DPI for printing, explained
DPI (dots per inch) tells you how densely an image's pixels are packed when printed at a given size. Print a 3000-pixel-wide photo across 10 inches and you get 300 DPI, the gold standard for sharp photo prints. Stretch that same image to 30 inches and it drops to 100 DPI, where individual dots become visible.
DPI = pixels ÷ print inches
How many DPI do you need?
300 DPI is the reference for photo books, magazines, and anything viewed up close. 150 to 200 DPI is fine for posters seen from a step back, and large banners can look acceptable at 100 DPI or lower because you never stand right next to them. Use the calculator above to check the largest size your image can print before it softens.
To see a print size at true scale before you commit, open the actual-size ruler and measure the dimensions on screen.