LED Resistor Calculator - Series Resistor Value
Calculate the required series resistor for an LED from supply voltage, LED forward voltage, and forward current, with nearest standard value.
Required Resistance
350.0 Ω
Nearest Standard Value
330 Ω
Power dissipation: 0.140 W (calculated) / 0.132 W (nearest value)
How to Calculate an LED Current-Limiting Resistor
LEDs need a series resistor to limit current to a safe level, since they don't have significant internal resistance of their own. Using Ohm's Law, the required resistance is the voltage dropped across the resistor (supply voltage minus the LED's forward voltage) divided by the desired forward current. The calculator then rounds to the closest standard E12-series resistor value, the common set of resistor values manufacturers actually produce.
Example
A 9V supply driving an LED with a 2V forward voltage at 20 mA needs R = (9 − 2) ÷ 0.02 = 350 Ω. The nearest standard E12 resistor value is 330 Ω, which would give a slightly higher current, or 390 Ω for a slightly lower, safer current — either is a reasonable practical choice.
Common Use Cases
- Choosing a resistor value for an LED in a hobby electronics or Arduino project.
- Verifying that an existing circuit's resistor is appropriately sized for the LED used.
- Learning how Ohm's Law applies to simple current-limiting circuits.
FAQs
What forward voltage should I use for my LED?
Forward voltage varies by LED color and type: roughly 1.8-2.2V for red/yellow, 3.0-3.4V for green/blue/white LEDs. Check your LED's datasheet for the exact value, since using an incorrect forward voltage will throw off the resistor calculation.
What is the E12 series, and why round to it?
The E12 series is a standardized set of 12 resistor values per decade (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, and their multiples of 10) that resistor manufacturers actually stock, so rounding your calculated value to the nearest E12 value gives you a resistor you can realistically buy.
What wattage rating does the resistor need?
Power dissipated is I² × R. Most small-signal LED circuits need well under a quarter watt, so a standard 1/4W resistor is usually sufficient, but always check the calculated power against your resistor's rated wattage, especially at higher currents.