Voltage Divider and LED Resistor Calculator

Work out a voltage divider (with or without a load), find the second resistor for a target voltage, or size the series resistor for an LED.

Leave empty if the output drives almost nothing, such as a microcontroller input.

How it works

A divider gives Vout = Vin × R2 ÷ (R1 + R2) when nothing is connected to the output. A load across R2 puts it in parallel with R2, which lowers the output: the calculator uses R2 ∥ Rload in place of R2. To find R2 for a target output with no load, R2 = R1 × Vout ÷ (Vin − Vout). For an LED, R = (Vsupply − n × Vf) ÷ I, then the nearest standard (E24) value is shown with the current it gives. The ideas are explained on the Circuit Basics and Diodes and LEDs pages.

Worked example

Divider: 5 V across R1 = 1 kΩ and R2 = 2 kΩ gives 3.33 V and carries 1.67 mA. A 10 kΩ load across R2 pulls the output down to 3.125 V (−6 %). Target: for 3.3 V from 5 V with R1 = 1 kΩ, R2 = 1 000 × 3.3 ÷ 1.7 = 1.94 kΩ; the nearest standard (E24) value is 2 kΩ, which gives 3.33 V. LED: a red LED (2.0 V) at 10 mA from 5 V needs (5 − 2.0) ÷ 0.010 = 300 Ω, which is a standard value, so the current is exactly 10 mA; a 330 Ω resistor gives 9.1 mA and dissipates 27 mW.

Things to know

LED forward voltages are typical and vary between parts and with current; treat the result as the starting point and check the brightness. A white or blue LED on a 3.3 V supply leaves almost nothing across the resistor, so the current varies a lot: use a higher supply or a constant-current driver.

A voltage divider is a signal tool, not a power supply: its output falls as the load draws current. For a steady supply voltage use a regulator.

Resistor values have tolerances (5 % for E24 parts), so the real output differs a little from the calculated value. Use 1 % resistors where the voltage matters, such as a divider for a battery monitor or an ADC.

The power figures assume a steady voltage. Choose a resistor rated for at least twice the dissipation, as the calculator suggests.

Common questions

Why is the output lower than expected when I connect something?

The load is in parallel with R2 and so lowers the lower resistance, pulling the output down. Add the load resistance in the calculator, use smaller divider resistors, or buffer the output with an op-amp.

What divider resistance should I use for an ADC input?

Large enough to save power, small enough that the ADC input (and its sample-and-hold capacitor) does not distort the reading. Often 10 kΩ to 100 kΩ in total; check the ADC’s recommended source impedance.

Should I round the LED resistor up or down?

Up. A slightly larger resistor gives a slightly dimmer LED that lasts longer. If you round down, check that the current stays below the LED’s maximum.

Can I put LEDs in parallel on one resistor?

It is better not to: LEDs do not share current evenly, and one may hog most of it. Give each LED its own resistor, or put LEDs in series.

Sources

  • Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press: voltage dividers and diodes.
  • LED manufacturers’ datasheets (for example Kingbright, Lite-On): forward voltage by color.
  • IEC 60063, preferred number series (E24) for resistors.

Last reviewed: October 22, 2026