The easiest way to get a handle on your metabolic rate is to calculate it based on your age, weight, and height. It will be an estimate, but measuring your metabolic rate requires equipment that you probably don’t have sitting around. The estimate is good enough to work with for the purpose of balancing your food intake and preventing weight gain. You can also learn more about metabolism here.
You can estimate your metabolic rate with the calculator below and then figure out what is the most you can eat in a day and not gain weight. Then I dare you to see if you can stay under that number for a week.