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LED Voltage Formula
The LED voltage calculator uses the voltage relationship for a simple series LED circuit with one external current-limiting resistor.
V_{LED} = V_S - I RThe same relationship can be rearranged to solve for the other missing values:
R = \frac{V_S - V_{LED}}{I}I = \frac{V_S - V_{LED}}{R}V_S = V_{LED} + I R- VLED = LED forward voltage, in volts
- VS = source or supply voltage, in volts
- I = LED current, in amps
- R = external series resistor, in ohms
Enter any three values and the missing value is calculated from the matching rearranged formula. If the LED voltage is missing, the calculator subtracts the resistor voltage drop from the source voltage. If the resistor is missing, it divides the voltage left over after the LED by the current. If the current is missing, it divides the resistor voltage drop by the resistance. If the source voltage is missing, it adds the LED voltage and resistor voltage drop.
This is a simplified DC series-circuit calculation. Real LED forward voltage changes with LED color, current, manufacturing tolerance, and temperature, so use the result as a circuit estimate unless you have a datasheet value measured at the intended current.
Typical LED Forward Voltage Ranges
Common LED forward voltage ranges are shown below. Actual values can vary by part number and operating current.
| LED type or color | Typical forward voltage | Common note |
|---|---|---|
| Red | 1.8 V to 2.2 V | Often used for indicators |
| Amber / Yellow | 2.0 V to 2.2 V | Similar to many red LEDs |
| Green | 2.0 V to 3.3 V | Depends strongly on LED chemistry |
| Blue | 3.0 V to 3.5 V | Usually higher than red or yellow |
| White | 3.0 V to 3.6 V | Typically a blue LED with phosphor |
| Infrared | 1.2 V to 1.7 V | Common in remotes and sensors |
Common Current Levels for Small LEDs
| LED current | Use case | Comment |
|---|---|---|
| 1 mA to 2 mA | Low-power indicator | Dim but efficient for many modern LEDs |
| 5 mA | General indicator | Often bright enough for panels and breadboards |
| 10 mA | Brighter indicator | Common design current for standard LEDs |
| 20 mA | Typical maximum for many 3 mm and 5 mm LEDs | Check the datasheet before using continuously |
LED Voltage Examples
Example 1: Find LED voltage
You have a 5 V source, a 330 Ω series resistor, and 10 mA of LED current.
V_{LED} = V_S - I RV_{LED} = 5 - (0.010)(330)V_{LED} = 1.7\text{ V}The LED voltage is 1.7 V.
Example 2: Find the series resistor
You have a 9 V source, an LED forward voltage of 2.0 V, and a desired current of 20 mA.
R = \frac{V_S - V_{LED}}{I}R = \frac{9 - 2}{0.020}R = 350\ \Omega
The required series resistor is 350 Ω. In a real circuit, you would normally choose the nearest standard resistor value and check the resulting current.
LED Voltage Calculator FAQ
What is LED voltage?
LED voltage usually means the forward voltage across the LED while it is on. It is the voltage drop from the LED anode to cathode at a given current. A red LED may be around 2 V, while a white or blue LED may be around 3 V or more.
Why does an LED need a series resistor?
An LED does not behave like a normal resistor. Once it starts conducting, a small voltage change can cause a large current change. A series resistor limits the current by dropping the extra supply voltage. Without current limiting, the LED can overheat or fail.
Can this calculator be used for multiple LEDs in series?
Yes, if you replace VLED with the total forward voltage of all LEDs in series. For example, three identical LEDs with 2.0 V forward voltage each have a total LED voltage of 6.0 V. The same formula then applies using that total value.
