【How-to】How long does it take to gain 20 pounds

And it turns out, you’d have to take in a ridiculous amount of food to gain even just a pound in one day. “It’s virtually impossible to gain weight overnight, even if you really blew it on bar food,” says Cassetty. … You’d have to eat 3,500 additional calories, totaling 5,500 calories, to gain a single pound.