LED Series Resistor Calculator

An LED is a current-driven device, not a voltage-driven one: past its turn-on voltage its current rises almost vertically, so connecting it straight across a supply will let the current run away and destroy it. A series resistor is the simplest way to set a safe, steady current. Its value comes straight from Ohm's law: R = (V_supply − V_f) / I_LED, where V_f is the LED's forward voltage and I_LED is the desired current.

Worked reasoning

The resistor's job is to "absorb" whatever supply voltage the LED doesn't. For a red LED (V_f ≈ 2 V) at 20 mA on a 5 V supply, the resistor drops 5 − 2 = 3 V, so R = 3 V / 0.02 A = 150 Ω. Always round up to the next standard value (e.g. 150 Ω → 150 or 160 Ω) so the current stays at or below target.

Don't forget the power rating

The resistor also dissipates power: P = (V_supply − V_f) × I_LED. In the example that is 3 V × 0.02 A = 60 mW, comfortably within a ¼ W part. At higher currents or bigger voltage drops the dissipation grows quickly, so always check it and choose a resistor rated at least twice the calculated power.

Typical forward voltages & tips

Forward voltage depends on LED colour: red/amber ≈ 1.8–2.2 V, green ≈ 2.0–3.0 V, blue/white ≈ 3.0–3.4 V. For several LEDs, wiring them in series and using one resistor is more efficient than one resistor each, as long as the supply exceeds the sum of the forward voltages. For many LEDs or for maximum efficiency, a constant-current driver beats a resistor.

Common questions

Why not connect an LED directly to a battery? With no current limit the LED's current spikes and it burns out (or the battery does). Always limit the current.

Can one resistor drive LEDs in parallel? It's discouraged — small V_f differences make one LED hog the current. Give each parallel LED its own resistor, or wire them in series with a single resistor.