We’ve all been there.

You’ve been counting calories and carefully minding your portions when there’s a birthday/ Saturday night/ chipper intervening in your quest for svelte perfection. One of the greatest diet pitfalls is waking up and thinking you have ‘blown your diet’, but apparently it’s your following day that’s the real decider. According to Women’s Health, the secret clue to how much weight you can gain in one day is actually a lot less than you probably imagined… a single pound. And you need to eat or drink a LOT of calories to see that gain. Try 3,500 extra calories to your usual routine. According to nutrition expert Samantha Cassetty, you’ll need to eat a lot of extra calorific foods to gain just one pound overnight: “It’s virtually impossible to gain weight overnight, even if you really blew it on bar food. The reason comes down to calorie math. “Though it’s not 100 percent precise, the basic principle stands true: In order to gain weight, you’d have to eat 3,500 more calories than you typically eat and burn off to maintain your figure." With the average person eating 2,000 calories on a normal day, Cassetty estimates a regular person would need to eat 5,500 calories to gain a single pound. (And that’s before you take any physical activity into account.) So what’s the cause of your bigger belly? Cassetty blames the effects of bloating and a sluggish feeling as part of the problem. And if you do go off track for a day? The best bet to combat that weight gain is to start right back into your healthy eating and fitness routine. So you can have your cake and eat it too (occasionally), without it showing on the scales.