LED Resistor Calculator
Works out the series resistor an LED needs, the nearest standard value, and what that resistor will dissipate. Also flags when your supply cannot drive the number of LEDs you have in series.
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The formula
R = (Vsupply − Vforward) ÷ I
An LED is a diode, not a resistor — it does not limit its own current. The resistor takes the difference between the supply and the LED forward voltage, and that difference divided by the target current is its value.
What to watch out for
Four things that decide whether the circuit works:
- An LED has no internal current limit. Connected directly across a supply it draws until something fails. The resistor is what sets the current.
- Round up, not down. The nearest standard value above your calculated figure gives slightly less current and a slightly dimmer, much longer-lived LED.
- Headroom matters. With a supply close to the forward voltage, small variations in Vf cause large swings in current. Aim for at least 20% of the supply across the resistor.
- Series or parallel, not both casually. LEDs in series share the same current, which is good. LEDs in parallel on one resistor do not share equally — the lowest-Vf device takes most of the current. Give each parallel branch its own resistor.
Frequently Asked Questions
What resistor for a 2 V LED at 20 mA on 5 V?
150 Ω exactly, which is a standard value. It dissipates 60 mW, so any quarter-watt resistor is fine.
What resistor for an LED on 12 V?
For a 2 V red LED at 20 mA, 500 Ω — use 560 Ω as the nearest standard value up. Note the resistor now wastes 200 mW, over three times the LED power.
Can I put several LEDs on one resistor?
In series, yes — they share the same current. In parallel, no: forward voltages differ slightly and the lowest one hogs the current. Each parallel branch needs its own resistor.
Why is my resistor wasting more power than the LED?
Because it is dropping more voltage than the LED. On 12 V with a 2 V LED, the resistor drops 10 V and so takes five sixths of the power. A constant-current driver avoids this; a resistor cannot.