Work out the aspect ratio of any width and height, or scale a size up and down while keeping the same shape.
How to use
- Enter the original width and height.
- To resize, type either a new width or a new height. Leave the other empty.
- Press Calculate to see the ratio and the matching size.
Good to know
The ratio is simplified using the greatest common divisor, so 1920 by 1080 becomes 16:9. Everything runs in your browser. Explore more on the Tools page.
What is an aspect ratio?
An aspect ratio describes the shape of a rectangle by comparing its width to its height. It is written as two numbers separated by a colon, such as 16:9 or 4:3. The first number is the width and the second is the height, so a 16:9 screen is a little less than twice as wide as it is tall. The ratio has nothing to do with how many pixels there are. A tiny thumbnail and a huge poster can share exactly the same ratio, which is why the idea is so useful when you need to resize something without distorting it.
Our aspect ratio calculator takes a width and a height, reduces them to the simplest ratio, and gives you the decimal value as well. If you also type a new width or a new height, it works out the missing side so the new size keeps the same shape. It is a quick way to avoid stretched photos, squashed videos and cropped thumbnails.
How the calculation works
The tool finds the greatest common divisor of the width and height, then divides both numbers by it. For 1920 by 1080 the greatest common divisor is 120, so the ratio becomes 16 by 9. When you resize, the rule is simple. New height equals new width multiplied by original height and divided by original width. New width equals new height multiplied by original width and divided by original height. The decimal value, found by dividing width by height, lets you compare two sizes quickly. Two sizes with the same decimal value have the same shape.
Rounding matters. Pixels are whole numbers, so a resize such as 1000 wide from a 4:3 source gives 750 tall exactly, but a 1001 wide target gives 750.75, which rounds to 751. That tiny difference is normally invisible, and the calculator rounds to the nearest pixel for you.
Common aspect ratios and where they are used
Different screens and platforms have settled on a handful of ratios. Knowing them saves time when you export images or video.
- 16:9 is the standard for TVs, monitors, YouTube videos and most laptop screens. Typical sizes are 1280 by 720, 1920 by 1080 and 3840 by 2160.
- 4:3 is the older television and tablet shape and is still common in presentations and some cameras. Examples are 1024 by 768 and 1600 by 1200.
- 1:1 is a square, popular for profile pictures and classic social media posts, for example 1080 by 1080.
- 9:16 is the vertical version of 16:9 used by Reels, Shorts and Stories, usually 1080 by 1920.
- 4:5 is the tall portrait ratio that works well for feed posts on Instagram, usually 1080 by 1350.
- 3:2 is the shape of most DSLR and mirrorless camera photos and of many prints, for example 6000 by 4000.
- 21:9 is the ultrawide ratio used on cinema style monitors and some films.
- 1.91:1 is the landscape size often used for link preview images, usually 1200 by 630.
Real life examples
Suppose you have a photo that is 4000 by 3000 and your blog needs it 800 pixels wide. Type 800 as the new width and the calculator returns 800 by 600, so nothing gets stretched. Or imagine a video recorded at 1920 by 1080 that you want to show at 640 pixels wide inside a web page. The tool gives 640 by 360. If you only know the height you need, for instance 500 pixels for a banner, it works backwards and tells you the width that keeps the picture undistorted.
Designers also use ratios to plan layouts. If a card must always be 3:2, you can set its width in your stylesheet and let the height follow by using the CSS aspect-ratio property. Checking the numbers here first helps you choose sensible fallback sizes for images and placeholders.
Tips for resizing without losing quality
- Always resize from the largest original you have. Scaling a small image up makes it blurry.
- Keep the ratio locked when you only need a smaller copy. Change one side and compute the other with this tool.
- If the target ratio is different from the source, crop instead of stretching. Cropping removes edges, stretching distorts faces and circles.
- Export at the exact pixel size the platform recommends so that it does not recompress your file.
- Keep a copy of the original in case you need a different ratio later.
Aspect ratio and image or video formats
Many beginners confuse aspect ratio with resolution. Resolution is the total number of pixels, such as 1920 by 1080, while the ratio is just the shape. A 1280 by 720 video and a 1920 by 1080 video are different resolutions but the same 16:9 shape. When a video with a different ratio is shown on a screen, you see black bars. If the video is wider than the screen the bars appear at the top and bottom, which is called letterboxing. If it is narrower the bars appear at the sides, which is called pillarboxing. Matching the ratio ahead of time avoids both.
For images on websites, a fixed ratio also prevents layout shift. When the browser knows the width and height of an image before it loads, it can reserve the right amount of space and the page does not jump around. That is good for readers and it helps your page experience scores.
Frequently asked questions
What is the aspect ratio of 1920 by 1080?
It is 16:9, the most common shape for monitors, TVs and online video.
How do I keep the same ratio when I change the width?
Multiply the new width by the original height and divide by the original width. This tool does that for you when you fill in the new width.
Is aspect ratio the same as resolution?
No. Resolution counts the pixels, while the ratio only describes the proportion between width and height.
Why does my image look stretched after resizing?
The new width and height did not match the original ratio. Enter the original size here and use the suggested value for the other side.
Does this tool upload my numbers?
No. All the maths happens in your browser and nothing is sent to a server.
If you work with pictures often, you may also like our Image Resizer, Image Compressor and Image Converter, which pair well with this calculator.
Aspect ratios for social media and web
Every platform has preferred sizes, and uploading the wrong shape leads to automatic cropping. For a feed post, a square 1:1 image of 1080 by 1080 or a portrait 4:5 image of 1080 by 1350 gives you the most screen space. For stories and short videos, use 9:16 at 1080 by 1920. For a YouTube video or a website hero banner, 16:9 works best. For blog featured images and link previews, 1200 by 630 is a safe choice. Before you design anything, enter the target size here and check the ratio so you know the safe area where text and faces should stay.
Designing for several ratios at once
Creators often need the same picture in three or four shapes. A useful trick is to design on a large canvas, such as 4000 by 3000, and keep the important subject near the centre. Then crop for each platform. Keep text away from the edges by at least five percent of the width so that it survives cropping. Use this calculator to find the exact crop size for each target before you start. For example, a 4000 by 3000 photo cropped to 16:9 becomes 4000 by 2250, and to 1:1 becomes 3000 by 3000.
Glossary
- Letterboxing: black bars above and below a video that is wider than the screen.
- Pillarboxing: black bars on the sides of a video that is narrower than the screen.
- Crop: cutting away part of the picture to change its shape.
- Scale: making the whole picture larger or smaller without changing its ratio.
- Resolution: the number of pixels, written as width by height.
Quick checklist before exporting
- Confirm the original size and ratio.
- Decide the target ratio and check it with this tool.
- Crop first and scale second.
- Export at the recommended size for the platform.
- Preview on a phone and a computer.