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LED Resistor Reference

Every common LED color at every common low-voltage battery supply, computed by the same LED Series-Resistor Calculator math: the exact resistor the formula calls for, the nearest real E12 part, and the current and power that real part actually delivers.

Resistor by LED color and supply voltage (target 20mA)

LED colorForward voltageSupplyCalculatedNearest standard (E12)Actual currentActual power
Red2.0V3V50 Ω56 Ω17.86mA17.9mW
Red2.0V4.5V125 Ω150 Ω16.67mA41.7mW
Red2.0V6V200 Ω220 Ω18.18mA72.7mW
Red2.0V9V350 Ω390 Ω17.95mA125.6mW
Yellow2.1V3V45 Ω47 Ω19.15mA17.2mW
Yellow2.1V4.5V120 Ω120 Ω20.00mA48.0mW
Yellow2.1V6V195 Ω220 Ω17.73mA69.1mW
Yellow2.1V9V345 Ω390 Ω17.69mA122.1mW
Green2.1V3V45 Ω47 Ω19.15mA17.2mW
Green2.1V4.5V120 Ω120 Ω20.00mA48.0mW
Green2.1V6V195 Ω220 Ω17.73mA69.1mW
Green2.1V9V345 Ω390 Ω17.69mA122.1mW
Blue3.2V4.5V65 Ω68 Ω19.12mA24.9mW
Blue3.2V6V140 Ω150 Ω18.67mA52.3mW
Blue3.2V9V290 Ω330 Ω17.58mA101.9mW
White3.2V4.5V65 Ω68 Ω19.12mA24.9mW
White3.2V6V140 Ω150 Ω18.67mA52.3mW
White3.2V9V290 Ω330 Ω17.58mA101.9mW

Every column here comes straight out of the calculator’s own ledResistor()function — supply voltages of 3V, 4.5V, 6V, and 9V only (battery-based, never a mains-derived supply). Rows where the supply doesn’t exceed the LED’s forward voltage are left out rather than shown as broken math.

How to read the “actual” columns

The Calculated column is the exact answer to R = (Vsupply − Vforward) ÷ Iforward— a value like 345Ω that doesn’t exist as a real part. The Nearest standard (E12)column rounds that up to a resistor you can actually buy, which is always slightly larger than the exact calculation — rounding down would push more current through the LED than intended, so the calculator always rounds up instead.

Because the real resistor is a little larger than the exact math called for, it drops a little more voltage than strictly necessary, which pulls the actual current down a little below the 20mA target — that’s the Actual current column. The gap is usually small (a fraction of a milliamp to a couple of milliamps), but it grows when the E12 series happens to have a wide jump between two values near your target — compare the red LED at 4.5V (calculated 125Ω, but the nearest E12 step jumps straight to 150Ω, landing at 16.67mA) against the red LED at 9V (calculated 350Ω, nearest 390Ω, landing at 17.95mA, much closer to target).

Actual poweris what the real resistor dissipates at that real current — P = V × I using the voltage the resistor actually drops and the actual current through it. It’s always comfortably under the 0.25W (quarter-watt) rating of a standard resistor for every combination in this table, which is why a single through-hole resistor is the normal choice for a single-LED indicator circuit.

Frequently Asked Questions

Why do the "calculated" and "nearest standard" resistor columns differ?

Resistors aren't manufactured in every possible value — only a standard set (the E12 series: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, and their ×10 multiples). The calculated column is the exact math from R = (Vsupply − Vforward) ÷ Iforward; the nearest standard column rounds that up to the closest real part you can actually buy. Rounding up is the safe direction — it slightly under-drives the LED instead of over-driving it.

Why is the actual current always a little under 20mA instead of exactly 20mA?

Because the real resistor value is rounded up from the exact calculated one, it has slightly more resistance than the math called for — and more resistance means slightly less current for the same voltage. That's the same safe-rounding effect showing up as a number: every row's actual current sits at or just under the 20mA target, never over it.

Why does Blue and White skip the 3V row?

Blue and white LEDs have a forward voltage around 3.2V, higher than a 3V supply — there isn't enough voltage left over for a resistor to drop anything, so the circuit can't work at all. Those cells are left out of the table rather than shown with a nonsense negative resistance. The same LEDs work fine from 4.5V, 6V, or 9V.

Where do these forward voltage numbers come from?

They're typical, commonly-cited ranges for standard 5mm LEDs by color — roughly 2.0V for red, 2.1V for yellow and green, and 3.2V for blue and white. Forward voltage varies by manufacturer and by the exact current you drive the LED at, so treat this table as a solid starting point and check your specific LED's datasheet when one is available.

Forward voltages are typical values for standard 5mm LEDs, not a substitute for your specific part’s datasheet. This table covers low-voltage, battery-powered circuits only.

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