DPI / PPI and Upscaling - a visual guide

First: DPI or PPI?

People often say DPI when they mean PPI: pixels per inch.
Technically, DPI describes the dots a printer makes. Here, we are talking about image pixels.

Why use 300 PPI?

Always prepare your artwork at 300 PPI.
This is a common target for detailed prints viewed up close.
It balances image detail and file size. More pixels mean larger files, without always producing a visible improvement.

A — Midjourney upscaled does not mean A0-ready!

Our MJ Creative Upsacled image contains 1824 × 2592 pixels.
Placed on A0 canvas @ 300 PPI, it occupies only a small part of the A0 page.
The 🩷 pink area is uncovered canvas. It shows how much larger the page is than the current image.
To fill A0 at 300 PPI, we need 9933 × 14043 pixels.

B — We can print it at 300 PPI but it would be much smaller than A1 🤣

On the A1 page, the current artwork is small, leaving lots of blank paper.
In this example, each source pixel lines up with one square on the red "actual pixels" grid.
You can print it at 300 PPI but it will be very small, but will be nice and sharp on the paper (just way too small).

C — Enlarging to fit does not add detail and makes it blurry ❌

Enlarging the original artwork fills the A1 page, but spreads the same pixels over a larger area.
Each source pixel now spans roughly four grid squares on the red "actual pixels" grid. We have simply stretched the pixels to make them bigger!
The image provides only about 78 PPI at A1 which is well below our 300 PPI target 🎯.
You can print it but it will likely be blurry and lead to angry customers 😡.

D — AI upscaling adds pixels ✅

A 4 × upscale multiplies both width and height:
1824 × 2592 → 7296 × 10368 pixels.
That gives 16 × the total pixel count.
A1 at 300 PPI needs about 7016 × 9933 pixels, so this upscaled image now has more than enough.
🚨 AI estimates extra detail. 🚨 Check the result for altered shapes, faces, lettering or edges.
When we look at the source pixels they once again line up with one square on the red "actual pixels" grid. We have added pixels by upscaling!

E — Why does the file still say 72 PPI?