No matter how you do the math, 20 lbs. is a lot of weight to attempt to lose in one month. It's possible to drop that much weight, but you'll need to really work at it, almost certainly with a combination of a low-calorie diet plus a huge amount of exercise. Losing more than 2 lbs. a week is considered unhealthy, so plan your weight loss under the supervision of a doctor.
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Weight Loss and Calories
To lose 20 lbs. in one month, you'll need to burn through 70,000 more calories than you consume. Most people need to eat about 2,000 to 2,500 calories each day just to stay at the same weight. If you're a woman, you should cut your caloric intake to no less than 1,200 calories each day, and if you're a man, don't drop down below 1,500 calories each day. Doing this should help you lose close to 2 lbs. each week.
To carve the most calories from your diet without depriving yourself of important nutrition, cut way back on fat and emphasize fresh fruits, vegetables and whole grains. Simple moves such as switching to non-fat dairy products instead of full-fat or part-skim products can help you meet your goal. Avoid highly processed flour products; instead reach for high-fiber whole-grain products to get enough fiber in your diet, which may also help you lose weight because they can make you feel fuller.
If you want to lose those 20 lbs., you'll need to get very active and spend plenty of time working out over the next month -- you're depending on exercise to burn about two-thirds of that weight. Plan to spend at least two hours each day in the gym or outside, exercising hard. Running, biking and rowing all can burn enough calories for you to reach your goal weight. Do not do this unless your doctor OKs it and you are already in good shape.
Although it's possible to lose 20 lbs. in one month, you'd be much better off spreading out your efforts over two or three months. Slow and steady weight loss produces better long-term results than super-fast weight loss. By pushing yourself to reach such a lofty goal, you risk injuring yourself from all that exercise, especially if you're not used to such a vigorous routine. You also risk burning out on your routine before the month has passed and quickly regaining whatever weight you've lost.