LED Resistor Calculator
Resistor value, wattage, and actual current for any LED.
What it does
LEDs pass whatever current the circuit allows, so a series resistor sets it by dropping the leftover volts. Supply minus LED voltage divided by target current gives exact ohms, rounded up to the nearest standard value.
The tool above takes supply, LED forward voltage, and target current. It returns the E12 resistor, wattage rating with margin, and actual current.
Forward voltage varies by color from the same current: reds near two volts while blues and whites need three plus. Datasheets pin the exact value per part number.
Wattage doubles as insurance. Resistors running near rating drift with heat, so ratings at least double the dissipation keep values stable.
How to use
- Enter the supply voltage feeding the LED.
- Enter the LED forward voltage from its datasheet.
- Enter target current, usually 20 mA.
- Buy the listed standard resistor at the wattage shown.
Formula
- supply - LED
- Volts the resistor must drop.
- E12
- Standard values: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 per decade.
- 2x P
- Heat margin rule for the wattage rating.
Parallel LEDs each want their own resistor. Sharing one resistor unbalances brightness as parts warm differently.
Worked example
Example: 9 V supply, 2 V LED, 20 mA
Battery 9 V, red LED at 2 V, target 20 mA.
- Exact: (9 - 2) / 0.020 = 350 Ω.
- Standard up: 390 Ω in E12.
- Actual: 7 / 390 = 17.9 mA.
- Power: 0.02² x 390 = 0.156 W, so 1/2 W rating with margin.
Reference table
| Supply | Red 2 V | White 3.2 V |
|---|---|---|
| 5 V | 150 Ω | 140 Ω -> 150 Ω |
| 9 V | 350 Ω -> 390 Ω | 290 Ω -> 330 Ω |
| 12 V | 500 Ω -> 560 Ω | 440 Ω -> 470 Ω |
Limits
- Single LED math. Strings need summed voltages.
- Datasheet values assumed exact.
- High power LEDs need drivers, not resistors.
- Heat sinking excluded from resistor math.
Common mistakes
- Skipping the resistor on low voltage logic.
- Rounding resistor values down instead of up.
- Sharing one resistor across parallel LEDs.
- Sizing watts exact with no heat margin.
- Guessing LED voltage by color alone.
Try it yourself
Quick checks. Answers sit tucked under each question.
5 V, 2 V LED, 20 mA?
(5-2)/0.02 = 150 Ω exact. Use 150 Ω.
12 V, 3.2 V white, 20 mA?
(12-3.2)/0.02 = 440 Ω. Use 470 Ω.
Questions
Why do LEDs need resistors?
LEDs limit no current themselves. The resistor drops extra volts at a safe current.
What resistor for 5 V and red LED?
Near 150 Ω at 20 mA: (5-2)/0.02.
Can I use a bigger resistor?
Yes, dimmer but safer. Smaller risks burnout.
What wattage do I need?
At least double the dissipation: 0.156 W needs 1/2 W rating here.
Do all colors need the same?
No. Forward voltages differ by chemistry, so compute per part.
Why is actual current lower?
Rounding the resistor up drops a little more volts, trimming current.