18 × 24 inch poster at 72 DPI is 1296 × 1728 pixels

The working

Pixels are inches multiplied by the resolution.

• Width: 18 in × 72 = 1,296, which rounds to 1296 pixels.
• Height: 24 in × 72 = 1,728, which rounds to 1728 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 1314 × 1746 pixels.

Is your photo big enough?

To fill 18 × 24 inch poster at 72 DPI an image needs at least 1296 × 1728 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 72 DPI the pixels of a print blend together from about 47.7 inches (121 cm) away. 72 DPI is the old screen convention: early Macintosh displays showed 72 pixels to the inch, matching the 72 points in an inch of type. Many image files still carry a 72 DPI tag for that reason.