LED Resistor Calculator
Free LED resistor calculator — exact and standard (E24) resistor values for any LED circuit, with power rating and live circuit diagram.
Everyday Uses
Arduino and hobby projects
The single most common beginner electronics question — which resistor for this LED — answered with the exact part to grab.
Custom lighting builds
Series LED strings for cabinets, models, or cosplay — get resistor value and wattage for any supply.
12 V automotive LEDs
Adding LEDs to a car? Calculate for 12–14.4 V automotive supplies safely.
Learning Ohm's law hands-on
The live circuit diagram shows exactly where the voltage drops — theory made visible.
Frequently Asked Questions
Why does an LED need a resistor?
LEDs are current-driven devices with almost no internal resistance — connect one straight to a battery and current rises until the LED burns out, often instantly. The series resistor limits current to a safe level: R = (Vs − Vf) / I, where Vs is the supply voltage, Vf the LED forward voltage, and I the desired current. A 5 V supply driving a red LED (2 V, 20 mA) needs (5−2)/0.02 = 150 Ω.
What forward voltage does my LED have?
It depends on the chemistry, which sets the color: red and yellow LEDs typically drop 1.8–2.2 V, green (modern InGaN) 2.9–3.3 V, and blue and white 3.0–3.4 V. Check the datasheet when you have it; otherwise the preset buttons here use safe typical values. Running the math with a slightly high Vf errs on the safe side.
Why use the next-higher standard resistor value?
Resistors come in standard series (E12, E24) — you cannot buy a 137.5 Ω resistor. Choosing the next value UP (e.g. 150 Ω) means slightly less current than target: marginally dimmer but safe. Choosing the next value down would overdrive the LED and shorten its life. This calculator picks the next E24 value up and shows the actual resulting current.
How do I wire multiple LEDs?
In series (chained): forward voltages add — three red LEDs need 6 V of headroom plus resistor margin, but current stays at one LED's worth, making series efficient. In parallel: each string needs its OWN resistor, because tiny Vf differences make LEDs sharing one resistor divide current unevenly — the hottest LED hogs more, gets hotter, and fails first.