12-gauge extension cord for a 15-amp window AC.

12-gauge, minimum. Don’t use 14-gauge.

A 15-amp air conditioner pulls close to the maximum for a standard household circuit. At 50 feet, a 14-gauge cord will drop voltage enough to make the compressor work harder and heat up the wire. That’s a fire risk and a waste of electricity. 12-gauge handles the load safely with acceptable voltage drop.

10-gauge is fine but overkill for 50 feet—you’d only need it for runs over 100 feet or so. Stick with 12-gauge, make sure the cord is rated for outdoor use (even if inside, because it’ll be heavier-duty), and plug it directly into a wall outlet, not another extension cord or power strip.

Don’t cheap out on the cord—this is a fire hazard, not a phone charger.