When to Use This Calculator
Use this calculator for a simple DC LED branch where one resistor limits current through one LED or several LEDs in series. It is most useful for indicators, hobby circuits, panel lights, and quick datasheet checks.
Enter Values
Formula Summary
R = (Vsupply - N × Vf) ÷ I
Presistor = (Vsupply - N × Vf) × I
Current must be converted from milliamps to amps before using the equations.
Practical Examples
12 V indicator LED
One 2 V LED at 20 mA from a 12 V source requires at least 500 Ω. The resistor dissipates 0.2 W.
Three LEDs in series
Three 3.2 V LEDs at 15 mA from a 12 V source leave 2.4 V across the resistor, requiring about 160 Ω and 0.036 W.
Selecting a Real Resistor
Choose the next higher standard resistance value to keep current at or below the target. Select a power rating comfortably above the calculated dissipation.
For the example, a 510 Ω resistor is a common practical choice, and a 0.5 W rating provides useful margin.
Important LED Behavior
Forward voltage varies by LED color, current, temperature, and part. Use the datasheet value at the intended operating current. Do not place LEDs directly in parallel behind one shared resistor because current may not divide evenly.
The supply voltage must remain higher than the total LED forward voltage with enough headroom for current regulation.
Common Mistakes
- Using milliamps directly in the formula instead of converting to amps.
- Choosing a resistor with no power-rating margin.
- Using a typical forward voltage when the datasheet maximum or temperature range matters.
- Putting parallel LEDs behind one shared resistor and expecting equal current sharing.
Reference Notes
The calculation uses Ohm's law and resistor power dissipation. LED forward voltage and recommended current should come from the LED manufacturer datasheet for the specific part and operating condition.