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LED Resistor Calculator
Use this free LED resistor calculator to find the right series resistor for any LED. Enter your supply voltage, pick the LED color or type in its forward voltage, and choose the current you want. The calculator gives the exact resistor value, the nearest standard E12 values, the power the resistor must handle, and the wattage rating to buy — with the full working shown.
Most small indicator LEDs are rated for 20 mA. High-brightness and power LEDs can take much more — always check the datasheet.
How it works
An LED is not like a light bulb. A bulb limits its own current because its filament has resistance, but an LED has almost no resistance once it starts conducting, so it will happily draw far more current than it can survive. The fix is a series resistor: a simple component placed in line with the LED that drops the extra voltage and sets the current to a safe value. The whole calculation is one application of Ohm's law.
First, find the voltage the resistor must drop. The supply voltage is shared between the resistor and the LED: the LED takes its forward voltage, and the resistor takes whatever is left. With a 9 V supply and a 2 V red LED, the resistor drops 7 V. Then divide that voltage by the current you want, in amps, to get the resistance in ohms. At 20 mA, which is 0.02 A, that gives 7 / 0.02 = 350 ohms. This is the exact value; real resistors come in standard steps, so you round to the nearest available one.
Forward voltage depends on the LED's color, because it is set by the semiconductor material. Red and amber LEDs are about 2.0 V, yellow about 2.1 V, green about 2.2 V, and blue and white about 3.2 V. The presets in the calculator use these typical values, but real parts vary by a few tenths of a volt, so for a precision design check the datasheet and use the custom option. The current is your brightness control: 10 mA gives a dim glow with a long life, 20 mA is the standard for small indicator LEDs, and 30 mA is very bright. High-power LEDs for lighting can take hundreds of milliamps or more, and those need a proper driver rather than a simple resistor.
Resistors are sold in standard series, and the most common is E12, which has twelve values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82, multiplied by powers of ten. The calculator shows the nearest E12 value above and below your exact result. Always round up: a 390 ohm resistor in place of a calculated 350 gives a slightly lower current, so the LED runs a little dimmer and cooler, which is safe. Rounding down to 330 ohms would push the current above your target and risk the LED.
Finally, check the power rating. The resistor turns the voltage it drops into heat, and the power is that voltage times the current: 7 V × 0.02 A = 0.14 W. A resistor run at its maximum rating gets very hot and fails early, so the rule is to choose a rating at least twice the dissipated power. Twice 0.14 W is 0.28 W, so the next standard rating, 1/2 W, is the right buy. The common ratings are 1/8 W, 1/4 W, 1/2 W, 1 W, and 2 W, and the calculator picks the smallest one that meets the doubled-power rule.
Worked example
You have a 9 V battery, a red LED with a forward voltage of 2.0 V, and you want it to run at the standard 20 mA. Which resistor do you need, and what wattage?
Step 1: Voltage across the resistor — 9 V − 2 V = 7 V.
Step 2: Convert the current to amps — 20 mA = 0.02 A.
Step 3: Resistance — R = 7 / 0.02 = 350 Ω.
Step 4: Nearest standard E12 values — 330 Ω below and 390 Ω above, so buy the 390 Ω resistor.
Step 5: Power dissipated — P = 7 V × 0.02 A = 0.14 W.
Step 6: Wattage rating — double the power for safety, 2 × 0.14 = 0.28 W, so use a 1/2 W resistor.
Enter 9 V supply, red LED, and 20 mA in the calculator and it gives exactly the same result: 350 Ω, nearest E12 up 390 Ω, power 0.14 W, recommended 1/2 W. With the 390 Ω resistor fitted, the actual current is 7 / 390 = 17.9 mA, slightly under the 20 mA target, which is exactly the safe direction. Try a 5 V supply with a white LED at 20 mA as a second check: (5 − 3.2) / 0.02 = 90 Ω, nearest E12 up 100 Ω.
FAQ
What resistor do I need for an LED?
Subtract the LED forward voltage from the supply voltage, then divide by the LED current in amps: R = (supply voltage - forward voltage) / current. For a 9 volt supply, a 2 volt LED, and 20 mA, that is (9 - 2) / 0.02 = 350 ohms. Then pick the next standard value up, which is 390 ohms in the E12 series.
What is the forward voltage of an LED?
Forward voltage depends on the LED color: red and amber LEDs are about 2.0 volts, yellow is about 2.1 volts, green is about 2.2 volts, and blue and white LEDs are about 3.2 volts. The exact value varies by part, so check the datasheet when you need precision, and use the typical value here for a quick estimate.
Can I use a bigger resistor than calculated for an LED?
Yes. A larger resistor lowers the current, so the LED runs dimmer but cooler and lasts longer. That is why you round up to the next standard resistor value instead of down. A smaller resistor than calculated pushes more current than planned, which can overheat and destroy the LED, so never round down unless you have checked the new current.
What wattage resistor do I need for an LED?
Multiply the voltage across the resistor by the current to get the power it dissipates, then choose a resistor rated for at least twice that power. For a 9 volt supply with a 2 volt LED at 20 mA, the resistor drops 7 volts times 0.02 amps = 0.14 watts, so a 1/2 watt resistor is the safe choice, since twice 0.14 is 0.28 watts.
What happens if I connect an LED without a resistor?
An LED without a current-limiting resistor will draw far more current than it can handle and usually burn out within seconds, sometimes with a pop and a puff of smoke. LEDs do not limit their own current the way a light bulb does, so a series resistor, or a proper constant-current driver, is always required.