Voltage drop is not your problem with LED holiday lights.

It’s rarely an issue. Modern holiday lights are almost all LED, and they draw so little current that voltage drop over a reasonable extension cord length is negligible. You can run a hundred feet of 16-gauge cord and not notice a difference.

If you’re still using old incandescent string lights (the ones that get hot), then yes, length and wire gauge matter. For incandescent, keep extension cords under 100 feet and use at least 16 gauge. For 200+ feet, step up to 14 or 12 gauge. But honestly, if you’re buying lights today, they’re LEDs.

If your lights look dim, it’s almost always a bad bulb, a loose connection, or a failing transformer—not the cord. Check those first before buying a thicker extension cord.

Future you deserves a simple rule: LED lights, long cord fine. Incandescent, keep it short and fat.