Depending on where you get your definition of what is “sin”, it is not included on any biblical list.
Is master bating a sin?
Masterbating as such is not mentioned as a specific sin. God's displeasure regarding the act is assumed based on Gen 38, vs 8 thru 10. A man named Onan disobeyed God and was eventually slain by God. Not enough room here to explain it all in detail. You will have to read a bible.
In my opinion, no. It's perfectly normal. Albeit some religions will probably disagree with my opinion. Personally, I think it's fine, and just a natural thing. I say do what you want, wheather that's masturbaiting or not haha.