Assuming a pixel has a fixed size in millimetres, when its physical size only exists once you pick a print or display size.
Resolution and scaling
Pixel
Also called px, picture element or pixels. Here is what it means, when it changes what you export, and what people get wrong.
Resolution and scalingRuns on your deviceUpdated 10 August 2026
The short answer
A pixel is the smallest single dot of colour an image is made of. A digital photo is a rectangular grid of them, each storing one colour value, usually 8 bits each for red, green and blue. A pixel has no physical size until you print or display it.
Updated
The short version
Pixel at a glance
| Quick fact | Detail |
|---|---|
| Short form | px |
| Typical depth | 8 bits per channel, 256 levels each |
| Colours from 8-bit RGB | 16,777,216 |
| Physical size | None until an output size is chosen |
| Shape in a photo | Square sample on a regular grid |
Slide the table sideways to see every column.
How much information does a single pixel hold?
In a standard 8-bit photo each pixel stores three numbers, one each for red, green and blue, and each number runs from 0 to 255. That is 256 levels per channel and 16,777,216 possible colours per pixel. Formats that carry transparency add a fourth number, alpha, for opacity. Editing formats such as TIFF and camera raw often use 16 bits per channel, which leaves more room to push exposure and colour before banding shows up.
Why does a pixel have no physical size?
A pixel is a sample, not an object. The file stores a grid of values and says nothing about how wide each one should be. Size only appears at output. Print a 2000 pixel wide image across 10 inches and each pixel occupies 1/200 of an inch; print the same file across 5 inches and each pixel is half that. On a high-density screen one image pixel is often drawn across two or three hardware pixels.
Is an image pixel the same as a screen pixel?
No. An image pixel is one sample in a file. A screen pixel is a physical cell in a panel, normally built from three subpixels, one red, one green and one blue, sitting side by side. Text rendering sometimes drives those subpixels individually to fake extra horizontal resolution, which is why a screenshot of text can show coloured fringes when you zoom in. Those fringes live in the capture, not in the original image.
What people get wrong about pixel
Each one is a real failure mode, not a style preference.
Confusing an image pixel with a screen subpixel, the individual red, green or blue element inside a display cell.
Expecting a pixel to hold part of two colours; it stores exactly one value, which is why diagonal edges look stepped.
Questions people ask
Pixel, answered
The follow-up questions people search for once they have the definition.
How many pixels are in a 4000 x 3000 photo?
Twelve million. Multiply width by height: 4000 x 3000 = 12,000,000 pixels, which is 12 megapixels. Every one of those pixels stores its own colour value, so pixel count is the main driver of how much raw data an image contains before compression.
Is a pixel always square?
In photographs and web images, yes. Some legacy video formats used non-square pixel aspect ratios, where each sample was stored wider or taller than it was displayed, but every common still image format today assumes square pixels on a regular grid.
Can you have half a pixel?
Not in a stored file. Pixel counts are always whole numbers. When you resize or rotate, the software computes fresh whole pixels at new positions from the old ones, which is why a resize is never a perfectly reversible operation.
Same cluster
More from the glossary
Neighbouring entries, so every term in the set is one hop from every other.
Pixelation
Pixelation is when individual pixels become visible as square blocks because an image is shown larger than its pixel count supports.
PDF image
A PDF image is a picture placed inside a PDF page rather than an image format in its own right.
PNG
PNG (Portable Network Graphics) is a lossless image format that stores every pixel exactly as it was saved and supports full alpha transparency.
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