You could use a mug to measure a cup of ingredients in a recipe.
When using a mug for a measuring cup you can take a tablespoon and scoop up 16 tablespoons of the ingredients and then place them in the mug.
It takes 16 tablespoons of the ingredients to equal a cup.
So if you put 16 tablespoons of the ingredients into the mug then you would have a cup of the ingredients.
But just don't use the mug to scoop a cup of ingredients because a mug is not the same size as an accurate measuring cup.
It's best to buy a measuring cup or a set of measuring cups, spoons etc to make sure you're getting the accurate amount of ingredients to make your recipe.
Getting too little ingredients in a recipe could make the food or baked goods or whatever you're cooking turn out bad just like too much of the ingredients can do the same.