Aspect Ratio Image Video Calculator

Aspect ratio for image and video

The aspect ratio is just the proportion width:height. To boil a resolution down to its simplest ratio, divide both numbers by their greatest common divisor. So 1920×1080 ÷ 120 lands on 16:9. The ones you bump into most: 16:9 (HDTV, Full HD, 4K UHD, and the default for YouTube and streaming), 21:9 (~2.39:1, the ultrawide of cinema and gaming monitors), 4:3 (legacy CRT TVs and older computer monitors), 1:1 for the classic square Instagram feed post, and 9:16 for vertical video on Reels, TikTok and Shorts. A couple more examples: 3840×2160 reduces to 16:9 (GCD 240), while 2560×1080 comes out to 64:27 ≈ 21:9.

Applications: responsive design and video editing

You'll see it in responsive web design, where CSS aspect-ratio: 16/9 holds space for an embed so the page doesn't jump as it loads. It shows up again when you set up video editing sequences in Premiere Pro, Final Cut Pro or DaVinci Resolve. Photographers lean on it to plan sensor crop modes, and creators use it to preview a vertical 9:16 cut before posting.

FAQ

How do I resize without distortion? Hold the ratio steady. Take new_height = new_width · (H/W) from the original frame and the picture stays in proportion.

What does 21:9 mean exactly? Honestly, it's a marketing label. It covers ratios somewhere around 2.37–2.39:1, the kind you find in cinema and on ultrawide monitors like 2560×1080 and 3440×1440.

Why does my 16:9 video show black bars? Either the player viewport isn't 16:9 or the encoded resolution doesn't line up with a true 16:9. The bars are letterboxing, added to keep the source ratio intact.