LED Resistor Calculator

What is an LED resistor calculator?

Every LED needs a current-limiting resistor to prevent it from drawing too much current and burning out. This calculator determines the correct resistor value based on your supply voltage, LED forward voltage, and desired LED current. Without the resistor, the LED would try to draw unlimited current and destroy itself almost instantly.

The formula is simple: R = (Vsupply - Vforward) / Iled. For a typical red LED (2.0V forward voltage) on a 5V supply at 20mA: R = (5 - 2) / 0.020 = 150 ohms. The calculator also shows the power dissipated in the resistor so you can choose the correct wattage rating.

LED forward voltages by color

• Red: 1.8–2.2V typical. The lowest forward voltage of common LEDs.
• Yellow/Orange: 2.0–2.2V. Similar to red LEDs.
• Green: 2.0–3.5V. Varies widely depending on the specific LED chemistry.
• Blue: 3.0–3.5V. Higher voltage due to the wider bandgap semiconductor.
• White: 3.0–3.5V. White LEDs are actually blue LEDs with a phosphor coating.

How to use this tool

Enter your supply voltage, the LED forward voltage (check the datasheet or use the typical values above), and the desired LED current (typically 10–20mA for standard LEDs). The calculator shows the required resistance and the nearest standard resistor values. It also displays power dissipation for both the resistor and the LED.

Multiple LEDs in series

You can drive multiple LEDs with one resistor by connecting them in series. Add the forward voltages together: three red LEDs at 2.0V each need (5V - 6.0V) / 0.020 = negative, which means 5V is not enough for three red LEDs in series. You need a supply voltage higher than the sum of all LED forward voltages.

Worked example

Drive a red LED (forward voltage 2.0 V, current 20 mA) from a 5 V supply. The resistor must drop 5 − 2.0 = 3.0 V at 0.02 A, so R = 3.0 ÷ 0.02 = 150 Ω. Its power dissipation is 3.0 × 0.02 = 0.06 W, so a standard quarter-watt resistor is comfortably rated. Pick the nearest standard value, 150 Ω, to stay safe.

Common mistakes

The biggest mistake is connecting an LED without any resistor; its current then rises uncontrollably and burns it out almost instantly. Another is using the supply voltage instead of the voltage across the resistor in Ohm's law — you must subtract the LED's forward voltage first. Different LED colours have different forward voltages, so a value that works for red may under-drive a blue LED.

Frequently asked questions

What happens if I use a resistor that is too large?

The LED will be dimmer than intended because less current flows through it. A slightly larger resistor is always safer than a smaller one — a dim LED is better than a dead LED. Many circuits deliberately use a larger resistor to extend LED life or reduce power consumption.

Can I use an LED without a resistor on a 3V coin cell battery?

In practice, many hobbyists do this because coin cell batteries have high internal resistance that limits current naturally. However, this is not recommended for a reliable design — the LED may be over-driven initially and the brightness will drop rapidly as the battery discharges. A small resistor (47–100 ohms) is always a better practice.

Can I put several LEDs on one resistor?

In series you can, and it is efficient — one resistor limits the shared current, as long as the supply exceeds the sum of the LEDs' forward voltages. Wiring LEDs in parallel on a single resistor is discouraged, because small differences between LEDs make them share current unevenly, and one may hog most of it.