LED Resistor Calculator
Pick a resistor that sets the right current through an LED.
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LED Resistor Calculator by TheCalcTools
How the LED Resistor Calculator works
Enter the supply voltage, the LED’s forward voltage from its datasheet, the current you want and how many LEDs are in series.
The resistor must drop the voltage that the LEDs do not use.
Divide that voltage by the current to get the resistance.
The formula
Worked example
Using the default values shown in the calculator:
| Supply voltage (V) | 5 |
|---|---|
| LED forward voltage (V) | 2 |
| LED current (mA) | 20 |
| LEDs in series | 1 |
- Voltage across the resistor = 5 − 2 = 3 V.
- R = 3 ÷ 0.020 = 150 Ω.
- It dissipates 60 mW, so a ¼ W resistor is ample.
Result:
| Resistor (Ω) | 150.0 |
|---|---|
| Resistor power (mW) | 60.0 |
| Minimum rating (mW, 2× margin) | 120 |
| Total power from supply (mW) | 100.0 |
Frequently asked questions
What if the exact value is not available?
Choose the next higher standard value (E12 or E24 series). The LED will be slightly dimmer but safe.
What forward voltage should I use?
Typical values are about 1.8–2.2 V for red, 3.0–3.4 V for white and blue. Always check the datasheet.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.