24 × 36 inch poster at 150 DPI is 3600 × 5400 pixels

The working

Pixels are inches multiplied by the resolution.

• Width: 24 in × 150 = 3,600, which rounds to 3600 pixels.
• Height: 36 in × 150 = 5,400, which rounds to 5400 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 3638 × 5438 pixels.

Is your photo big enough?

To fill 24 × 36 inch poster at 150 DPI an image needs at least 3600 × 5400 pixels. Cropped to fill the sheet:

• A 12 megapixel phone photo (4000 × 3000) gives 111 DPI: short of 150, so it would be upscaled or printed softer.
• A 24 megapixel camera photo (6000 × 4000) gives 167 DPI: enough.
• A 48 megapixel photo (8000 × 6000) gives 222 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.