This article explains the basics of image size, resolution and DPI, so you can upload images to your photography website with confidence and prepare them correctly for printing.
What does 300 dpi mean?
300 dpi (dots per inch) means that, when printed, 300 dots fit into every inch (2.54 cm). It is the standard resolution for professional-quality photo prints: at that density the eye cannot distinguish individual dots at a normal viewing distance.
What really matters is not the dpi value stored in the file, but the number of pixels in the image. These are the minimum pixels you need to print the most common sizes:
Print size | At 300 dpi (high quality) | At 150 dpi (acceptable quality) |
4 × 6 in (10 × 15 cm) | 1181 × 1772 px | 591 × 886 px |
5 × 7 in (13 × 18 cm) | 1535 × 2126 px | 768 × 1063 px |
6 × 8 in (15 × 20 cm) | 1772 × 2362 px | 886 × 1181 px |
8 × 12 in (20 × 30 cm) | 2362 × 3543 px | 1181 × 1772 px |
12 × 16 in (30 × 40 cm) | 3543 × 4724 px | 1772 × 2362 px |
16 × 24 in (40 × 60 cm) | 4724 × 7087 px | 2362 × 3543 px |
For example, a photo from a 24-megapixel camera (6000 × 4000 px) can be printed at 300 dpi up to about 20 × 13 in (50 × 34 cm).
To display an image on a website, on the other hand, dpi makes no difference: only pixels count. We explain why below.
Basic concepts
- Pixels (px): the individual dots of colour that make up a digital image.
- Dots: the individual dots of colour that make up a printed image. They are used to express density, usually in dots per inch.
- Resolution: in printing, it is the number of dots that can be printed per inch; print shops, photo labs and printers work with this figure. In digital formats, resolution means the number of pixels of a screen or an image.
- Size: in the physical world, size is measured in millimetres, centimetres or inches. In the digital world, image size is measured in pixels (width × height); it belongs to the image itself, wherever it is used. In a digital image, size and resolution mean the same thing.
- Weight: for paper, weight (grams per square metre, or gsm) describes the thickness of the material. For a digital image, it is the space the file takes up, usually in megabytes (MB).
- DPI, PPI and dot density: DPI stands for "dots per inch" and PPI for "pixels per inch"; in everyday use they are treated as equivalent. Both describe how many dots a device or printing system can fit into one inch. For example, 96 dpi means 96 dots along a line one inch (2.54 cm) long.
When uploading photos to your website, keep in mind that the maximum allowed size is 10,000 px on the longest side (width or height).
When to use each term
A physical print is not the same as a digital image. In printing it makes sense to talk about dpi; for digital images it does not, because an image simply has a size in pixels, and how large it looks depends on the density of the device where it is displayed or printed.
Practical examples
Example 1: the same print at different densities
A printed photo measures 6 × 4 in (15 × 10 cm). That physical size never changes, but depending on the printer it can be printed at 150, 200, 300 or even 1200 dpi. What changes is the digital resolution needed to print well at each density:
150 dpi: 886 × 591 px.
200 dpi: 1181 × 787 px.
300 dpi: 1772 × 1181 px.
1200 dpi: 7087 × 4724 px.
Example 2: the same image on different screens
An image of 1024 × 768 px shown at its native size (without scaling) looks bigger or smaller depending on the screen:
15-inch laptop, 1366 × 768 px (33.2 × 18.7 cm screen): the image measures about 24.9 × 18.7 cm.
iPhone 12, 6.1-inch screen, 1170 × 2532 px (about 7.15 × 14.67 cm), held vertically: about 6.3 × 4.5 cm.
27-inch 5K iMac, 5120 × 2880 px (about 59.5 × 33.5 cm): about 11.9 × 8.9 cm.
So the same photo does not have the same physical size on every device: it depends on the screen size and its pixel density. In practice, images are usually scaled to display as well as possible on each screen.
How to convert between dpi, size and pixels
Just remember that 1 inch = 2.54 cm. Round the result down, since there are no half pixels.
From size and dpi to pixels
Pixels = inches × dpi (or Pixels = (cm / 2.54) × dpi)
6 × 4 in (15 × 10 cm) at 200 dpi: (15 / 2.54) × 200 = 1181 px wide and (10 / 2.54) × 200 = 787 px high.
16 × 12 in (40 × 30 cm) at 300 dpi: (40 / 2.54) × 300 = 4724 px wide and (30 / 2.54) × 300 = 3543 px high.
From pixels and dpi to size
Inches = pixels / dpi (or cm = (pixels / dpi) × 2.54)
4000 × 3000 px at 200 dpi: 20 × 15 in (50.8 × 38.1 cm).
2500 × 1800 px at 300 dpi: 8.3 × 6 in (21.17 × 15.24 cm).
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