Voltage Divider Calculator

About the Voltage Divider Calculator

The voltage divider is one of the most fundamental circuits in electronics: two resistors in series produce an output voltage that is a fraction of the input. The output voltage is Vout = Vin × R2 / (R1 + R2), where R1 connects the input to the output node and R2 connects the output to ground. This simple relationship is used in sensor interfaces, biasing networks, level shifting, and signal attenuation.

However, the ideal voltage divider formula assumes no load — any current drawn from the output reduces the effective R2 and lowers the voltage. Understanding this load effect is critical in practical circuit design. The output impedance of a voltage divider is R1 ∥ R2, which sets a limit on how much current can be drawn before the voltage drops significantly.

This calculator works in two modes: calculate mode (given R1 and R2, find Vout) and design mode (given a target Vout, find R1 and R2 with E24 nearest standard values). Both modes include load analysis, tolerance sensitivity, and visual ratio display.

When This Page Helps

Designing a voltage divider often requires multiple iterations: calculate the ideal ratio, find standard resistor values, check the tolerance impact, and verify behavior under load. This calculator handles all steps in one tool, with automatic E24 value matching and a sensitivity table that shows how resistor tolerances affect the output.

How to Use the Inputs

1. Select the mode: Calculate (find Vout from R1/R2) or Design (find R1/R2 for target Vout).
2. Enter the input voltage Vin.
3. In Calculate mode, enter R1 (top) and R2 (bottom) resistor values in ohms.
4. Optionally add a load resistance to see how it affects the output.
5. In Design mode, enter the target Vout and desired total resistance (R1+R2).
6. Use presets for common divider configurations (5V→3.3V, 12V→5V, etc.).
7. Check the tolerance sensitivity table to assess output accuracy.

Example Calculation

Tips & Best Practices

• For voltage dividers feeding high-impedance inputs (op-amps, ADC reference pins), load effect is negligible — use high-value resistors to minimize power consumption.
• For low-impedance loads, use a buffer (voltage follower op-amp) after the divider to eliminate load effect.
• Keep total resistance R1+R2 between 1 kΩ and 100 kΩ for most applications — too low wastes power, too high picks up noise.
• E24 series (5% tolerance) is usually sufficient; for precision dividers, use E96 (1%) resistors or trimmer potentiometers.
• Watch for temperature coefficients: the ratio R2/(R1+R2) is stable only if both resistors have matching temperature coefficients.
• In audio applications, the divider forms an attenuator pad — the dB value shows the signal reduction.

Voltage Dividers in Sensor Interfaces

Many sensors (thermistors, photoresistors, strain gauges) are essentially variable resistors. Placing them in a voltage divider with a fixed resistor converts the resistance change to a voltage change that an ADC can read. The choice of fixed resistor value determines the sensitivity and linearity of the sensor interface — ideally, it should match the sensor's resistance at the midpoint of the measurement range.

AC Voltage Dividers and Impedance Matching

At AC frequencies, reactive components (capacitors, inductors) form frequency-dependent voltage dividers. A capacitive divider passes high frequencies; an inductive divider passes low frequencies. The combination of resistive and reactive dividers creates filters — low-pass, high-pass, and band-pass — that are the building blocks of analog signal processing.

Precision Voltage References

For applications requiring precise voltage ratios (DAC reference, ADC feedback networks), resistor networks with matched temperature coefficients are used. Integrated resistor dividers in a single package track each other thermally, maintaining ratio accuracy even as temperature changes. Thin-film resistor networks achieve ratio matching of 0.01% or better.

Frequently Asked Questions

• Why does loading change the output voltage?

  The load resistance appears in parallel with R2, reducing the effective bottom resistance. A lower R2_eff shifts the divider ratio, producing a lower output voltage. The effect is significant when R_load is within an order of magnitude of R2.

• Can a voltage divider replace a voltage regulator?

  Generally no. A voltage divider has high output impedance and cannot supply significant current without voltage drop. For powering circuits, use a proper voltage regulator (LDO, buck converter). Voltage dividers are for biasing, measurement, and reference — not for power supply.

• What are E24 standard values?

  The E24 series defines 24 preferred resistor values per decade (1.0, 1.1, 1.2, ... 9.1), each available in multiples of powers of 10. They are spaced approximately 10% apart and cover the 5% tolerance range. Using E24 values ensures your resistors are readily available from suppliers.

• How does resistor tolerance affect the output?

  With 5% resistors, the worst case is when R1 is 5% low and R2 is 5% high (or vice versa), causing the output to shift by roughly ±10% of the divider drop. The sensitivity table shows exact values for your specific configuration.

• What about capacitive voltage dividers?

  The same principle applies with capacitors: Vout = Vin × C1 / (C1 + C2). Note that the capacitor with the smaller value gets the larger voltage — opposite to resistive dividers. Capacitive dividers are used in AC coupling and charge-sharing circuits.

• What is the maximum power each resistor dissipates?

  P_R1 = I² × R1 and P_R2 = I² × R2, where I = Vin / (R1 + R2). For most signal-level dividers, the power is milliwatts or less. Check that each resistor stays within its power rating (typically 1/8 W or 1/4 W for SMD/through-hole).