I'm underweight

how do you know you are under weight, did you do a BMi (body mass index) check as they are very good, if you need a link i can  private message you one

i'm not being patronising btw

Hey henry, the only thing you need to increase your weight, is to be in a caloric surplus, that is to say, that you are eating more calories than you burn in a day. Since you are underweight and have fat thighs and calfs, it's safe to assume that your body type is that of a skinny person, however you have a higher body fat percentage. This boils down to genetics, thyroid function as well as the amount of muscle on your body. The healthiest way for you to gain weight would be to start doing resistance training with weights or your body weight, to pack on muscle, because muscle is more dense and weighs more than fat. Make sure you don't eat junk food to gain weight, as most of it will be dead weight, just focus on doing compound exercises that are multi joint exercises that will give the most amount of muscle growth in the shortest amount of time. I recomend starting with pushups, squats, and pull ups These three exercises will spur growth in your largest muscle groups, however make sure that your diet is filled with protein from sources such as meat or even beans, and carbohydrates from sources such as rice, or pasta and fruits. Fats are very important for your testosterone, so make sure you consume healthy fats, such as natural peanut butter, fish oils and avocados. Feel free to ask me any questions you may have about this process , good luck

Try eating healthy balanced meals three times day in combination with moderate exercise.

How do you define being underweight? Did a md tell you that you are underweight? Try checking your BMI. If being underweight is not causing health issues, I probably wouldn't worry about it too much.

Btw, to determine body mass index divide your weight in kg by your height in meters, then divide the answer by your height.

Hey, don't worry, It's not a big issue. You can increase your weight by having larger intake of milk products in your meal. But though you said that you have fat thighs and calfs, it means that your body structure is skinny. But anyways, you can consume the milk products and other fatty food which can help your weight to grow.