You need to resize a video or image to a specific width, but you need to know the corresponding height so it doesn't get distorted. Doing it by hand involves a simple rule of three, but repeating it every time a measurement changes is needlessly tedious for something a dedicated calculator solves instantly.
What aspect ratio is, in short
Aspect ratio is the proportion between an image or screen's width and height, normally expressed as two numbers (16:9, 4:3, 21:9). This proportion is what determines whether an image looks "normal" or distorted when resized: as long as the same ratio between width and height is kept, the image retains its correct visual proportions, no matter how much it grows or shrinks in absolute size.
How to calculate the right width or height
- Enter the aspect ratio you need (16:9, 4:3, 21:9, or a custom one).
- Enter one known dimension (for example, the width you need).
- The tool automatically calculates the other dimension (the corresponding height) that keeps the proportion.
You can do it free with the aspect ratio calculator on this site.
The most common aspect ratios and where they're used
- 16:9: the standard for modern video and screens (YouTube, TVs, widescreen monitors).
- 4:3: the classic TV and old monitor format, still present in some specific contexts.
- 21:9: ultrawide format, common in cinema and high-end gaming monitors.
- 1:1: square format, popular on social media like Instagram posts.
- 9:16: the vertical format for stories and short mobile videos (Reels, TikTok, Stories).
How the simplified ratio works
Any pair of dimensions (for example, 1920x1080) can be simplified to its most basic aspect ratio (16:9) by dividing both numbers by their greatest common divisor. This is useful for identifying which standard aspect ratio corresponds to specific dimensions you have in front of you, without having to memorize every possible combination.
Real use cases
- Resizing an image or video to a specific width without distorting it, calculating the correct height beforehand.
- Designing for multiple formats, calculating the exact dimensions needed to adapt the same content to different platforms (square for feed, vertical for stories).
- Setting up an element with a fixed proportion in CSS or design, knowing in advance what height corresponds to a given width.
Frequently asked questions
Can I use a custom aspect ratio, not just the standard ones? Yes, you can enter any numeric ratio, not just the most common ones.
How do I know the aspect ratio of an image I already have? Enter its current dimensions; the tool can show you the corresponding simplified ratio.
Does this resize the image directly? No, it calculates the correct dimensions; to apply the actual resize, use a tool like the image resizer.
Is anything I calculate saved? No, the calculation happens in your browser.
Calculate the right width or height while keeping the proportion for free with the aspect ratio calculator, for 16:9, 4:3, 21:9 and more.