LED Resistor Calculator
Calculate the correct resistor for your LED circuit. Works for single LEDs, series strings, parallel arrays, and mixed configurations. Beginner-friendly — hover any ? icon for explanations.
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How to Use This LED Resistor Calculator
This calculator sizes the series resistor an LED needs to operate safely on a given voltage source. Enter the supply voltage, select your LED (or enter the forward voltage and drive current manually), and the calculator returns the resistor value in ohms and the recommended power rating in watts. The math behind it is Ohm's Law applied to LED circuits, and the calculator uses the same formula a hand calculation would — it just looks up our catalog forward voltages and brightness data so you don't have to.
Every standard LED needs a current-limiting resistor when connected to a voltage source higher than its forward voltage rating. Without one, the LED draws more current than its rated drive current, heats up beyond the chip's thermal limits, and either burns out within seconds or degrades over hours. The resistor drops the excess voltage and limits current to the manufacturer's specified drive current — typically 20mA for standard through-hole and SMD LEDs.
The Math: Ohm's Law for LED Resistor Sizing
The formula for an LED current-limiting resistor is a direct application of Ohm's Law (V = I × R), rearranged to solve for resistance:
R = (Vsupply − Vforward) ÷ Idrive
Where:
- Vsupply is the source voltage (5V from an Arduino or USB, 9V from a battery, 12V from a car or DC adapter, etc.)
- Vforward is the LED's rated forward voltage, which varies by chip color and material (1.8–2.2V for red/orange/yellow, 3.0–3.4V for blue/green/white, 3.2–3.6V for UV, around 3.0V for pink)
- Idrive is the LED's rated drive current, almost always 20mA (0.020A) for standard 3mm and 5mm through-hole LEDs and most SMD packages
The power rating for the resistor follows from P = (Vsupply − Vforward) × Idrive. A standard 1/4 watt resistor handles most calculations below 12V comfortably; 1/2 watt is the safe choice at 15V or when running multiple LEDs through a shared resistor.
Forward Voltage by LED Color
Different LED semiconductor materials produce different colors and different forward voltage requirements. A quick reference for the most common LEDs we stock:
| LED Color | Forward Voltage | Wavelength | Typical Drive Current |
|---|---|---|---|
| Red | 1.8–2.1V | 620–630nm | 20mA |
| Orange / Amber | 1.9–2.1V | 605–620nm | 20mA |
| Yellow / Gold | 2.0–2.2V | 585–595nm | 20mA |
| Pure Green | 3.0–3.4V | 515–525nm | 20mA |
| Blue | 3.0–3.4V | 460–470nm | 20mA |
| Cool White / Warm White | 3.0–3.4V | n/a (white) | 20mA |
| Pink | 3.0–3.2V | n/a (white + filter) | 20mA |
| UV Purple (true UV) | 3.2–3.6V | 395–405nm | 20mA |
Forward voltage is also temperature-dependent — typical specifications are at 25°C. In a hot enclosure or near a heat source, the actual forward voltage may drift 0.05–0.1V lower, which causes higher current draw at the same supply voltage. For automotive and outdoor applications where temperatures swing 50°C or more, choose a resistor that's slightly larger than the calculated minimum so the LED stays well under its absolute maximum current rating.
Worked Examples
5V Supply (Arduino, USB, microcontroller)
Powering a red 3mm LED from a 5V Arduino GPIO pin: (5V − 2.0V) ÷ 0.020A = 150Ω. Power dissipated in the resistor: (5V − 2.0V) × 0.020A = 0.06W. A standard 1/4 watt (0.25W) resistor handles this with plenty of headroom.
Powering a blue 5mm LED from the same 5V source: (5V − 3.2V) ÷ 0.020A = 90Ω. The nearest standard value is 100Ω. Power: (5V − 3.2V) × 0.020A = 0.036W. Again, a 1/4 watt resistor is fine.
9V Supply (battery, 9V wall adapter)
Red LED on 9V: (9V − 2.0V) ÷ 0.020A = 350Ω. Standard value: 360Ω 1/4W. Power: 0.14W — well within the 0.25W rating.
White or blue LED on 9V: (9V − 3.2V) ÷ 0.020A = 290Ω. Standard value: 300Ω 1/4W. Power: 0.116W.
12V Supply (automotive, marine, DC adapter)
Red LED on 12V: (12V − 2.0V) ÷ 0.020A = 500Ω. Standard value: 510Ω 1/4W. Power: 0.20W — close to the 0.25W rating, so 1/4W is acceptable but 1/2W gives more margin.
