What Resistor Do I Need for an LED?
Last reviewed: January 2026
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LEDs require a current-limiting resistor to prevent burnout. Too little resistance = LED dies instantly. The formula: R = (Vsupply − Vforward) / Icurrent. Standard LED current is 10–20mA for indicator LEDs, 350mA–3A for high-power LEDs. Always use the nearest E12 (standard) resistor value above your calculated value for safety. For multiple LEDs, wire them in series (adding forward voltages) or in parallel (same voltage, multiply current). Series is simpler but requires higher supply voltage.
| LED Color | Forward Voltage (V) | Typical Current (mA) | Resistor (5V supply) |
|---|---|---|---|
| Red | 1.8–2.2 | 20 | 140–160Ω |
| Yellow | 2.0–2.4 | 20 | 130–150Ω |
| Green | 2.0–3.5 | 20 | 75–150Ω |
| Blue | 3.0–3.5 | 20 | 75–100Ω |
| White | 3.0–3.5 | 20 | 75–100Ω |
The current-limiting resistor value for an LED is calculated using R = (V_supply − V_forward) / I_desired. V_supply is your power source voltage (5V USB, 9V battery, 12V automotive, etc.), V_forward is the LED's forward voltage drop (varies by color), and I_desired is the target current (typically 20 mA for standard indicator LEDs). The resistor absorbs the excess voltage as heat, preventing the LED from drawing too much current and burning out. Without a resistor, an LED connected directly to a voltage source will draw excessive current, overheat, and fail within seconds — sometimes spectacularly.
| LED Color | Vf | 5V Supply | 9V Supply | 12V Supply |
|---|---|---|---|---|
| Red (2.0V) | 2.0V | 150Ω | 350Ω | 500Ω |
| Yellow (2.1V) | 2.1V | 145Ω | 345Ω | 495Ω |
| Green (2.2V) | 2.2V | 140Ω | 340Ω | 490Ω |
| Blue (3.2V) | 3.2V | 90Ω | 290Ω | 440Ω |
| White (3.3V) | 3.3V | 85Ω | 285Ω | 435Ω |
Values calculated at 20 mA. Always use the nearest standard E12 resistor value equal to or above the calculated value — never below.
LEDs can be wired in series (daisy-chained) or parallel, and the resistor calculation differs significantly. In series, forward voltages add up: three red LEDs in series require R = (V_supply − 3 × 2.0V) / 0.02A. With a 12V supply: R = (12 − 6) / 0.02 = 300Ω. All LEDs share the same current, ensuring uniform brightness. However, the supply voltage must exceed the total forward voltage — three blue LEDs at 3.2V each need at least 9.6V, making a 9V battery insufficient. In parallel, each LED needs its own resistor calculated independently, and they all receive the same voltage. Parallel is more wasteful of resistors but allows each LED to operate independently — if one fails, the others continue working.
High-power LEDs (1W, 3W, 5W, and higher) draw significantly more current than indicator LEDs — typically 350 mA to 3A depending on the wattage. At these current levels, a simple resistor wastes enormous power as heat. A 1W white LED (3.3V, 350 mA) on a 12V supply would need a resistor dissipating (12−3.3) × 0.35 = 3.045 watts — more power wasted in the resistor than consumed by the LED. For this reason, high-power LEDs use constant current driver circuits (buck converters or linear regulators) that efficiently step down the voltage while maintaining precise current control. These drivers also protect against voltage fluctuations that would cause brightness variations with a simple resistor. For automotive LED projects, constant current drivers are essential because vehicle voltage fluctuates between 11.5V and 14.5V during normal operation.
