LED Resistor Calculator

How to use

1. 1Pick a wiring mode: Single LED for one LED in series with one resistor, LEDs in series for a single string of several LEDs sharing one resistor, or Parallel strings for multiple identical strings side by side, each with its own resistor.
2. 2Choose an LED color preset to seed typical forward voltage and current values, or pick Custom to type your own from the datasheet.
3. 3Enter the supply voltage. Use the Common supplies chips for 3.3 V, 5 V, 9 V, 12 V, and 24 V rails.
4. 4If you chose LEDs in series or Parallel strings, enter the number of LEDs per string. For Parallel strings, also enter how many strings you have.
5. 5Read the result panel for the exact resistance, the nearest E12 and E24 preferred values (round up so the LED current stays at or below rated), the resistor voltage drop, the power dissipated, and the recommended resistor power rating.
6. 6Use Copy summary to grab a clean recipe block, or click Reset to start over.

About this tool

LED Resistor Calculator finds the exact current limiting (ballast) resistor needed to drive any light-emitting diode safely from a known supply voltage. The math is the standard Ohm's law derivation used in every electronics textbook: R equals the supply voltage minus the LED forward voltage, all divided by the LED forward current. The tool covers the three wiring patterns that account for almost every real-world circuit. Single LED is the classic Arduino, breadboard, pilot-light, or indicator circuit with one LED and one resistor. LEDs in series wires several LEDs end to end behind one shared resistor, which is the right pattern when the supply has enough headroom for the combined forward voltage; the calculator enforces the headroom constraint and warns when n times Vf is at or above the supply voltage. Parallel strings repeats the same series string side by side, with one resistor per string (never share a resistor across parallel strings or current will hog into the lowest-Vf branch and burn it out). Color presets seed the typical forward voltage and current for the most common LED chemistries: red around 2.0 V, orange and yellow around 2.1 V, standard green at 2.2 V, modern bright green at 3.2 V, blue and white around 3.2 V, UV around 3.4 V, infrared around 1.4 V, and the 1 W high-power star LED at 3.4 V and 350 mA. Override any value to match a specific datasheet. Output reports the exact calculated resistance plus the nearest higher member of the E12 (10 percent) and E24 (5 percent) preferred resistor series from IEC 60063, so you can pick a value that is actually on the shelf at any supplier. The result panel also shows the resistor voltage drop, the resistor power dissipation, and a recommended resistor power rating with a 2x safety factor (so a quarter-watt dissipation rounds up to the standard 0.5 W rating, never the bare-minimum 0.25 W part). For parallel mode the output also reports the number of resistors needed and the total current drawn from the supply, which is what sizes the input fuse, switch, and wire gauge. Common supply chips drop 3.3 V, 5 V (USB and Arduino), 9 V battery, 12 V automotive or RV, and 24 V into the field in one click. The Copy summary button exports a clean text block you can paste into a build log, a notebook, a schematic note, or a parts order. Useful for indicator LEDs on a PCB, current-limited LEDs on a microcontroller GPIO pin, simple LED panels and signs, prop and costume lighting, automotive accessory LEDs, RV and marine 12 V indicator lights, breadboard prototypes, and any moment you need to answer 'what resistor do I use with this LED?' without pulling out a calculator and a chart. Everything runs in your browser; no values are uploaded.

Free to use. Works in your browser. No signup, no login.