What aspect ratio means
The aspect ratio of a picture, video or screen is the proportional relationship between its width and its height, written as two numbers with a colon. A 1920 × 1080 image has a ratio of 16:9, because dividing both numbers by their greatest common divisor (120) leaves 16 and 9. The ratio says nothing about size: a 640 × 360 thumbnail and a 3840 × 2160 4K frame are both 16:9, so one can be scaled to the other without stretching or cropping.
Common ratios and where they appear
- 16:9 – widescreen video, YouTube, most TVs, laptops and monitors.
- 4:3 – older TVs and monitors, many tablets and some compact cameras.
- 3:2 – 35 mm film and many DSLR cameras, and some laptops.
- 1:1 – square social media posts and profile pictures.
- 4:5 and 9:16 – portrait posts and vertical video such as stories and Reels.
- 21:9 – ultrawide monitors and cinema-style video.
How to scale while keeping the ratio
To find a new height from a new width, multiply the width by the ratio's height and divide by its width. For 16:9 at a width of 1280, the height is 1280 × 9 ÷ 16 = 720. To work from a height, do the reverse. The second calculator does this for any ratio, which is helpful for sizing video embeds, responsive images, CSS boxes and slide layouts. Fractional results are shown with two decimals because actual pixels must be whole numbers, so round them when you build the final asset.
Close ratios and odd sizes
Screens often have sizes that are close to, but not exactly, a standard ratio. A 1366 × 768 laptop screen reduces to the awkward 683:384 but is sold as 16:9, since it is within a fraction of a percent. This calculator reports the exact reduced ratio and also names the closest standard ratio when one lies within 1%. In CSS, the aspect-ratio property can hold a box to a ratio, and padding-top percentages are an older trick that achieves the same.
When you need to change an image to a different ratio, scaling is not enough. You must crop or add padding. The image resizer on this site offers a fill-and-crop mode for this. Calculations run in your browser.