For an A1 print, an 18MP file is generally enough if you use the full image and don’t crop heavily. A common guideline for a large print like this is about 150 ppi, which is usually sufficient for good viewing quality at that size.

A1 is about 33.1 × 23.4 inches. At 150 ppi, that works out to roughly 4965 × 3510 pixels, or about 17.4MP. So your 18MP photo should be fine.

A few practical points:

• Use the highest-resolution original file you have.
• Don’t worry too much about chasing a specific DPI number alone; the actual pixel dimensions matter most.
• Printers often scale images well automatically, and the ideal output setting can depend on the printer.
• If you need slight upscaling, software may help a bit, but it usually isn’t necessary if your image already meets the target size.

So: for A1 canvas, your 18MP image should be suitable at around 150 ppi, assuming minimal cropping.