Aspect Ratio Calculator Online
Enter any width or height and the aspect ratio to instantly calculate the missing dimension. Includes presets for 16:9, 4:3, 1:1, 9:16, and more. No signup, runs entirely in your browser.
How the Aspect Ratio Calculator Works
- 1Select a preset ratio (16:9, 4:3, 1:1, etc.) or enter a custom ratio in the W:H inputs at the top.
- 2Enter the width — the height is calculated automatically. Or enter the height to calculate the width.
- 3The detected ratio shows the simplified ratio for any custom dimensions you enter.
- 4Click Copy to copy the dimensions and ratio to your clipboard.
Why Aspect Ratios Matter
Maintaining the correct aspect ratio prevents images and videos from appearing stretched or squashed. Video editors, designers, and developers constantly use aspect ratio calculations when exporting at different resolutions, designing responsive layouts, cropping thumbnails, or specifying canvas sizes. A 16:9 ratio at 1280×720 is the same shape as 1920×1080, 2560×1440, or 3840×2160 — they are all 16:9.
Aspect Ratio by Platform
YouTube & TV
16:9 is the standard for all HD and 4K video. Use 1920×1080 for Full HD or 3840×2160 for 4K. Older content may be 4:3.
Instagram Feed
Square 1:1 (1080×1080), portrait 4:5 (1080×1350), or landscape 1.91:1 (1080×566). Stories and Reels: 9:16 (1080×1920).
Twitter / X
In-stream images display at approximately 16:9 cropped to 2:1. OG images use 1.91:1 or 2:1. Cards use 1:1 for small previews.
Photography
Most DSLRs shoot 3:2 (the classic 35mm format). Micro Four Thirds sensors use 4:3. Medium format is often 1:1 or 4:5.
CSS Responsive Design
Use aspect-ratio: 16 / 9 in CSS to maintain a ratio automatically. Combine with width: 100% to fill a container.
A4/Letter paper is approximately 1:√2 ≈ 1:1.414. US Letter is 8.5:11 ≈ 1:1.294. Neither maps cleanly to screen ratios.
Frequently Asked Questions
What is an aspect ratio?
An aspect ratio is the proportional relationship between an image's width and height, expressed as two numbers separated by a colon (e.g. 16:9). It describes the shape of the image without specifying its actual size. Two images with the same aspect ratio have the same shape regardless of their pixel dimensions.
How do I calculate the height from a width and aspect ratio?
Height = Width × (ratio height / ratio width). For a 16:9 image that is 1280px wide: height = 1280 × (9/16) = 720px. This tool does this calculation automatically — just enter the width and the result appears instantly.
What aspect ratio should I use for YouTube videos?
YouTube uses 16:9 for standard videos. The minimum recommended resolution is 1280×720 (720p HD). For 1080p Full HD use 1920×1080, for 4K use 3840×2160. YouTube will add pillarboxes (black bars) to videos that are not 16:9.
What aspect ratio should I use for Instagram?
Instagram supports multiple ratios: 1:1 (square, the classic Instagram format), 4:5 (portrait, maximum vertical space in the feed), and 1.91:1 (landscape, similar to 16:9 but tighter). For Reels and Stories, use 9:16 (vertical full-screen).
How do I maintain an aspect ratio when resizing in CSS?
Use the CSS aspect-ratio property: .element { aspect-ratio: 16 / 9; width: 100%; }. For older browsers, the padding-top trick works: set padding-top: 56.25% (9/16 × 100%) on a container with position: relative, then absolutely position the content inside.
What does the detected ratio in the tool output mean?
After you enter width and height values, the tool calculates their greatest common divisor and reduces the ratio to its simplest form. For example, 1920×1080 reduces to 16:9. This tells you the canonical ratio for that resolution.