Supply voltage Vs, LED forward voltage Vf, and desired If in mA—ballast resistor (Ω) and resistor power (W).
Supply Vs must exceed LED forward Vf. Forward current is entered in mA and converted to amperes. Then R = (Vs − Vf) ÷ I and resistor dissipation P ≈ (Vs − Vf)×I (rough sizing only; pick a rated power above this estimate).
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