LED brightness is roughly proportional to current up to the rated maximum. Running an LED at 10 mA instead of 20 mA produces approximately 50% of maximum brightness but significantly extends lifespan and reduces heat. Many decorative and indicator applications look fine at 5–10 mA, and reducing current to this range can extend LED life from 50,000 hours to over 100,000 hours. Dimming LEDs below about 1 mA causes visible flickering in some types. PWM (Pulse Width Modulation) dimming switches the LED on and off rapidly at full current — varying the duty cycle controls perceived brightness without changing the operating current, producing smoother dimming and more consistent color temperature than current reduction.
The resistor must be rated to handle the power it dissipates: P = I² × R. A 150Ω resistor carrying 20 mA dissipates P = (0.02)² × 150 = 0.06W — well within a standard 1/4W (0.25W) resistor's rating. However, for high-current applications, power dissipation can exceed small resistor ratings. Always choose a resistor rated at least 2× the calculated power dissipation for reliability and thermal margin. For related electrical calculations, see our Ohm's Law Calculator and Wire Gauge Calculator.
→ Always round up to the next standard value. If the calculation gives 143Ω, use 150Ω (the next E12 value). Using a slightly higher resistance reduces current slightly, which is always safer than exceeding the LED's rating.
→ Use series wiring when possible. Series LEDs share one resistor and guarantee equal current through each LED. Parallel LEDs with a single shared resistor will have unequal brightness due to manufacturing variations in forward voltage.
→ Check the LED datasheet. Forward voltage varies by manufacturer. A "white LED" might have Vf anywhere from 2.8V to 3.6V. Using the actual datasheet value produces more accurate calculations.
See also: Ohm's Law · Wire Gauge · Electricity Bill · Binary Calculator
LED strip lights (5V or 12V flexible strips) use built-in SMD resistors for each LED cluster, but voltage drop over long runs causes the LEDs at the far end to appear dimmer. For strips longer than 5 meters, power injection at both ends or at intermediate points is necessary to maintain uniform brightness. A 5-meter 12V strip drawing 1A per meter experiences approximately 0.5V drop per meter through the copper traces — by the fifth meter, voltage has dropped from 12V to about 9.5V, causing noticeably dimmer and color-shifted LEDs. For addressable LED strips like WS2812B, power distribution is even more critical because each LED passes power through to the next, and a single failed LED can break the data chain. Professional installers use 18–16 AWG power injection wires every 2–3 meters for consistent brightness across long installations. For wire sizing calculations, use our Wire Gauge Calculator.
Whether you are building a simple indicator circuit with a single LED and a battery, designing custom automotive accent lighting, creating an Arduino-controlled LED matrix, or installing architectural LED lighting, understanding the relationship between supply voltage, forward voltage, current, and resistance is essential for reliable, long-lasting LED circuits that perform as intended.
The simple resistor-LED circuit is often the first project for electronics beginners, but the underlying Ohm's Law calculation remains relevant through every level of complexity — from single-LED indicators on breadboards to professional architectural lighting installations with hundreds of high-power LEDs requiring careful thermal and electrical design.
See also: Wire Gauge Calculator · Ohm's Law Calculator · Resistor Color Code Calculator · Electrical Power Calculator
→ LEDs need current limiting. Without a resistor, an LED draws excessive current and burns out instantly. The resistor value = (Vsupply − Vled) / Iled. For 5V supply, red LED (2V), 20mA: R = (5−2)/0.02 = 150Ω.
→ LED color determines forward voltage. Red/yellow: 1.8–2.2V. Green: 2.0–3.0V. Blue/white: 3.0–3.5V. UV: 3.3–3.8V. Always check the datasheet for your specific LED.
→ Power LEDs need heat sinking. A 1W LED (350mA × 3.2V) generates significant heat. Mount on an aluminum heat sink and use a constant-current driver instead of a simple resistor for efficiency.
→ Series strings share one resistor. Three red LEDs in series (6V total drop) on 12V need R = (12−6)/0.02 = 300Ω. This is more efficient than giving each LED its own resistor. See our Ohm's Law Calculator for basic circuit math.
See also: Ohm's Law · Voltage Divider · Electricity Cost · Wire Gauge