BYTETOOLS

LED Resistor Calculator

Calculate the series resistor for any LED from supply voltage, forward voltage and current. Get the nearest E12/E24 standard value and required wattage.

150 Ω
Exact resistor
3 V
Resistor drop
60 mW
Dissipation
1/8 W
Suggested rating

Standard resistor values (rounded up for safety)

E12: 150 Ω — actual LED current ≈ 20 mA

E24: 150 Ω — actual LED current ≈ 20 mA

R = (Vs − Vf) ÷ I = (5 − 2) ÷ 0.02 = 150 Ω. Standard values are rounded up so the LED never exceeds your target current.

What is the LED Resistor Calculator?

The ByteTools LED Resistor Calculator finds the current-limiting resistor an LED needs: R = (Vsupply − Vled) ÷ Iled.

  • R = (Vs − Vf) / I with full working shown
  • Forward-voltage presets for red, yellow, green, blue, white, IR and UV LEDs
  • Nearest E12 and E24 standard resistor suggestions
  • Actual current recomputed for each suggested resistor
  • Resistor power dissipation with a recommended wattage rating
  • 100% private — runs entirely in your browser

How to use the LED Resistor Calculator

  1. 1

    Enter the supply voltage (e.g. 5 V or 12 V).

  2. 2

    Pick an LED color preset or enter the forward voltage from your datasheet.

  3. 3

    Set the desired LED current in mA (20 mA is typical for indicator LEDs).

  4. 4

    Read the exact resistance, nearest E12/E24 values and resistor wattage.

  5. 5

    Copy the result or adjust values to compare options.

About the LED Resistor Calculator

The ByteTools LED Resistor Calculator finds the current-limiting resistor an LED needs: R = (Vsupply − Vled) ÷ Iled. Enter your supply voltage, pick an LED color preset (or type a custom forward voltage) and a target current, and the tool returns the exact resistance, the nearest E12 and E24 standard resistor values, and the power the resistor must dissipate.

LED forward voltages in the presets are typical values — around 2.0 V for red, 2.1 V for yellow, 2.2 V for green and 3.2 V for blue and white — but real parts vary by manufacturer, so check your datasheet for precision work. The calculator also recomputes the actual current you will get with each suggested standard resistor.

Everything runs 100% locally in your browser with nothing uploaded, so it is fast, private and usable offline at the workbench.

Frequently asked questions

How do I calculate the resistor for an LED?

Subtract the LED's forward voltage from the supply voltage, then divide by the desired current: R = (Vs − Vf) ÷ I. For a red LED (2 V) on a 5 V supply at 20 mA, R = (5 − 2) ÷ 0.02 = 150 Ω — which is conveniently a standard E24 value.

What happens if I use a bigger resistor than calculated?

The LED current drops, so the LED runs dimmer but cooler and lasts longer. Rounding up to the next standard value is the safe choice. Rounding down increases current and can shorten the LED's life or destroy it if you go too far.

What forward voltage should I use for my LED?

Typical values are about 1.8–2.2 V for red, orange and yellow, 2.0–3.0 V for green, and 3.0–3.4 V for blue, white and UV. These presets are estimates — the exact figure is on the LED's datasheet and varies with current and temperature.

What wattage resistor do I need for an LED?

The resistor dissipates P = (Vs − Vf) × I. In the 5 V red LED example that is 3 V × 0.02 A = 0.06 W, so a common 1/4 W (0.25 W) resistor has plenty of margin. A good rule is to pick a rating at least double the calculated dissipation.

Can I run several LEDs from one resistor?

LEDs in series through one resistor work well: add up their forward voltages and use the same formula, as long as the supply exceeds the total. Parallel LEDs sharing one resistor is poor practice, because manufacturing differences make one LED hog the current.

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