LED Resistor Calculator

Size the series resistor for an LED from the supply voltage, LED forward voltage, and target current using R = (Vsupply - Vforward) / I.

This LED resistor calculator is for anyone wiring up an LED and wanting it to be bright without burning out. That includes electronics beginners on their first breadboard, makers adding indicator lights to a project, and hobbyists building displays or signage. An LED needs a resistor in series to limit its current, and choosing the wrong value either leaves the LED dim or destroys it in an instant. This tool gives the correct resistor in one step, plus the power that resistor must handle, so you can pick a part that will not overheat.

An LED drops a roughly fixed voltage when lit, called its forward voltage, and it is brightness is set by the current through it, not the voltage across it. The series resistor soaks up the difference between your supply and the LED's forward voltage. The formula is R = (Vsupply - Vforward) / I, where I is your target current in amps. The calculator subtracts the forward voltage from the supply to find the voltage the resistor must drop, converts your milliamp current to amps, and divides to get the resistance. It then multiplies the voltage across the resistor by the current to show how much power the resistor dissipates, so you can choose a part with enough headroom.

Suppose you run a standard red LED from a 9 volt battery and want 20 milliamps through it. A red LED drops about 2 volts, so the resistor must absorb 9 minus 2, which is 7 volts. Dividing 7 volts by 0.02 amps gives 350 ohms, so pick the next common value at or above that, which is 360 or 390 ohms. The power in the resistor is 7 volts times 20 milliamps, or 140 milliwatts, so a standard quarter-watt (250 milliwatt) resistor is fine with room to spare. Drop the supply to 5 volts at 10 milliamps and the resistor becomes 300 ohms.

LEDs behave the same in every country, so the formula does not change. What varies is the typical forward voltage by LED color and type, which is a property of the part, not the region: red and yellow LEDs sit around 1.8 to 2.2 volts, green and blue closer to 3 to 3.4 volts, and white LEDs around 3 to 3.6 volts. Check the datasheet for your exact part. Mains adapters differ by region too, but you should always work from the DC voltage your LED actually sees, not the wall voltage.

• Connecting an LED with no resistor at all. Without one, the LED draws far too much current and fails almost instantly.
• Entering current in amps when the field expects milliamps, or vice versa, which throws the resistor value off by a factor of a thousand.
• Using the wrong forward voltage. A blue or white LED needs a different value than a red one, so use the datasheet figure for your color.
• Ignoring the resistor's power rating. The right resistance can still cook if the wattage is too low for the power it dissipates.
• Rounding down to a smaller resistor, which raises the current above your target. Always round up to the next standard value.

There is no deadline, but there is a sequence worth following before you power on: confirm the LED's forward voltage and maximum current from its datasheet, calculate the resistor, round up to a standard value, and check the resistor's power rating against the figure shown here. Most small indicator LEDs are happy between 10 and 20 milliamps, and many look plenty bright at less, which also saves power and runs cooler. When in doubt, choose a slightly larger resistor and a little less brightness.

• LED circuit - Reference (retrieved 2026-06-11)