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MasterMath

Megapixel Print Size

A 6000×4000 photo is 24 megapixels and prints at quality up to 50.8 × 33.9 cm. Work out how large you can print before the pixels show.

Megapixels

—

Megapixels—
Quality print size—
As a poster, at 150 ppi—
Total pixels—
Printed width—
Printed height—

How this was worked out

    The formula

    printed side = pixels on that side ÷ print resolution

    Where it comes from

    Megapixels say nothing on their own: what matters is how many pixels per inch you have when printing. The same 24-megapixel photo looks perfect at A3 and pixelated on a two-metre poster, because the same pixels spread over more area.

    How to work it out by hand

    1. Multiply width by height and divide by a million for the megapixels
    2. Divide each side by the print resolution you want
    3. The result is inches: multiply by 2.54 for centimetres
    4. If the size falls short, lower the ppi or use a larger image

    What is worth knowing

    The 300 dots per inch reference comes from print and holds for things looked at close: a book, a magazine, a photo in the hand. But a poster is looked at from metres away, and there 150 ppi or even less is enough: hence the same image allows twice the size. That is why billboards are printed at resolutions that would be unacceptable on paper. And a warning about megapixels: going from 24 to 48 does not double the printable size, it multiplies it only by the square root of two, because the pixels spread across two dimensions.

    Frequently asked questions

    How many megapixels do I need to print A4?

    A4 at 300 ppi is about 2480×3508 pixels, which is 8.7 megapixels. Any modern phone comfortably exceeds it.

    Why 300 ppi?

    Because at that density the eye no longer picks out the dots at normal reading distance. For anything viewed from a distance, far less is enough.

    Does doubling the megapixels double the print size?

    No: it multiplies it by the square root of two, around 1.41. The pixels spread across width and height at once.

    Can I enlarge a small photo?

    You can, but by inventing pixels that do not exist. AI upscalers do it quite well these days, though the real detail does not appear from nowhere.