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Image Size and Print Size

Calculate the pixels needed for A4 and A3 at 300 ppi, and the largest size a photo can be printed sharply.

The Two Formulas

Pixels needed = print size in inches \times ppi Largest sharp print size in inches = pixels \div ppi 1 inch = 2.54 cm (25.4 mm). To change centimetres into inches, divide by 2.54.

ISO paper sizes and the pixels needed at 300 ppi
ISO paper sizes and the pixels needed at 300 ppi

Each A size is exactly half of the one before it: two A4 sheets make one A3.

More pixels, more detail

This picture has 32 × 32 = 1,024 pixels.

Print size calculator

Width (pixels)Height (pixels)

Resolution

Prints at 21.0 × 29.7 cm (8.27 × 11.69 in). Uncompressed at 24-bit colour: about 26.1 MB.

Try it:

Scroll to the print calculator. Enter 3508 \times 4961 at 300 ppi — it should print at A3 (about 29.7 \times 42 cm). Then try the 1600 \times 1067 photo.

In Photoshop, Image \to Image Size shows the pixel dimensions and the print size. With Resample switched off, changing the resolution only changes the print size — no pixels are added or removed. Use this to set 300 ppi for printing. With Resample switched on, Photoshop adds or removes pixels. Reducing size is fine (for the web). Enlarging makes the computer guess the new pixels, which looks soft.

Practice

A photo is 1800 pixels wide. What is its largest sharp print width at 300 ppi, in inches?

Your answer

Paper Sizes

ISO paper sizes and the pixels needed at 300 ppi
ISO paper sizes and the pixels needed at 300 ppi

Each A size is exactly half of the one before it: two A4 sheets make one A3.