If you want to start selling t-shirts online on a print-on-demand website like Merch by Amazon, Society6, Redbubble, Threadless, or Spreadshirt, then you need a profit margin calculator. Why? Easy! To calculate how much money you can make by selling t-shirts online on a print-on-demand website. This way, you will be able to know if it is profitable to venture down this path, but also discover and compare which is the best alternative that will generate the highest profits for you.