What is Aspect Ratio Calculator?

Enter any width and height to find the simplified ratio. Lock it, then change one dimension and the other scales to match. Free and private — your numbers never leave your device.

The ratio is reduced using the greatest common divisor, so 1920 x 1080 returns 16:9 instead of an awkward 1920:1080. Lock the ratio and a new width snaps the height to the nearest whole pixel, which is what you actually need when resizing for a UI or a video timeline.

How to use

1. Enter width and height values to calculate the simplified aspect ratio (e.g., 1920x1080 gives 16:9).
2. Lock the ratio, then change either width or height — the other dimension updates proportionally.
3. Use the preset ratios (16:9, 4:3, 1:1, 21:9) for quick calculations or enter custom ratios.

When to use

• Resizing a YouTube thumbnail or banner to 16:9 from arbitrary source dimensions.
• Cropping a 4:5 Instagram portrait while keeping the proportions identical to the original.
• Picking the right canvas size for an ultrawide 21:9 desktop wallpaper.

Result

Enter 1920x1080 to see the ratio is 16:9. Lock the ratio and change width to 1280 — height auto-calculates to 720. Common presets: 4:3 for presentations, 16:9 for video, 1:1 for social media.

FAQ

What is an aspect ratio in plain terms?

It is the proportional relationship between the width and the height of a rectangle. 16:9 means the width is 16 units for every 9 units of height, regardless of whether those units are pixels, inches, or centimetres.

How is the ratio reduced to its simplest form?

The calculator finds the greatest common divisor (GCD) of width and height, then divides both by it. So 1280 x 720 becomes 16:9 because gcd(1280, 720) = 80, and 1280/80 : 720/80 gives 16:9.

Why does the height not always come out as a round number when I lock the ratio?

Because aspect ratios are rational, not magic. When you type a width that is not a multiple of the locked ratio's width term, the calculator rounds the height to the nearest whole pixel to keep the image usable.

Which aspect ratio should I pick for social media posts?

Use 1:1 for Instagram feed squares, 9:16 for stories and reels on Instagram or TikTok, 16:9 for YouTube videos, and 4:5 for Instagram portrait feed posts that get more vertical space without being cropped.

Can the same ratio look different on different screens?

The proportional shape is identical, but pixel density and screen size change the apparent size. A 1:1 image will fill a square on every device, just at the resolution that device offers.

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