18 × 24 inch poster at 150 DPI is 2700 × 3600 pixels

The working

Pixels are inches multiplied by the resolution.

• Width: 18 in × 150 = 2,700, which rounds to 2700 pixels.
• Height: 24 in × 150 = 3,600, which rounds to 3600 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 2738 × 3638 pixels.

Is your photo big enough?

To fill 18 × 24 inch poster at 150 DPI an image needs at least 2700 × 3600 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 150 DPI the pixels of a print blend together from about 22.9 inches (58 cm) away. 150 DPI is a common minimum for posters and large prints seen from a step or two away, and a usual setting for scanning documents that only need to be read.