How can I gain weight in a fast but healthy way?

Well since you don’t want to say you’re weight or anything I’ll give you the resources to figure out some simple calculations that will help you.

YOUR VITALS A. Your Weight in pounds B. Multiply A by 11 to get your basic calorie needs (how much your body burns just by existing) C. Multiply B by 1.6 to estimate your resting metabolic rate (rate of calorie burn when you factor in daily activities) _

YOUR EXERCISE D. Aerobic Training - Multiply the number of minutes per week that you run, cycle or play sports by 8 E. Strength Training - If you’re doing a total body exercise plan, add 600 to your answer from D. If you’re doing 3 sets, add 840 to your answer from D. F. Divide that by 7 _

YOUR CALORIE NEEDS G. Add C and F to get your daily calorie needs H. If you’re happy with your weight stop here. Otherwise, subtract 500 from G This is your estimated daily calorie budget if you want to lose 1 lb per week. Subtract 1,000 to lose 2 lbs per week. Anything more and you’ll be losing more muscle than fat.

*Now all you need to do is ADD instead of subtract. Add 500 to your daily calorie budget to gain a pound a week. Or 1000 to gain 2 pounds. Anything more than that and it’ll be unhealthy. Remember though, that although you’re trying to gain weight - what you eat still needs to be healthy.