Enter the series resistor value (ohms), the LED current (amps), and the source voltage (volts) into the calculator to determine the LED forward voltage (volts). 

LED Voltage Calculator

Enter any 3 values to calculate the missing variable

Note: This calculator uses the simple series-circuit relationship VLED = VS − I·R where R is the external series (current‑limiting) resistor. LEDs are non‑ohmic devices; forward voltage depends on LED type, current, and temperature.

LED Voltage Formula

The following formula is used to calculate the LED forward voltage for a simple series circuit with a current-limiting resistor.

V_{LED} = V_S - I \cdot R
  • Where VLED is the LED voltage (volts)
  • R is the series (current-limiting) resistor value (ohms) 
  • I is the LED current through the series circuit (amps) 
  • VS is the source voltage (volts) 

To calculate the LED voltage, multiply the series resistor by the current to find the resistor voltage drop, then subtract that drop from the source voltage.

How to Calculate LED Voltage?

The following example problems outline how to calculate the LED Voltage.

Example Problem #1

  1. First, determine the series resistor value (ohms). In this example, the resistor value is 150 Ω.
  2. Next, determine the LED current (amps). For this problem, the LED current is 0.020 A (20 mA).
  3. Next, determine the source voltage (volts). In this case, the source voltage is 5 V.
  4. Finally, calculate the LED Voltage using the formula above: 

VLED = VS – I·R

Inserting the values from above into the equation gives: 

VLED = 5 – (0.020 × 150) = 2 (Volts)

FAQ

What factors affect LED voltage drop?

An LED’s forward voltage drop is primarily affected by the LED’s semiconductor material/bandgap (which correlates with color/type), the forward current, and junction temperature (forward voltage typically decreases as temperature rises). There is also normal part-to-part variation between LEDs.

How can I increase the lifespan of my LEDs?

To increase the lifespan of LEDs, ensure they are operated within their recommended current and voltage ranges. Using a current-limiting resistor (or a constant-current driver) and avoiding high temperatures can also help prolong their life.

Can I connect multiple LEDs in series with a single resistor?

Yes, you can connect multiple LEDs in series with a single resistor, but all LEDs in the series will have the same current flowing through them. Ensure the power supply voltage is high enough to account for the total forward voltage drop across all LEDs plus the resistor.