Buying too much ice for your drink cooler feels like a waste, but it doesn't feel as bad as coming up short. Luckily, a physics professor has applied years of mathematical training to the matter of making sure your cans stay cold.
Photo by lendog64.
Rhett Allain, associate professor of physics at Southeastern Louisiana University, puts it right upfront that your conditions will vary from those he used for his formulas. Walking through the physics of a mass of material at or around 0C keeping a bundle of other materials very close to that temperature, Allain teaches us a good bit about the imperfect maths of heat transfer. More helpfully for your weekends, however, he also gives up a good starting point:
This says that you need about 1 ten pound bag of ice for a 12 pack. Remember, my calculation was for the case where all the ice melted. You probably don't want that.
So, if you'd like to serve very cold drinks and expect to completely blow out your ice buy, start with that ratio. You can scale that number up, obviously, if you're only outside for a few hours, and (I'm assuming) if you're packing more soft drink than beer, which you don't necessarily want near the freezing point.