LED Resistor Calculator
Size the current-limiting resistor for an LED from the supply voltage, LED forward voltage and desired current — with the nearest standard value.
How it works
R = (V_supply − V_forward) ÷ I · P_resistor = (V_supply − V_forward) × I · (I in amps)
An LED is a diode with a nearly fixed forward voltage, so you cannot connect it straight to a supply — you must limit the current with a series resistor. The resistor drops the leftover voltage (supply minus forward voltage) at the current you want, which is a direct application of Ohm’s law. Sizing the resistor’s power rating with headroom keeps it from overheating.
Worked example
A red LED (2 V forward, 20 mA) on a 5 V supply needs R = (5 − 2) ÷ 0.020 = 150 Ω, which is already a standard value. It dissipates (5 − 2) × 0.020 = 0.06 W, so a common ⅛ W resistor is plenty.
Frequently asked questions
What happens without a resistor?
The LED draws whatever current the supply can deliver, which almost always exceeds its rating and burns out the junction in an instant. The series resistor sets a safe, predictable current.
How do I find my LED’s forward voltage?
Check the datasheet, or use a rough guide by colour: red/orange ≈ 1.8–2.2 V, yellow/green ≈ 2.0–2.4 V, and blue/white ≈ 3.0–3.4 V. Higher forward voltage leaves less across the resistor.
Can I run several LEDs on one resistor?
LEDs in series share one resistor — add their forward voltages and subtract from the supply. LEDs in parallel should each have their own resistor, because tiny forward-voltage differences make them hog current otherwise.
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This calculator is for educational purposes. Double-check important results before acting on them.