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LED Series Resistor Calculator

Size the current-limiting resistor for one or more LEDs in series, with the nearest standard (E12) resistor value and power dissipation.

Voltage across resistor
7 V
Exact resistance needed
350 Ω
Nearest standard resistor (E12)
390 Ω
Resistor power dissipation
140 mW

How it works

An LED needs a series resistor to limit current to its rated forward current — without one, a near-vertical current-vs-voltage curve means a tiny voltage change causes a huge current spike. The resistor absorbs whatever voltage the LED string doesn't: voltage across resistor = supply voltage − (forward voltage × number of LEDs in series). Ohm's law then sizes the resistor from that voltage and the desired forward current: R = V ÷ I.

The exact resistance computed almost never matches a resistor you can actually buy, so the calculator also reports the nearest value from the E12 series — the 12-values-per-decade set (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 × a power of ten) that off-the-shelf resistors come in — always rounding up, never down, since a smaller resistance would push more than the intended current through the LED.

Power dissipated by the resistor, P = V × I, determines its wattage rating: most through-hole LED circuits only need a standard 1/4 W resistor, but it's worth checking, especially with a high supply voltage or current.

FAQ

Why does the calculator round the resistor value up instead of to the nearest value?

Rounding down to a smaller standard resistance would let more current through than the forward current you specified, which can shorten the LED's life or damage it. Rounding up slightly undershoots the target current instead — always the safer direction for an LED.

What if I'm driving several LEDs in parallel instead of in series?

This calculator assumes LEDs in series, where the same current flows through all of them and their forward voltages add. LEDs in parallel each need their own resistor (or close to it, since no two LEDs are perfectly identical) — run this calculator once per parallel branch with "Number of LEDs in series" set to however many are in that branch.

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