How to use the Aspect Ratio Calculator
- Enter original width and height.
- Type a new width or height to resize.
- Or click a preset ratio.
About this tool
Aspect ratio is width divided by height. Getting it right avoids stretched thumbnails and black bars: YouTube and most monitors use 16:9, Instagram feed posts favour 4:5 or 1:1, Reels/TikTok/Shorts use 9:16, and ultrawide monitors are 21:9.
Enter the original dimensions, then type a new width or height and the other value is calculated to preserve proportions — handy for responsive images, video exports and CSS aspect-ratio values.
Frequently asked questions
What's the YouTube thumbnail size?
1280×720 (16:9).
Can CSS keep ratios?
Yes: aspect-ratio: 16 / 9; on the element.