Aspect Ratio Calculator

What an aspect ratio really is

An aspect ratio describes the relationship between an image's width and its height, written as width:height. It is a proportion, not a size: a 16:9 frame looks identical whether it is 1280×720 or 3840×2160 pixels, because both keep the same width-to-height relationship. That is why understanding the ratio matters more than memorising specific pixel dimensions — once you know an image is 16:9, you can scale it to any resolution without distorting it.

This calculator turns a pair of pixel dimensions into the simplest whole-number ratio by dividing both numbers by their greatest common divisor (GCD), the largest integer that divides both evenly. It also shows the decimal form (width divided by height) so you can compare ratios that do not reduce to clean integers.

How the simplification works — a worked example

Suppose you enter a width of 1920 and a height of 1080. The tool computes the GCD of 1920 and 1080 using the Euclidean algorithm: it repeatedly replaces the larger number with the remainder of dividing the two, until the remainder is zero.

• 1920 mod 1080 = 840
• 1080 mod 840 = 240
• 840 mod 240 = 120
• 240 mod 120 = 0 → the GCD is 120

Dividing both dimensions by 120 gives 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the simplified ratio is 16:9, and the decimal form is 1920 ÷ 1080 = 1.7778. A messier input like 1366×768 reduces to 683:384 — a reminder that not every common screen size lands on a "nice" ratio, which is exactly why the decimal value is shown alongside it.

Reading the equivalent sizes table

Once a ratio is set, the table lists standard video heights — 240p, 360p, 480p, 720p, 1080p, 1440p, and 2160p — and computes the matching width for your ratio. The named broadcast resolutions (1080p = 1920 wide, 1440p = 2560 wide, 4K = 3840 wide) are shown as their canonical 16:9 widths, while the smaller rows multiply the height by your ratio. If you feed in a 4:3 image and look at the 720p row, you will see roughly 960×720 rather than the 16:9 1280×720 — the table adapts to whatever proportion you typed.

Scaling to a target dimension

The Scale section answers the everyday question "I need this at a specific width — what height keeps it undistorted?" It works both directions, and the labels matter:

• Target Width → Height: enter a width and the tool divides it by the ratio (width ÷ height) to return the height. For a 16:9 source, a target width of 800 yields 800 ÷ 1.7778 ≈ 450 px tall.
• Target Height → Width: enter a height and the tool multiplies it by the ratio to return the width. A target height of 450 yields 450 × 1.7778 ≈ 800 px wide.

Results are rounded to whole pixels, so for awkward ratios you may be off by a pixel — harmless on screen, but worth knowing if you are slicing a sprite sheet where exact alignment counts.

Practical use cases

• Responsive web design: reserve space for an embedded video or hero image so the page does not jump as media loads. Knowing the ratio lets you set a CSS aspect-ratio box.
• Video and social media: 16:9 for YouTube and TVs, 9:16 for vertical Reels and Stories, 1:1 for square feed posts, 4:5 for portrait posts, and 21:9 for ultrawide cinematic crops.
• Print and photography: camera sensors are usually 3:2, while many print sizes assume 4:3 or 5:4 — checking the ratio first tells you how much will be cropped.
• UI mockups: resize a placeholder to a precise width while keeping the original proportions intact.

Common mistakes to avoid

• Confusing ratio with resolution. 720p and 1080p share the same 16:9 ratio; one is not "wider" than the other, just larger. Pick the ratio for shape and the resolution for sharpness.
• Stretching instead of scaling. If you change width and height independently, you break the ratio and your subjects look squashed. Always scale both by the same factor — which is exactly what the Scale fields enforce.
• Assuming 16:10 is 16:9. Many laptops are 16:10 (1.60) and look almost square next to 16:9 (1.78); a small ratio difference is very visible across a full screen.
• Forgetting rounding. When a scaled dimension lands on a half-pixel, browsers and editors round it. For pixel-perfect work, prefer target sizes that divide cleanly by your ratio's terms.

Everything here is computed instantly in your browser with simple arithmetic — no image is uploaded and nothing is sent to a server.