Many people wrongly assume that the BMR = maintenance calories. That is not so. BMR just means the amount of calories that your body needs just to function, that means to sustain all the vital organs like the heart and the basic activities like breathing. Most people, besides breathing and staying at rest, also do other things during the day, like walking and sometimes even being active. 🙂 All these activities require a greater expenditure of calories than the BMR. So in order to gain weight you need to find what your MAINTENANCE calories are. Maintenance calories are your BMR calories + the amount of calories that you expend throughout the day on going around your daily functions (for example walking around or doing sports). You should then eat more calories than your maintenance calories. For people that are trying to lose weight, they should calculate their BMR and their maintenance calories and should eat less calories than their maintenance calories, but still more than their BMR.
Here are two calculators: