What DPI means
For printing, DPI here is the number of image pixels available per printed inch. More pixels per inch usually means more detail at normal viewing distance.
300 vs 150 DPI
300 DPI is a common high-quality photo target. Large posters are often viewed farther away, so lower effective DPI can still look good.
No resampling
This calculator does not invent pixels; it only explains what your existing pixel dimensions mean at a chosen print size.