADC inputs on microcontrollers, which typically have input impedances of 10 kΩ to 1 MΩ, keeping R1 + R2 below 10 kΩ gives reliable results.

Common Voltage Divider Applications

Voltage dividers appear throughout electronics at every level of complexity:

• ADC scaling — scaling a 0–12 V signal down to 0–3.3 V for a microcontroller ADC input. A 22 kΩ / 8.2 kΩ divider gives approximately 0–3.3 V from 0–12 V.
• Transistor biasing — setting the base voltage of an NPN transistor to establish the operating point. A common design uses a 100 kΩ / 47 kΩ divider on a 12 V rail to bias the base at about 3.8 V.
• Sensor signal conditioning — NTC thermistors and LDRs (light-dependent resistors) are typically one arm of a voltage divider; as the resistance changes with temperature or light, V_out changes proportionally.
• Volume control — a potentiometer is a mechanically adjustable voltage divider. Rotating the knob changes the R1/R2 ratio continuously.
• Level shifting — converting 5 V logic signals to 3.3 V for UART, I2C, or SPI interfaces between different-voltage devices. Use our parallel resistor calculator to quickly find equivalent resistance when combining multiple resistors.

Safety Note

This calculator is intended for low-voltage electronics design (signal-level and logic circuits, typically 3.3 V to 48 V DC). Do not use a resistor voltage divider to reduce mains voltage (120 V AC or 240 V AC) or high-voltage DC circuits. Mains-connected dividers can be lethal and must be designed by a licensed electrical engineer in accordance with applicable codes. For any circuit connected to mains power, consult a qualified professional and your local Authority Having Jurisdiction (AHJ) before construction.