Free LED Resistor Calculator – Size a Current-Limiting Resistor

Sizing a Current-Limiting Resistor for an LED

An LED needs a series resistor to survive, because it behaves like a diode, not a fixed resistance. Wire one straight across a supply and it will gulp far more current than it can handle and burn out in an instant. A resistor in series soaks up the excess voltage and holds the current to a safe level. This calculator works out the right resistor with the formula R = (V_supply − V_LED) ÷ I_LED, taking the guesswork out of a step that trips up almost every beginner in electronics. Each term has a clear meaning. V_supply is the voltage of your power source, such as a 9-volt battery. V_LED is the LED's forward voltage — the voltage it naturally drops when lit, which depends on its colour. I_LED is the current you want to run through it, given in amps in the formula (so 20 milliamps becomes 0.02 A). The tool subtracts the LED's forward voltage from the supply to find the voltage the resistor must drop, then divides that by the target current. Typical forward voltages are about 1.8 V for red, 2.1 V for yellow, 2.2 V for green, and around 3.2 V for blue and white, and 20 mA is a safe current for a standard 5 mm LED. Here is a worked case. Drive a red LED (forward voltage 2 V) from a 9-volt supply at 20 mA. The resistor must drop 9 − 2 = 7 volts, and at 0.02 amps that means R = 7 ÷ 0.02 = 350 ohms. Since 350 is not a standard value, you would pick the nearest standard part at or above it, commonly 360 ohms in the E24 series, which nudges the current a touch below 20 mA for safety. The resistor dissipates about 7 × 0.02 = 0.14 watts, so a common quarter-watt resistor is comfortably rated for the job. With 360 ohms fitted, the actual current settles at 7 ÷ 360 = 19.4 milliamps, close enough that the LED brightness is indistinguishable. Run the same red LED from a 5-volt board instead and you need only (5 − 2) ÷ 0.02 = 150 ohms. This is bread-and-butter work for indicator lights, hobby electronics, Arduino projects, decorative lighting, and panel LEDs. The tool reports the exact value, the nearest standard E24 value you can actually buy, the power the resistor burns, and a suggested minimum power rating with headroom. Choosing the standard value just above the calculated one keeps the LED safely under its maximum current and lengthens its life. Consider three concrete jobs. An Arduino builder adding a status LED to a 5-volt pin uses 150 ohms for a red LED at 20 mA and keeps the pin well inside its current limit. Someone lighting a model railway signal from a 12-volt supply wires three white LEDs in series, adds their forward voltages to 9.6 volts, and needs only (12 − 9.6) ÷ 0.02 = 120 ohms for the whole string, dissipating a tiny 0.048 watts. A panel builder driving an indicator from a 24-volt industrial rail lands on 1,100 ohms and 0.44 watts, which means a half-watt resistor is the minimum sensible choice. A few guardrails keep results sensible. If the LED's forward voltage is equal to or greater than the supply, there is not enough headroom to light it, and the tool says so rather than returning nonsense; it also rejects a current of zero. For several LEDs, calculate per branch — LEDs in series add their forward voltages and share one resistor, while parallel LEDs each need their own resistor to avoid uneven current sharing. The classic beginner error is entering the current in milliamps where the formula expects amps — typing 20 instead of 0.02 gives a resistance a thousand times too small and destroys the LED on first power-up. Also resist the temptation to push current to the absolute datasheet maximum; running a little under keeps the LED cooler and dramatically extends its life. Treat the figures as a design guide, check your LED datasheet for true ratings, and if the circuit connects to mains in any way, follow local electrical code and have a qualified electrician handle that side. Everything runs locally in your browser.

LED Resistor Calculator Formula & Method

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