A3 at 150 DPI is 1754 × 2480 pixels

A3 (297 × 420 mm) at 150 DPI is 1754 × 2480 pixels, which is 4.3 megapixels.

Portrait1754 × 2480 pixels
Landscape2480 × 1754 pixels
With 3 mm bleed1789 × 2516 pixels
Megapixels4.3
Aspect ratio1.41:1 (1 to √2 before rounding)
File size12 MB as uncompressed 8-bit RGB; a JPEG or PNG is smaller
Paper size297 × 420 mm (11.69 × 16.54 in)

The working

Pixels are inches multiplied by the resolution, and an inch is 25.4 mm.

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 3 mm on every side of an A-series page, which makes the file 1789 × 2516 pixels.

Is your photo big enough?

To fill A3 at 150 DPI an image needs at least 1754 × 2480 pixels. Cropped to fill the sheet:

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.

A3 at other resolutions

ResolutionPixelsWith bleedMegapixelsUncompressed
72 DPI842 × 1191859 × 120812.9 MB
96 DPI1123 × 15871145 × 16101.85.1 MB
100 DPI1169 × 16541193 × 16771.95.5 MB
150 DPI1754 × 24801789 × 25164.312 MB
200 DPI2339 × 33072386 × 33547.722 MB
240 DPI2806 × 39692863 × 40251132 MB
300 DPI3508 × 49613579 × 50311750 MB
400 DPI4677 × 66144772 × 67093189 MB
600 DPI7016 × 99217157 × 1006370199 MB
1200 DPI14031 × 1984314315 × 20126278797 MB

Other sizes at 150 DPI

A3 is two A4 pages side by side, used for small posters, drawings and spreads. See A3 in pixels at every resolution or everything at 150 DPI.