LED Resistor Calculator

Supply voltage (V)

LED color

LED current (mA)

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Resistor needed (exact)

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Nearest standard (E12)

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Power dissipated

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Wattage rating (2× margin)

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Last updated: October 2026

What resistor do I need for my LED?

Short answer: for a red LED on 5V at 20mA you need a 150Ω resistor rated 1/8W or higher — (5−2)÷0.02 = 150Ω, and it dissipates just 0.06W. Always pick the nearest standard E12 value equal to or above the calculated resistance, then choose a wattage rating with at least double the dissipated power. The calculator above does all of this for you.

How to calculate the resistor for an LED

The formula is Ohm's law applied to the voltage the resistor must drop:

R = (Vs − Vf) ÷ I

Vs — your supply voltage (5V, 12V, 3.3V…)
Vf — the LED's forward voltage (depends on color: red ≈ 2.0V, white/blue ≈ 3.2V)
I — LED current in amps (20mA = 0.02A for a standard LED)

Resistors aren't sold in every value, so round up to the nearest standard E12 preferred value. Rounding down would push the current above your target. For background on the parts involved, see the light-emitting diode and resistor articles on Wikipedia.

What resistor do I need for LED Arduino 5V projects?

The classic Arduino case: a red LED (Vf ≈ 2.0V) driven from a 5V pin at 20mA. Applying the formula: (5 − 2) ÷ 0.02 = 150Ω — which happens to be a standard E12 value. It dissipates 3V × 0.02A = 0.06W, so a 1/8W resistor (0.125W ≥ 2 × 0.06W) is the safe, standard pick. For a blue or white LED on 5V: (5 − 3.2) ÷ 0.02 = 90Ω → round up to the E12 value 100Ω.

LED resistor calculator for 12V

On a 12V supply, a single red LED needs (12 − 2) ÷ 0.02 = 500Ω → E12 560Ω. It dissipates 10V × 0.02A = 0.2W, so use a 1/2W resistor (0.5W ≥ 2 × 0.2W) — a 1/4W resistor would run too hot. For 12V it's more efficient to wire LEDs in series so they share one resistor; see the next section.

How to calculate resistor for multiple LEDs in series

When LEDs are wired in series, the same current flows through all of them and their forward voltages add up: R = (Vs − N×Vf) ÷ I. Three white LEDs (3 × 3.2V = 9.6V) on 12V at 20mA need (12 − 9.6) ÷ 0.02 = 120Ω — one resistor for the whole string. The calculator handles this with the "N in series" tab. Important: if Vs is less than N×Vf, no resistor can make it work — you need a higher supply or fewer LEDs in the string.

How to use the calculator

1. Enter your supply voltage in volts. 2. Pick your LED color — the forward voltage fills in automatically, or choose "Custom…" and enter Vf from your LED's datasheet. 3. Set the LED current (20mA is standard for 5mm LEDs). 4. Choose the wiring: single LED, N LEDs in series, or N parallel strings of one LED. 5. Read off the exact resistor, the nearest buyable E12 value, the power dissipated, and the recommended wattage rating.

Note on parallel wiring: give each string its own resistor — the calculator assumes one resistor per string and reports the total supply current. See the FAQ below for why a single shared resistor is a bad idea.

Frequently asked questions

Why should I use the nearest higher E12 value instead of the closest one?

Because resistors are only sold in standard E12 values, and rounding down would push more current through your LED than you intended, shortening its life. Rounding up to the next standard value slightly lowers the current — the brightness difference is imperceptible, and your LED stays within its safe rating.

What wattage resistor do I need for an LED?

Multiply the voltage across the resistor by the LED current: P = Vdrop × I. For a red LED on 5V at 20mA that's 3V × 0.02A = 0.06W, so choose a rating at least double that — a 1/8W (0.125W) resistor is the standard safe pick. The calculator above applies this 2× safety margin automatically.

Can I use one resistor for several LEDs wired in parallel?

Give each parallel string its own resistor. LEDs never share current perfectly — tiny differences in forward voltage make one LED hog most of the current, burn out, and dump its share onto the rest until they all fail. In series, however, one resistor works fine because the same current flows through every LED.

My supply is only 3.3V. What resistor do I need?

The same formula still applies. A red LED (2.0V) at 20mA on 3.3V needs (3.3 − 2) ÷ 0.02 = 65Ω, so use the nearest higher standard value, 68Ω. With white or blue LEDs (about 3.2V) there's barely any voltage left to drop — use the calculator result but expect the current to be sensitive to small voltage changes, and consider a dedicated LED driver for stable brightness.

Is my data private?

Completely. Every calculation runs locally in your browser with JavaScript — nothing you enter is sent to a server, stored, or tracked.

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