LED Resistor Calculator
Calculate the correct current-limiting resistor for any LED circuit. Choose single or series LED mode, pick an LED color preset, and get the exact resistor value, nearest E24 standard value, and power dissipation — all calculated privately in your browser.
How the LED Resistor Calculator Works
Every LED requires a current-limiting resistor to prevent burnout. LEDs are current-driven devices with a very low internal resistance once forward-biased, so without a resistor, the current would spike and destroy the LED almost instantly. This calculator uses Ohm's Law to determine the correct resistance value. For a single LED, the formula is R = (Vsupply - VLED) / I, where Vsupply is your power source voltage, VLED is the LED forward voltage drop, and I is the desired forward current in amps. For multiple LEDs wired in series, the total LED voltage drop is multiplied by the number of LEDs: R = (Vsupply - n * VLED) / I. The calculator also computes the power dissipated by the resistor using P = I2 * R, which helps you select a resistor with the correct power rating (typically 1/4W for standard circuits).
LED Resistor Formulas
Single LED: R = (Vsupply - VLED) / I
Series LEDs: R = (Vsupply - n × VLED) / I
Power: P = I² × R
Where R is resistance in ohms, V in volts, I in amps, P in watts, and n is the number of LEDs.
Understanding E24 Standard Resistor Values
Resistors are not manufactured in every possible ohm value. Instead, they follow standard series defined by the IEC. The E24 series provides 24 values per decade (1 to 10), with a tolerance of 5%. When you calculate a resistor value like 473 ohms, you cannot buy that exact value. The nearest E24 standard value would be 470 ohms. Using the nearest standard value slightly changes the actual current flowing through the LED, but the difference is usually negligible. This calculator automatically finds the closest E24 value and shows you the actual current that will flow with that standard resistor, so you can verify the LED will operate safely within its rated specifications. Always round up rather than down when choosing resistors, as slightly more resistance means slightly less current, which extends LED lifespan.
LED Color Forward Voltages Explained
Different LED colors use different semiconductor materials, which produce different forward voltage drops. Red LEDs typically drop 1.8 to 2.2 volts and use aluminium gallium arsenide (AlGaAs). Green LEDs drop 2.0 to 3.0 volts using gallium phosphide (GaP) or InGaN. Blue and white LEDs use indium gallium nitride (InGaN) and drop 3.0 to 3.6 volts. UV LEDs have the highest forward voltage at 3.2 to 3.8 volts. The forward voltage also varies with current and temperature. The preset values in this calculator use typical mid-range values for standard 5mm through-hole LEDs at 20mA. For high-power LEDs (1W or above), always check the datasheet as forward voltages and currents differ significantly. The standard 20mA forward current applies to most indicator LEDs, while high-brightness LEDs may require 350mA or more with appropriate heat sinking.
Series LED Circuits vs Parallel
When connecting multiple LEDs, series wiring is preferred because all LEDs share the same current, ensuring uniform brightness. In a series circuit, you need only one resistor, and the supply voltage must exceed the total voltage drop of all LEDs combined. For example, three blue LEDs at 3.3V each require at least 9.9V supply plus some headroom for the resistor. If your supply voltage is too low for the desired number of series LEDs, you must split them into parallel branches, each with its own current-limiting resistor. Never connect LEDs in parallel sharing a single resistor, as manufacturing variations mean LEDs have slightly different forward voltages, causing uneven current distribution and potential failure. This calculator handles the series case, and you can use it for each parallel branch by treating each branch as an independent series string.