The formula
Choose the next standard value above the calculated resistance so the current is never higher than intended, and a resistor rated at least twice the power it dissipates.
Why use 14.4 V for "12 V"?
A 12 V vehicle or leisure system sits at 13.5–14.6 V while charging. A resistor sized for exactly 12 V would let the LED run over its rated current whenever the engine, solar or charger is on. Sizing at 14.4 V keeps it safe, at the cost of slightly lower brightness when the battery is resting.
LED strips and lamps
12 V LED strips and replacement bulbs already have resistors or drivers built in. Connect them straight to 12 V. This calculator is for individual LEDs.
Frequently asked questions
What resistor do I need for an LED on 12V?
For a 20 mA white LED (3.2 V) sized at 14.4 V: (14.4 − 3.2) ÷ 0.02 = 560 Ω, a standard value. Use a 1/2 W resistor.
Can I put LEDs in parallel with one resistor?
It's not recommended. LEDs share current unevenly. Give each LED (or each series string) its own resistor.