LED resistor calculator

An LED is not a resistor: its voltage barely moves while the current climbs, so connecting one straight across a supply either does nothing or destroys it. The series resistor exists to take the difference between the supply and the diode's forward voltage, and its value follows from Ohm's law applied to that difference — not to the whole supply, which is the mistake that halves the brightness or doubles it.

How it is calculated

R = (supply − forward voltage) ÷ current

Only the leftover voltage appears across the resistor, so only that leftover goes into the division. The answer is then rounded up to the next value in the E12 series, because rounding down means more current than you asked for, and the last figure is the power the resistor turns into heat.

Source: NIST SP 811 (2008), Appendix B.8 — ohm (Ω), the SI unit of resistance: 1 Ω = 1 V/A

Questions people ask

Where do I find the forward voltage?

On the datasheet, and it depends on the colour rather than on the brand: red and yellow sit near 1.8 to 2.2 V, green and blue near 3 to 3.4 V, white near 3.2 V. Guessing costs brightness or lifetime, so if the part came without a datasheet, measure it — put a 1 kΩ resistor in series with a 9 V supply and read the voltage across the diode.

Why round up instead of to the nearest value?

Because a smaller resistor passes more current than you asked for, and current is what kills an LED. Rounding up costs a little brightness you will not notice; rounding down eats into the margin the datasheet gave you. The exact figure is on the page too, if your resistor drawer is better stocked than E12.

Can I put several LEDs on one resistor?

In series, yes — add their forward voltages together and enter the total, as long as the sum stays below the supply. In parallel, no: two diodes never conduct identically, so the one with the slightly lower forward voltage takes most of the current and fails first, after which the other one takes all of it. Parallel strings each need their own resistor.

What does "the supply is below the forward voltage" mean?

That the circuit cannot work at all. A diode does not conduct until the voltage across it reaches its forward voltage, so a 3.2 V white LED on a 3 V coin cell stays dark no matter what resistor you fit. The answer is a higher supply or a different colour of LED, not a smaller resistor.

Does the resistor go before or after the LED?

It makes no difference. The same current flows through everything in a series loop, so the resistor limits it whichever side of the diode it sits on. What does matter is that every LED has one — and that you check the polarity of the diode, which the resistor cannot fix.

Related tools