LED Resistor Calculator

How It Works

1. 1

   Enter supply voltage, LED V_f, and I_f

   Look up the LED's datasheet for forward voltage and current. For unmarked indicator LEDs, start with V_f = 2 V and I_f = 10 mA as a safe underdrive.

2. 2

   Select single, series, or parallel configuration

   Series multiplies the forward drop by LED count, so the supply must exceed n × V_f. Parallel requires one resistor per branch — choose parallel and the calculator reports the per-branch resistor plus total source current.

3. 3

   Pick the nearest standard value and power rating

   Use the E24 or E12 recommendation for a part you can actually buy. Rate the resistor at 2× the calculated dissipation — 0.1 W calls for a 1/4 W resistor, 0.25 W for a 1/2 W part.

LED current-limiting resistors, in under four minutes

An LED is a diode, and a diode does not obey Ohm's law. Above its forward voltage, an LED's current climbs exponentially with voltage, so a supply held 0.2 V above V_f can push many times the rated current through the junction and cook it. The fix is a current-limiting resistor in series, a textbook application of Kirchhoff's voltage law: the supply voltage V_s equals the LED's forward drop V_f plus the resistor voltage I·R, so R = (V_s − V_f) / I_f solves for the value that sets the current to exactly I_f. The formula dates back to the first practical red LEDs Nick Holonyak Jr. demonstrated at General Electric in 1962; the circuit topology has not changed since. Typical forward voltages are 1.8–2.2 V for red, yellow, and green indicators, and 3.0–3.3 V for blue and white LEDs that use indium gallium nitride. Typical indicator current is 10–20 mA. High-brightness LEDs draw 350 mA or more. After picking a resistance, round up to the nearest E12 or E24 standard value (IEC 60063). Rounding up lowers current slightly — safer than rounding down. The resistor also dissipates power: P_R = (V_s − V_f) × I_f. Rate the part at twice that dissipation so it runs cool. For parallel LEDs, never share a single resistor across branches — forward-voltage mismatch causes one LED to hog current until it fails.

Common pitfalls

• Sharing one resistor across parallel LEDs. Forward-voltage mismatch, even a few tens of millivolts, makes one LED conduct harder, heat up, drop further in V_f, and steal current until it burns out and takes the rest with it. Every parallel branch needs its own resistor.

• Running a series chain with V_s ≤ n × V_f. Each LED drops V_f; the resistor needs V_s − n × V_f volts across it to limit current. Three 3.3 V blue LEDs in series need a supply above 9.9 V; a 9 V battery cannot drive them and a 12 V rail leaves only 2.1 V for the resistor.

• Picking a resistor rated at the calculated dissipation. A 0.1 W dissipation on a 0.1 W part runs hot and fails early. Pick the next power rating up (0.25 W in this case, IEC 60115). The rule of thumb is 2x the calculated power.

• Using the typical V_f from the datasheet as gospel. V_f binning can vary +/- 0.2 V within a reel, and V_f also drops about 2 mV/°C as the junction heats. For mass production, use a constant-current driver (CAT4101, AL8860) instead of fixed resistors.

• Forgetting that PWM dimming keeps peak current at the resistor-sized value. A 1 kHz 10% PWM of a 20 mA LED still pushes 20 mA peak through the resistor; do not downsize based on the 2 mA average.

Frequently Asked Questions

Related Tools

Related Reading