18 × 24 inch poster at 96 DPI is 1728 × 2304 pixels

The working

Pixels are inches multiplied by the resolution.

• Width: 18 in × 96 = 1,728, which rounds to 1728 pixels.
• Height: 24 in × 96 = 2,304, which rounds to 2304 pixels.

Bleed is extra image past the trim line, so that a cut that lands slightly off still leaves ink to the edge. Print shops usually ask for 0.125 in on every side of a US size, which makes the file 1752 × 2328 pixels.

Is your photo big enough?

To fill 18 × 24 inch poster at 96 DPI an image needs at least 1728 × 2304 pixels. Cropped to fill the sheet:

• A 12 megapixel phone photo (4000 × 3000) gives 167 DPI: enough.
• A 24 megapixel camera photo (6000 × 4000) gives 222 DPI: enough.
• A 48 megapixel photo (8000 × 6000) gives 333 DPI: enough.

At 96 DPI the pixels of a print blend together from about 35.8 inches (91 cm) away. 96 DPI is the reference resolution of Windows and of the web: CSS defines one inch as 96 pixels, so a web page sized in inches or millimetres is laid out at 96 pixels to the inch.