Skip to content
TabBench

Aspect Ratio Calculator

Calculate aspect ratios, proportional dimensions, and social media image presets.

Runs in your browser. Nothing you add is uploaded.

What the Aspect Ratio Calculator does

Aspect ratio is the proportion of width to height — 16:9 for most video, 4:5 for tall Instagram posts, 9:16 for Reels and Shorts. This calculator reduces any pixel size to its simplest ratio (1920 × 1080 becomes 16:9) and works out the missing side when you resize, so a 1600-pixel-wide image keeps its shape at exactly 900 pixels tall. Presets cover common video resolutions and social media formats, and a preview box shows the shape before you commit to it.

How to calculate an aspect ratio

  1. Enter the original width and height in pixels to see the simplified ratio.
  2. To resize, type the new width and the matching height is calculated, or type the height to get the width.
  3. Pick a preset such as 1080p, 4K, Instagram portrait or YouTube thumbnail to fill in standard sizes.
  4. Check the preview box to see the proportions at a glance.
  5. Copy the dimensions into your editor, export settings or CSS.

The Aspect Ratio Calculator runs entirely in your browser — nothing you enter is uploaded, stored, or logged.

How it works

Proportional Dimension Formula

New Height = (New Width × Original Height) / Original Width

Calculates proportional dimension preserving the original aspect ratio without distortion.

Example: Scaling 1920x1080 to a width of 1280 yields a proportional height of 720.

When to use it

Resizing for social media

A 3000 × 2000 photo is 3:2, which Instagram crops when posted as a 4:5 portrait. Knowing the ratio first tells you whether to crop, pad or choose a different format before you upload.

Video exports and thumbnails

YouTube expects 16:9. A 1280 × 720 thumbnail and a 3840 × 2160 video share that ratio, so neither is letterboxed. Checking the ratio avoids black bars and stretched faces.

Responsive web layouts

Reserving space for images and embeds with the CSS aspect-ratio property stops the page jumping as they load. Calculate the ratio of your media once and use it in the stylesheet.

Good to know

  • Scaling an image down keeps quality; scaling up beyond the original pixel size makes it blurry whatever the ratio.
  • Changing the ratio always means cropping or adding bars. Decide which part of the picture matters before converting 16:9 to 9:16.
  • Screens are 16:9 or 16:10, most phone cameras shoot 4:3 photos, and most DSLRs shoot 3:2.
  • For print, the ratio of the paper matters too: A-series paper is about 1:1.414, which neither 4:3 nor 3:2 matches exactly.

Guides that use this tool

Frequently asked questions

What is the aspect ratio for YouTube thumbnails?

YouTube thumbnails use a 16:9 aspect ratio with a recommended resolution of 1280x720 pixels (minimum width 640px).

How is an aspect ratio simplified?

By dividing both sides by their greatest common divisor. For 1920 × 1080 the divisor is 120, giving 16:9. Some sizes do not reduce to familiar numbers; 1366 × 768 is really 683:384, which is approximately 16:9.

What size should an Instagram post be?

Square posts are 1:1 (1080 × 1080), portrait posts 4:5 (1080 × 1350) and Reels and Stories 9:16 (1080 × 1920). Portrait takes up the most space in the feed.

Is 16:9 the same as 1.78:1?

Yes. A ratio can be written as two whole numbers or as width divided by height: 16 ÷ 9 = 1.78. Cinema formats are usually written the second way, such as 2.39:1.

Related tools