Posters are often viewed from farther away, so 100–150 DPI is common while 300 DPI is for close viewing.
Recommended DPI for poster
Start around 150 DPI, then adjust for size and viewing distance. Fine detail and text need higher DPI than soft photographic backgrounds.
Pixels still matter most
Pick the final print size first, multiply by target DPI, and check whether your file has enough pixels on the short side.
FAQ
What DPI do I need for poster?
Often around 150 DPI, but large pieces viewed from farther away can use less.