LED Series Resistor Calculator
Finds the correct series resistor value and power rating to safely drive one or more LEDs in series from a given supply voltage. Use it whenever you connect LEDs to a voltage source without a dedicated driver.
About this calculator
An LED is not a simple resistor — its forward voltage (V_f) is nearly fixed by its semiconductor material (typically 1.8–2.2 V for red, 2.9–3.4 V for white or blue), and without current limiting it will draw excessive current and burn out instantly. A series resistor drops the excess voltage and limits current to the desired forward current (I_f). The formula is: R = (V_supply − V_f × N) / I_f, where V_supply is the power supply voltage, V_f is the LED forward voltage, N is the number of LEDs in series, and I_f is the desired forward current in amperes. Power dissipated by the resistor is P = (V_supply − V_f × N) × I_f, and you should choose a resistor rated for at least twice that value for safety margin. This approach applies to any single-color LED circuit powered by a DC source.
How to use
You want to drive 3 white LEDs (V_f = 3.2 V each) in series from a 12 V supply at 20 mA (0.020 A). Step 1: Total LED voltage = 3.2 × 3 = 9.6 V. Step 2: Voltage across resistor = 12 − 9.6 = 2.4 V. Step 3: R = 2.4 / 0.020 = 120 Ω. Enter supply_voltage = 12, led_voltage = 3.2, led_current = 20, num_leds = 3 — the calculator returns 120 Ω. Step 4: Power in resistor = 2.4 × 0.020 = 0.048 W — a standard ¼ W resistor is more than adequate. Choose the nearest standard E24 value (120 Ω is a standard value).
Frequently asked questions
What happens if I omit the series resistor in an LED circuit?
Without a current-limiting resistor, the LED is connected directly across the supply voltage. Because an LED's forward voltage is nearly fixed, any supply voltage above V_f drives theoretically unlimited current, constrained only by the tiny internal resistance of the LED and wiring. This current is far beyond the LED's rated maximum (typically 20–30 mA for standard LEDs), causing the junction to overheat within milliseconds and permanently destroying the device. In some cases the LED may briefly flash very brightly before failing. Always include a resistor or use a constant-current LED driver.
How do I calculate the power rating needed for my LED series resistor?
The power dissipated in the resistor is P = I² × R, or equivalently P = V_R × I, where V_R is the voltage dropped across the resistor and I is the LED forward current. For example, a 120 Ω resistor carrying 20 mA dissipates 0.020² × 120 = 0.048 W. Always select a resistor with a power rating of at least twice the calculated dissipation to ensure long-term reliability and avoid overheating. Common ratings are ⅛ W, ¼ W, ½ W, and 1 W; for most low-power LED circuits a standard ¼ W metal-film resistor is sufficient.
Can I connect multiple LEDs in parallel instead of in series with one resistor?
Parallel wiring is generally discouraged without individual resistors for each LED branch, because even small manufacturing variations in forward voltage cause unequal current sharing — the LED with the lowest V_f hogs most of the current and fails first. If you must use parallel LEDs, give each one its own series resistor calculated for that branch's current. Series wiring (the configuration this calculator targets) is preferable because the same current flows through every LED, ensuring uniform brightness and balanced thermal stress. For large arrays, a constant-current LED driver IC is the most reliable solution.