LED Circuit Calculator

LED Forward Current is your target — the current you want the LED to run at — but resistors only exist in standard E24 values, so the calculator rounds the ideal resistance up to the nearest available value and then recalculates what the current will actually be with that real resistor in place. Because it rounds up (not to the nearest value in either direction), Actual Current will typically be slightly lower than your entered Forward Current, which is deliberate since a slightly dimmer LED is safer than an overdriven one.

Each additional series LED adds its forward voltage to the total, so if enough LEDs are chained that their combined forward voltage exceeds your supply voltage, there's no voltage left for the resistor to drop and the circuit can't work — the calculator returns a resistor value of 0 in that case rather than a negative resistance, since a negative resistor isn't physically meaningful. You'd need either a higher supply voltage or fewer LEDs in the series string.

The Ω value only sets how much current flows — it says nothing about how much heat the resistor itself must survive. Min Resistor Rating is calculated from P = I²R using the actual current through the chosen standard resistor, then mapped up to the next common wattage rating (1/8W, 1/4W, 1/2W, 1W, or 2W); using an underrated resistor can cause it to overheat, discolor, or fail open even though its resistance value is exactly right for the LED.

Not directly — this calculator's series-string model assumes one shared resistor drops the leftover voltage across a single current path, but parallel LED branches each need their own individual resistor sized to that branch's own current, since even LEDs of the same type can have slightly different forward voltages and would otherwise hog an uneven share of current. Run the calculator once per parallel branch with ledCount set to however many LEDs are in series within that specific branch.