The Aspect Ratio Calculator finds the missing dimension that keeps an image, video, or screen in proportion. Pick an aspect ratio (or type a custom one), choose whether you are solving for width or height, enter the dimension you know, and it gives you the other — plus the simplified ratio and a set of common sizes at that ratio.
It is the quick tool for resizing a video to 16:9, fitting a photo to a 4:3 frame, sizing a banner, or working out a resolution without stretching anything. Everything is calculated in your browser.
How to use the aspect ratio calculator
- Choose an aspect ratio from the presets (16:9, 4:3, 21:9…) or type your own.
- Pick whether you want to solve for the width or the height.
- Enter the dimension you already know, in pixels.
- Read the matching dimension, the full size, and the simplified ratio.
What is an aspect ratio?
An aspect ratio is the proportional relationship between width and height, written as width:height — so 16:9 means 16 units wide for every 9 units tall. It describes shape, not size: a 1280×720 image and a 1920×1080 image are both 16:9.
Keeping the ratio fixed when you resize is what stops images and videos from looking stretched or squashed. To scale without distortion, both dimensions must change by the same factor.
Common aspect ratios and where they are used
Different media settled on different ratios. The table below lists the ones you will meet most often.
Common aspect ratios and typical uses.
| Ratio | Example size | Commonly used for |
|---|---|---|
| 16:9 | 1920 × 1080 | HD/4K video, YouTube, monitors, TVs |
| 9:16 | 1080 × 1920 | Phone screens, Reels, Shorts, Stories |
| 4:3 | 1024 × 768 | Older screens, many cameras, slides |
| 3:2 | 1080 × 720 | DSLR photos, prints |
| 21:9 | 2560 × 1080 | Ultrawide monitors, cinematic video |
| 1:1 | 1080 × 1080 | Square social posts, profile images |
How to calculate an aspect ratio
To find a missing side, multiply the dimension you have by the ratio of the other side. For a 16:9 ratio with a known width, height = width × 9 ÷ 16; with a known height, width = height × 16 ÷ 9.
To simplify a resolution into a ratio, divide both numbers by their greatest common divisor. For 1920×1080 the greatest common divisor is 120, giving 16:9.
Frequently Asked Questions
How do I calculate width from height for 16:9?
Multiply the height by 16 and divide by 9. For a height of 720, the width is 720 × 16 ÷ 9 = 1280, giving 1280×720.
What is the aspect ratio of 1920×1080?
It is 16:9. Divide both numbers by their greatest common divisor (120): 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9.
What does 16:9 mean?
16:9 describes the shape of a rectangle that is 16 units wide for every 9 units tall. It is the standard for HD and 4K video, YouTube, monitors, and TVs.
How do I keep an image in proportion when resizing?
Change both the width and height by the same factor so the ratio stays constant. Set the aspect ratio here, enter the new width or height, and use the matching dimension the calculator returns.
What aspect ratio should I use for social media?
Use 1:1 for square feed posts, 9:16 for full-screen vertical video (Reels, Shorts, Stories), 4:5 for portrait feed posts, and 16:9 for landscape video.