# LED Resistor Calculator

Calculate the resistor needed for an LED circuit. Enter supply voltage, LED color, and current to get the exact and nearest standard E24 resistor value with power rating.

## What this calculates

Every LED needs a current-limiting resistor to avoid burning out. This calculator tells you exactly what resistor to use based on your supply voltage, LED type, and desired brightness. It also rounds up to the nearest standard E24 value and checks the power rating so your resistor does not overheat.

## Inputs

- **Supply Voltage** (V) — min 0 — Common values: 3.3V, 5V, 9V, 12V
- **LED Color (preset)** — options: Custom voltage, Red (2.0V), Orange (2.1V), Yellow (2.1V), Green (2.2V), Blue (3.2V), White (3.2V), UV (3.4V), Infrared (1.5V)
- **LED Forward Voltage** (V) — min 0 — Used when LED Color is set to Custom
- **LED Forward Current** (mA) — min 0
- **Number of LEDs in Series** — min 1, max 20

## Outputs

- **Exact Resistance** (Ω) — Calculated resistance value
- **Nearest Standard Resistor (E24)** (Ω) — Closest E24 series standard resistor value
- **Actual Current (with standard R)** (mA) — Current when using the nearest standard resistor
- **Power Dissipated by Resistor** (mW) — Power the resistor must handle
- **Suggested Power Rating** — formatted as text — Minimum resistor power rating to use

## Details

## The Formula

**R = (V_supply - V_LED) / I_LED**

The resistor drops the excess voltage so the LED receives its rated forward voltage at the correct current. Without it, the LED draws too much current and burns out almost instantly.

### Common LED Forward Voltages

| LED Color | Forward Voltage | Typical Current |
|-----------|----------------|-----------------|
| Red | 1.8 - 2.2V | 20 mA |
| Orange/Yellow | 2.0 - 2.1V | 20 mA |
| Green | 2.0 - 3.0V | 20 mA |
| Blue/White | 3.0 - 3.4V | 20 mA |
| Infrared | 1.2 - 1.6V | 20 - 50 mA |

### Worked Example

Running a red LED (2.0V, 20 mA) from a 5V Arduino pin:

- R = (5V - 2V) / 0.020A = 150 ohms exactly
- Nearest standard value: 150 ohms (it's in the E24 series)
- Power dissipated: 3V x 0.02A = 60 mW (a 1/8W resistor is fine)

### Multiple LEDs in Series

When wiring LEDs in series, their forward voltages add up. Three red LEDs in series need 6.0V, so a 5V supply is not enough. You would need at least 7V or more. This calculator handles that math automatically.

## Frequently Asked Questions

**Q: Why do LEDs need a resistor?**

A: LEDs are current-driven devices with very low internal resistance. Without a resistor, the current shoots up uncontrollably and destroys the LED within milliseconds. The resistor limits current to a safe level (typically 20 mA for standard LEDs). Think of it as a speed limiter for electrons.

**Q: Should I round the resistor value up or down?**

A: Always round up. A slightly larger resistor means slightly less current, which makes the LED marginally dimmer but keeps it safe. Rounding down pushes more current through the LED than intended, shortening its lifespan. This calculator automatically rounds up to the nearest standard E24 series value.

**Q: What is the E24 resistor series?**

A: E24 is the most common set of standard resistor values. Each decade (1-10, 10-100, etc.) has 24 values spaced roughly 10% apart. Common values include 100, 150, 220, 330, 470, 680 ohms. These are what electronics stores stock, so using a standard value means you can actually buy the part.

**Q: Can I use one resistor for multiple LEDs in parallel?**

A: Technically yes, but it is a bad idea. LEDs have slightly different forward voltages even within the same batch. With a shared resistor, one LED hogs more current and burns brighter (and hotter) while others stay dim. Use individual resistors for each parallel LED, or wire them in series with a single resistor if your voltage is high enough.

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Source: https://vastcalc.com/calculators/physics/led-resistor
Category: Physics
Last updated: 2026-04-08