LED Series-Resistor Calculator
Enter your supply voltage and the LED’s forward voltage and current to get the resistor value, its power rating, and the nearest standard part.
Resistor needed
Sizing the resistor and checking its power rating
Once you know the resistance, check the power it needs to dissipate too: P = I² × R. Most through-hole resistors are rated for a quarter watt (0.25W), which comfortably covers the vast majority of single-LED indicator circuits, but it’s worth checking — higher current or a bigger voltage drop across the resistor can push past that rating.
If you’re driving several LEDs from the same supply, each one generally gets its own series resistor sized the same way, rather than sharing a single resistor across LEDs wired in parallel.
Frequently Asked Questions
Why does an LED need a series resistor at all?
An LED's voltage barely changes once it starts conducting, so without something to limit the current, it will draw more and more current as the supply pushes past its forward voltage — usually destroying the LED within moments. A series resistor is the simplest way to cap that current at a safe level.
How is the resistor value calculated?
Subtract the LED's forward voltage from the supply voltage to get the voltage the resistor must drop, then divide by the target current: R = (V_supply − V_forward) ÷ I_forward. A 9V supply driving a 2V red LED at 20mA needs a 350Ω resistor.
Why round up to the nearest standard resistor value?
Resistors are manufactured in a standard set of values (the E12 series: 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 ×10 multiples), so an exact calculated value like 350Ω usually isn't available off the shelf. Rounding up to the next standard value (390Ω) slightly reduces the current below the target, which is the safe direction — rounding down would push the LED harder than intended.
Where do I find an LED's forward voltage?
It depends on the LED's color and material: roughly 1.8–2.2V for red and amber, 2.0–2.2V for yellow and green, and around 3.0–3.4V for blue and white. Check the datasheet for your specific part where possible — this is only a typical range.
Educational tool for low-voltage LED circuits only. Always check your specific LED’s datasheet for its actual forward voltage and maximum current rating.