The average size for a 15 year old is a little less than 5 inches erect. But don't worry. You still have some more growing to do....
"For a mature adult, the average size for a non-erect penis is between 3-4 inches. When a mature adult penis is fully erected it can reach up to an average of 5.5-6.5 inches (see illustration above). Teenage boys will have to wait a further one to two years after they have stopped growing in height before they will be able to clearly know their final penis size."
At your age, almost any size is normal. There are 15 year olds that have attained their adult size; others that are still growing, even just starting to grow. When did you start puberty? If it's just recently, then you've got lots of growing to do.
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