How to use the led resistor calculator
Choose the series resistor for an LED. Enter supply voltage, led forward voltage, led current in the fields above; the resistor value, resistor power are computed instantly and plotted on the chart so you can see how the answer changes.
Formula
Variables
- Vs — Supply voltage (V)
- Vf — LED forward voltage (V)
- I — LED current (mA)
Worked example
With supply voltage = 5 V, led forward voltage = 2 V, led current = 20 mA, the calculator gives resistor value ≈ 150 Ω; resistor power ≈ 60 mW.
Frequently asked questions
What is the formula for the led resistor calculation?
Resistor value (Ω) = (Vs-Vf) / (I / 1000). Resistor power (mW) = (Vs-Vf) × I. Here Vs is supply voltage (V), Vf is led forward voltage (V), I is led current (mA).
What inputs do I need for the led resistor calculator?
You need supply voltage in V, led forward voltage in V, led current in mA. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use this calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.