LED Series Resistor Calculator

Pick your supply, LED, and current — get the exact resistor, the nearest standard value, and the power rating you need.

Supply voltage V

LED

Forward voltage V

LED current

LEDs in series

Parallel strings

Exact resistor

Standard value

Actual current

Resistor power

Why an LED needs a resistor

An LED is not a light bulb: above its forward voltage its current rises almost vertically, so connecting one straight across a supply lets the current run away until something burns. A series resistor sets the operating point. The resistor absorbs whatever voltage the LED doesn't:

R = (V_supply − n × V_f) / I_LED

where n is the number of LEDs in series. This calculator goes three steps further than the formula: it rounds up to the nearest standard E24 value you can actually buy (rounding up keeps the current at or below your target), tells you the actual current with that standard part, and sizes the power rating with the usual 2× safety margin.

Worked example

A red LED (V_f = 2.0 V) at 15 mA from a 5 V rail: R = (5 − 2.0) / 0.015 = 200 Ω, which is itself a stock E24 value. It dissipates 3 V × 15 mA = 45 mW — an eighth-watt part is already 2.7× headroom, and a common ¼ W resistor is plenty.

Typical forward voltages

LED type	Typical V_f
Infrared	1.2 – 1.6 V
Red	1.8 – 2.2 V
Orange / Yellow	2.0 – 2.2 V
Green (classic GaP)	2.0 – 2.4 V
Green (bright InGaN)	3.0 – 3.4 V
Blue / White	3.0 – 3.6 V
UV	3.1 – 3.5 V

These are starting points — check the datasheet, and remember V_f varies between individual LEDs and with temperature.

Frequently asked questions

Does the resistor go before or after the LED?: Electrically it makes no difference — it's a series circuit, the same current flows everywhere. Put it wherever the layout is cleaner.
Can I run several LEDs off one resistor?: In series, yes — that's what the "LEDs in series" field is for. Avoid sharing one resistor across parallel LEDs: forward-voltage mismatch makes one LED hog the current. Give each parallel string its own resistor, which is exactly how this calculator figures it.
My supply is barely above the LED voltage. Is that OK?: It works, but with little voltage across the resistor the current becomes very sensitive to V_f spread and temperature. The calculator warns when the resistor drops under 20% of the supply; a higher supply or a constant-current driver gives more predictable brightness.
How do I dim an LED?: Preferably with PWM (rapid switching) rather than a bigger resistor — PWM dims without shifting the LED's color and works over a huge range.
Where does the resistor value math come from?: Plain Ohm's law applied to the voltage left over after the LEDs take their share.