How many days does it take to gain weight?

Eating 1,000 extra calories a day for five days did not lead to any significant changes in weight, fat mass, or fasting blood sugar levels. But chronic overeating—eating 1,000 extra calories a day over the course of a month—was linked to a fat-mass increase of about 3 pounds, as well as increases in blood sugar.

How can I gain weight overnight?

Eating a lot of carbs generally can cause weight gain, but specifically, eating a lot of carbs one day when you normally don’t can also cause you to retain weight and weigh in heavier the next morning. One gram of a carbohydrate retains three grams of water, so it’s a really easy way to gain a lot of weight quickly.