Starlight Tools

Voltage Divider Calculator

Calculate Vout or solve Vin, R1, and R2, including loaded output, standard resistor values, current, power, and tolerance. For a two-resistor divider (also called a potential divider), the output is the fraction of Vin across the lower resistor.

Vout = Vin × R2 / (R1 + R2)
Voltage divider circuit Vin enters resistor R1, Vout is taken at the junction, and resistor R2 connects to ground. Optional load RL connects from Vout to ground in parallel with R2. VinR1R2 VoutRL(optional)Ground

Calculator

Calculator mode
Output model
V
DC supply or signal voltage.
V
Voltage at the R1/R2 junction.
Between Vin and Vout.
Between Vout and ground.
Load, tolerance & design options
Resistance from Vout to ground; required in Loaded mode.
%
µA

Answer

Choose a mode and enter the labeled values, then calculate.

Common voltage-divider presets

Select a starting point. Each preset uses standard resistor values and calculates a worked result.

Choose a preset to see its substitution, standard values, current, power, loading, and tolerance result.

How to design and troubleshoot a voltage divider

Derive the divider formula

With no load, R1 and R2 carry the same current: I = Vin / (R1 + R2). Ohm’s law across R2 gives Vout = I × R2; substituting the current produces Vout = Vin × R2 / (R1 + R2).

Choose R1 and R2

First choose the ratio for the required voltage, then choose the resistance scale. Lower values draw more current and generally drive a load or sampling capacitor more firmly; higher values waste less power but make leakage, noise, and capacitance more important. Use the standard-pair mode to balance voltage error against a preferred total resistance or current range.

Account for a connected load

A resistive load RL is in parallel with R2, so Rlower = R2 ∥ RL and Vout,loaded = Vin × Rlower / (R1 + Rlower). In loaded solve mode this calculator designs directly for the requested loaded output; it rejects a requested equivalent lower resistance that cannot be produced by R2 in parallel with RL.

Know when a divider is appropriate

Dividers work well for attenuation, biasing high-impedance inputs, and resistive sensing. They are not regulated power supplies: changing load current changes Vout and can overheat the resistors. Use a buffer, regulator, or power converter when a device needs meaningful or variable current.

Check ADC input behavior

For an ADC, compare R1 ∥ R2 with the device’s specified source impedance, input leakage, sample-and-hold acquisition time, and external capacitance guidance. Also check pin clamps, maximum input voltage, power-off injection current, bandwidth, and settling in the actual datasheet.

Common mistakes and measured deviations

  • Swapping R1 and R2 reverses the ratio; R1 is above Vout and R2 is below it.
  • A meter, ADC, or next circuit stage can act as RL and pull Vout below the ideal result.
  • Source resistance, resistor tolerance and temperature coefficient, leakage, wiring, and supply error all affect a measurement.
  • Worst-case tolerance is a ratio calculation using opposing resistor extremes; it is not automatically equal to the sum of the two tolerance percentages.

Voltage divider FAQ

How do I calculate Vout?

For an unloaded divider, multiply Vin by R2/(R1 + R2). With a load, replace R2 with R2 ∥ RL.

How do I calculate R1 or R2?

Ideal formulas are R1 = R2(Vin/Vout − 1) and R2 = R1Vout/(Vin − Vout). Select Loaded to solve using the requested loaded Vout and RL instead.

What changes when a load is connected?

RL sits in parallel with R2, reducing the effective lower resistance. R1 supplies both the R2 branch current and the load current, so branch currents and resistor powers must be calculated separately.

Why is measured Vout lower than expected?

The measuring device or receiver may load the divider. Also check Vin under load, resistor values and tolerances, leakage, connection order, and ADC acquisition requirements.

Can a voltage divider power a device?

Usually not. A divider is suitable when the receiver current is negligible and predictable. Use a buffer or regulated supply when the load draws meaningful or changing current.

How do resistor tolerances affect Vout?

Vout is lowest when R1 is high and R2 is low, and highest when R1 is low and R2 is high. The calculator evaluates those conservative combinations, including RL when Loaded is selected.

Which resistor values should I choose for 5 V to 3.3 V?

An E24 starting pair is R1 = 5.1 kΩ and R2 = 10 kΩ, giving about 3.31 V unloaded. Confirm tolerance, input leakage, source impedance and maximum input conditions for the receiving device.

Review, assumptions and sources

Technical author and reviewer
Starlight Robotics Electronics Team — electronics and embedded-systems engineering.
Reviewed and updated 15 July 2026
Calculation assumptions
DC, positive, purely resistive divider; ideal zero-impedance voltage source; RL is resistive and fixed; nominal values unless tolerance is enabled. Wiring, source error, temperature coefficient, noise, and transient behavior are outside the model.

Technical sources

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