Almost every LED circuit needs a current-limiting resistor in series, and this calculator works out the right value in a moment along with the power it will dissipate and the nearest standard resistor to use. Enter the supply voltage, the LED's forward voltage and the current you want to run it at, and it returns the required resistance, the power rating to look for, and the closest standard E12 value at or above the calculation, updating as you type. The reason a resistor is essential comes down to how an LED behaves. Unlike an ordinary resistor, an LED barely limits current itself once it turns on, so wiring it straight across a battery or supply lets the current run away and burns the LED out almost instantly. The series resistor takes up the leftover voltage and sets the current to a safe level. The maths is a direct application of Ohm's law: take the supply voltage, subtract the LED's forward voltage, which is the bit the LED itself drops, and divide what is left by the current you want, in amps. The calculator also reports the power the resistor must handle, the leftover voltage times the current, so you can pick a resistor with enough wattage, and it rounds up to a real, buyable value rather than leaving you with an awkward number. That makes it genuinely useful for hobbyists, makers and students building electronics projects, wiring up indicator lights, panels or Arduino circuits, and for anyone who needs a quick, safe resistor value without reaching for the formula. Typical LED forward voltages run from about 1.8 volts for red up to around 3.4 volts for blue and white, and common currents are 10 to 20 milliamps. The formula and a worked example are explained clearly below.
The resistor value is the supply voltage minus the LED forward voltage, divided by the current in amps (milliamps divided by 1,000). The power dissipated in the resistor is the voltage across it, supply minus LED voltage, times the current. The nearest standard value is the next E12 resistor at or above the calculated resistance.
For a 9 V supply, a 2 V LED and 20 mA: the resistor is (9 minus 2) divided by 0.02, which is 7 over 0.02, giving 350 ohms. The nearest standard value at or above is 390 ohms. The power is 7 times 0.02, which is 0.14 watts, so a quarter-watt resistor is fine.
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