Video resolution explained

Resolution is how many pixels make up each frame, written as width × height. 1920 × 1080 is what people mean by 1080p. The second number is the one the shorthand uses.

The key relationship is that pixel count goes up with the square of the dimensions. 4K is not twice 1080p — it is four times, because both width and height doubled. That is why a 4K clip is so much larger than the number 4K suggests.

Resolution is not sharpness

A blurry 4K video is still blurry. Resolution sets the ceiling on detail; focus, lighting, lens and bitrate decide whether you get near it. A well-lit, well-encoded 720p clip looks better than a dim, heavily-compressed 4K one.

What to deliver at

• Phone or messaging — 720p is ample. Most people watch on a screen where 1080p is indistinguishable.
• Web page or embed — 1080p if it can go full-screen, 720p if it is background or thumbnail-sized.
• Social media — match the platform's shape more than its resolution. 1080 × 1920 for vertical feeds.
• TV or projector — 1080p minimum; this is where lower resolutions become obvious.

Resolution and aspect ratio are different things

Changing resolution keeps the shape and changes the pixel count. Changing aspect ratio changes the shape, which means something has to give — letterbox bars, or cropping. Resize Video asks how to fit: Fit adds bars, Fill crops the edges, Stretch distorts. If you want to choose what survives rather than letting the centre win, crop deliberately instead.

Mistakes worth avoiding

• Recording at 4K for something that will only be watched on a phone.
• Upscaling to meet a platform's "recommended" resolution. It adds size, not quality.
• Using Stretch to change shape. It distorts faces and is almost never what you want.