LED Resistor Calculator
The series resistor, rounded up to a standard value.
Work out LED Resistor. The series resistor, rounded up to a standard value. Answers in more than one unit, because SI is not always the legible one.
About 2 V for red, 3.2 V for blue and white.
Series resistor (Ω)
150
Use 150 Ω — the next standard value up
R = (supply − forward voltage) ÷ current. The standard value chosen is always the next one *up*, never the nearest: rounding down raises the current above the LED's rating, so the safe direction is unambiguous. At 60 mW a common 250 mW resistor is comfortable, but the recommended rating leaves headroom — a resistor run at its limit gets hot enough to matter.
How the LED Resistor Calculator works
R = (supply − forward voltage) ÷ current. The standard value suggested is always the next one *up*, never the nearest: rounding down raises the current above the LED's rating, so the safe direction here is unambiguous.
Also known as: resistor for an led · led series resistor calculator · current limiting resistor calculator · led resistor formula
The calculation itself
R = (supply voltage − LED forward voltage) ÷ current. A 5 V supply, a 2 V red LED and 20 mA needs (5 − 2) ÷ 0.02 = 150 Ω.
The LED's forward voltage is roughly fixed by its chemistry — about 2 V for red, 3.2 for blue and white — which is why an LED cannot simply be connected to a supply. Without the resistor, current is limited only by the LED's own tiny resistance, and it destroys itself.
Why round up, never to nearest
Standard resistors come in fixed steps, so the calculated value rarely exists as a part. The obvious move is to pick the nearest one, and it is the wrong move.
Rounding down lowers the resistance and raises the current above the LED's rating, shortening its life or destroying it. Rounding up costs a little brightness, which is almost always invisible.
The asymmetry is total: one direction costs nothing you will notice, the other costs the component. So this page always suggests the next value up.
Frequently asked questions
What resistor does my LED need?
Subtract the LED's forward voltage from the supply, then divide by the desired current. A 5 V supply, a 2 V red LED and 20 mA needs 150 Ω.
What is an LED's forward voltage?
Roughly 2 V for red, 2.1 for yellow, 3.2 for blue, white and green. It varies by part, so the datasheet is worth checking when precision matters.
Why round up rather than to the nearest value?
Because rounding down increases the current beyond the LED's rating. Rounding up costs a little brightness; rounding down costs LED lifetime.
What power rating do I need?
Multiply the voltage across the resistor by the current. At 60 mW an ordinary 250 mW resistor is comfortable — running one near its rating makes it hot enough to matter.
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