White or blue LED on 12V: (12V − 3.2V) ÷ 0.020A = 440Ω. Standard value: 470Ω 1/4W. Power: 0.176W.
For 12V automotive use specifically, consider an LED with a built-in resistor instead. Lighthouse LEDs stocks 5mm and 3mm cube LEDs with the resistor pre-installed in the package — rated for 7–15V operation directly without any external components.
15V Supply (automotive charging-system worst case)
Automotive electrical systems labeled "12V" actually run at 13.5–14.5V with the engine running, and can spike to 15V during alternator regulation. Sizing for 15V is the safe choice for any dashboard or under-hood circuit.
Red LED on 15V: (15V − 2.0V) ÷ 0.020A = 650Ω. Standard value: 680Ω 1/2W. Power: 0.26W — this exceeds 1/4W rating, so 1/2W is required.
White or blue LED on 15V: (15V − 3.2V) ÷ 0.020A = 590Ω. Standard value: 620Ω 1/2W. Power: 0.236W.
Series vs Parallel LED Circuits
When wiring multiple LEDs, you have two choices: in series (one after another) or in parallel (side by side). The calculation differs significantly between them, and one approach is almost always wrong.
Series Wiring (Preferred)
In a series circuit, the LEDs share a single current path. The forward voltages add together: three red LEDs in series on 12V drop 3 × 2.0V = 6.0V across the LEDs, leaving 6.0V for the resistor. Calculation: (12V − 6.0V) ÷ 0.020A = 300Ω. One resistor sizes the current for all three LEDs simultaneously.
Series is the preferred wiring method because it uses one resistor for the whole chain, dissipates less total power, and guarantees identical current through each LED. The downside: total forward voltage must be less than supply voltage minus a few volts of headroom for the resistor to drop.
Parallel Wiring (Use With Caution)
In parallel, each LED has its own current path. Manufacturing variation in forward voltage means LEDs in parallel never share current equally — the LED with the lowest forward voltage grabs disproportionately more current and burns out first, leaving the rest to handle even more current. The result is a cascade failure.
The correct approach for parallel LEDs: give each LED its own resistor. Yes, that means more components, but it's the only configuration that guarantees long LED life. Each branch is a separate calculation using the formula above.
Frequently Asked Questions
What happens if I use a smaller resistor than calculated?
The LED draws more current than its rated drive current. In the short term it appears brighter; over time the LED chip overheats and degrades. A 20mA-rated LED at 30mA will lose perhaps half its rated lifespan; at 50mA it can burn out within hours. The brief brightness boost isn't worth the reduced life.
What happens if I use a larger resistor?
The LED draws less current and operates dimmer. This is actually a useful technique for dimming — choose a resistor 50–100% larger than the minimum and the LED runs at 10–15mA, drawing less power and lasting significantly longer. The brightness reduction is roughly proportional to the current reduction, so 50% current gives roughly 50% brightness.
Do I need a resistor for LEDs in series if their total forward voltage matches the supply?
Yes — always. Even when the forward voltages appear to match the supply exactly, manufacturing tolerance, temperature drift, and supply voltage variation mean current can swing wildly without a resistor in the circuit. A small resistor (50–100Ω) acts as a current-stabilizing ballast even when the math says you don't need one.
What about LEDs with built-in resistors?
LEDs sold as "12V" or "with built-in resistor" have a current-limiting resistor inside the LED package or in-line with the lead, sized for the rated supply voltage. These plug directly into the supply with no external components. Lighthouse LEDs stocks pre-wired SMD LEDs with built-in resistors for 9–18V DC operation and 5mm through-hole LEDs with built-in resistors for 7–15V. The trade-off is they're locked to a single voltage range; for lower voltages or higher currents, a standard LED with a sized external resistor remains the right choice.
Can I use the calculator for LED strips?
No — LED strips already have resistors built into each LED segment, sized for the strip's rated voltage (usually 12V or 24V). Powering an LED strip is purely a matter of supplying the correct voltage at sufficient amperage; no external current-limiting resistor is required. The amperage needed depends on the strip length and LED density, not on forward voltage calculations.
What standard resistor values should I keep on hand?
For 1/4 watt resistors covering common LED applications: 150Ω, 220Ω, 300Ω, 360Ω, 470Ω, 510Ω, 680Ω, and 1kΩ will handle the majority of 5V, 9V, and 12V LED circuits. Add 1/2 watt 470Ω through 1kΩ for 15V automotive work where the power dissipation is higher. Lighthouse LEDs sells these values individually and as assortment packs.