LED Resistor Calculator | Size a Current-Limiting Resistor with Ohm's Law
Work out the series resistor that protects an LED from your supply voltage, the LED's forward voltage and its target current. Alongside the exact figure you also get the nearest real-world E24 value, the power the resistor dissipates, a recommended wattage, and the 4-band color code.
💡 About this tool
An LED can't limit its own current. Connect it straight to a supply above its forward voltage and it draws far more than its rated current, then burns out almost instantly. The fix is a series resistor that drops the extra voltage and sets the current.
The whole thing comes down to one equation: R = (Vs − n × Vf) / I, where Vs is the supply voltage, Vf is the forward voltage of one LED, n is how many LEDs sit in series, and I is the target current in amps. Drive a single red LED (Vf = 1.8 V) at 20 mA from 5 V and you get R = (5 − 1.8) / 0.02 = 160 Ω.
The calculator also handles the part beginners trip over: buying the part. Your exact value is usually an odd number, so it snaps to the closest E24 value — the 24-step-per-decade standard you actually find on the shelf. It computes the heat the resistor dissipates (P = I² × R) and recommends a wattage (1/16 W, 1/8 W, 1/4 W and up) with headroom. If the supply sits at or below the LEDs' combined forward voltage, it warns you that a resistor alone can't light them.
🧐 Frequently Asked Questions
Is a bigger resistor always safer? A larger resistor lowers the current, which reduces burnout risk but also dims the LED. Aim for the recommended current on the datasheet — usually around 20 mA for standard 5 mm LEDs.
Why use an E24 value instead of the exact result? Odd values like 160 Ω or 213 Ω often aren't stocked. E24 is the set of standard values that retailers actually carry, so picking the nearest one keeps your build reproducible.
How do I handle several LEDs? In series the forward voltages add up (n × Vf), so enter the count in "LEDs in series" and the tool subtracts the total before solving for R. For a parallel arrangement, give each branch its own resistor and calculate one branch at a time.
What is the gold band in the color code? On a 4-band resistor the fourth band is the tolerance. This tool assumes the common gold band (±5%). The first two bands are significant digits and the third is the multiplier (a power of ten).
How should I read the recommended wattage? Pick a rating with margin above the dissipated power. A common rule of thumb is to choose roughly double the calculated wattage so the resistor runs cool and lasts.
📚 Fun Facts
Forward voltage tracks color because it is set by the semiconductor's band gap. Red and yellow LEDs sit near 1.8–2.2 V, while green, blue and white land around 3 V. That is why the same supply and resistor push a different current through a blue LED than a red one. Datasheet Vf is only nominal — it drifts with temperature and from part to part — so when you need precisely matched brightness, a constant-current driver is often the better tool than a single resistor.