1Red LED on 5V (Vf=2V, 20mA)
Inputs
Supply5 V
Forward V2.0 V
Current20 mA
Result
Standard Resistor150Ω
Exact150Ω
Power60 mW
R = (5-2)/0.02 = 150Ω. Nearest E24 value is 150Ω. P = 0.02² × 150 = 60mW.
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LED Resistor Calculator
Resistor Value
150Ω
Exact
150Ω
Power
60 mW
Standard Resistor Value
150Ω
Exact: 150Ω
Actual Current
20.0 mA
Power
60 mW
What You'll Need
300 Pcs 3mm & 5mm LED Diode Assortment Kit
$7.99
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DiCUNO 450pcs 5mm LED Diode, LED Light Kit Box 5 Colors, for DIY Project
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EEEEE 420pcs 6 Colors 5mm LED Diode Assortment LED Light Kit Box Multi Colored mini LEDs White Red Yellow Orange Green Blue Light Emitting lamp diodes Components Parts ledlights Circuit
$9.99
View on Amazon300 Pcs 3mm & 5mm LED Diode Assortment Kit
$7.99
View on AmazonDiCUNO 450pcs 5mm LED Diode, LED Light Kit Box 5 Colors, for DIY Project
$11.99
View on AmazonEEEEE 420pcs 6 Colors 5mm LED Diode Assortment LED Light Kit Box Multi Colored mini LEDs White Red Yellow Orange Green Blue Light Emitting lamp diodes Components Parts ledlights Circuit
$9.99
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Price as of Jul 24, 2026. Product prices and availability are accurate as of the date indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
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Example Calculations
2White LED on 12V (Vf=3.2V, 20mA)
Inputs
Supply12 V
Forward V3.2 V
Current20 mA
Result
Standard Resistor470Ω
Exact440Ω
Power176 mW
R = (12-3.2)/0.02 = 440Ω. Next E24 value is 470Ω. P = 0.02² × 470 = 188mW.
Frequently Asked Questions
QHow do I calculate the resistor for an LED?
R = (Vsupply - Vforward) / Iled. For a 5V supply, red LED (2V forward, 20mA): R = (5-2)/0.02 = 150Ω. Always use the next higher standard value for safety.
- R = (Vsupply - Vf) / If
- 5V, red LED: (5-2)/0.02 = 150Ω
- 12V, white LED: (12-3.2)/0.02 = 440Ω
- Use next higher E24 standard value
| LED Color | Forward Voltage | Typical Current | Resistor (5V) | Resistor (12V) |
|---|---|---|---|---|
| Red | 2.0V | 20mA | 150Ω | 510Ω |
| Green | 2.1V | 20mA | 150Ω | 510Ω |
| Blue | 3.3V | 20mA | 82Ω | 430Ω |
| White | 3.2V | 20mA | 91Ω | 430Ω |
QWhat happens if I use too small a resistor?
Too little resistance means too much current. The LED overheats, dims prematurely, and eventually burns out. Even a few milliamps over rated current can halve LED lifespan.
- Excess current = overheating
- LED lifespan drops exponentially
- No resistor = instant LED death on most supplies
- Always round up to next standard value
QCan I use one resistor for multiple LEDs?
For LEDs in series: R = (Vsupply - N×Vf) / If, one resistor works. For parallel LEDs, each needs its own resistor because Vf varies slightly between LEDs.
- Series: one resistor for the string
- Series max LEDs: floor(Vsupply / Vf)
- Parallel: each LED needs its own resistor
- Parallel with shared resistor: uneven brightness
QWhat resistor power rating do I need?
P = I² × R. For 20mA through 150Ω: P = 0.02² × 150 = 60mW. A standard 1/4W (250mW) resistor is fine. Derate by 50% for reliability.
- P = I² × R in watts
- 20mA + 150Ω = 60mW
- 1/4W (250mW) covers most LED circuits
- For high-power LEDs: use 1/2W or 1W resistors
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Last Updated: Jul 24, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.