Ohm's Law Calculator · Practical guide
What Size Resistor Do I Need for an LED?
One Ohm's Law calculation picks the right current-limiting resistor for any LED circuit.
At a glance
The Short Answer
Use R = (supply voltage − LED forward voltage) ÷ LED current. For a typical red LED (2 V, 20 mA) on a 9 V battery: R = (9 − 2) ÷ 0.02 = 350 Ω. Since 350 Ω is not a standard value, use the next size up — 390 Ω — or the common 330 Ω for slightly more brightness.
Interactive tool
Ohm's Law Calculator
Enter your own values, then compare the result with the examples below.
Ohm's Law Results
Common LED Resistor Values
| Supply | LED (2 V, 20 mA) | Calculated R | Standard Choice |
|---|---|---|---|
| 3 V coin cell | Red LED | 50 Ω | 51 Ω |
| 5 V (USB/Arduino) | Red LED | 150 Ω | 150 Ω or 220 Ω |
| 9 V battery | Red LED | 350 Ω | 330 Ω or 390 Ω |
| 12 V (car) | Red LED | 500 Ω | 510 Ω |
Why the Resistor Is Not Optional
An LED barely limits its own current: connect one directly to a supply above its forward voltage and it draws a destructive surge within seconds. The resistor absorbs the leftover voltage — 7 V in the 9 V example — and Ohm's Law fixes the current at the LED's rated 20 mA. Blue and white LEDs have higher forward voltages (about 3–3.4 V), so recalculate with the LED's actual spec.
Check the Resistor Wattage Too
The resistor also dissipates heat: P = I² × R = 0.02² × 350 = 0.14 W in the 9 V example, safely inside a standard ¼-watt (0.25 W) resistor. For high-current circuits, verify the power rating with the calculator above — enter the voltage across the resistor and the current, and it returns resistance and wattage together.
Frequently Asked Questions
What happens if I use no resistor with an LED?
The LED draws far more than its rated current, overheats, and burns out — often in seconds. The only common exception is a 3 V coin cell, whose internal resistance sometimes limits current enough for a brief test.
Should I round the resistor value up or down?
Rounding up to the next standard value keeps current safely below the LED's rating at a small cost in brightness. Rounding down increases brightness but shortens LED life — stay at or above the calculated value when in doubt.
Do LEDs in series share one resistor?
Yes. Add the forward voltages of the series LEDs, subtract from the supply, and divide by the current. Two 2 V LEDs on 9 V need R = (9 − 4) ÷ 0.02 = 250 Ω.
What is LED forward voltage?
It is the voltage the LED itself drops when lit — roughly 1.8–2.2 V for red and yellow, around 3–3.4 V for blue, white, and green. The exact figure is in the LED's datasheet.