Aspect Ratio Calculator
This aspect ratio calculator does two jobs. Resize a width or height while it holds the ratio steady, so your image or video never ends up stretched, or feed it two pixel dimensions and it'll hand back the simplified ratio. Pick a preset like 16:9 or type your own, and you'll see the answer with a scaled preview that updates as you go.
- Resize keeping the ratio
- Pixels to ratio
- 16:9, 4:3, 21:9 presets
- Simplified ratio
- Live preview
Last updated June 16, 2026 ratio = width ÷ height, reduced Reviewed by the Calcowa math team
Enter positive numbers above 0.
Decimals are fine. Pixel answers are rounded to whole numbers.
What is an aspect ratio?
An aspect ratio is the proportion between an image's width and its height, written as two numbers joined by a colon, such as 16:9 or 4:3. It's the shape of a rectangle, not its size, so a phone thumbnail and a cinema screen share a ratio whenever their proportions match. Get the ratio right and your media fits its frame cleanly; get it wrong and you'll see black bars or a stretched, distorted picture. That's the whole point of matching it.
How do you calculate an aspect ratio?
Turning a pixel size into a ratio takes one division and one reduction. Here's how it works for a 1920 by 1080 screen, the same steps the calculator runs:
- 1
Divide width by heightA 1920 by 1080 image gives 1920 ÷ 1080 = 1.778, the decimal form of the ratio.
- 2
Find the greatest common factorThe largest number that divides both 1920 and 1080 is 120.
- 3
Reduce both sidesDivide each by 120: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9.
- 4
To resize, scale one sideKeep that ratio and pick a new width or height; the other side follows, so the shape stays the same.
Resizing while you keep the aspect ratio
When you scale an image or video, both sides have to move together or it'll look wrong. Lock the ratio, set the side you care about, and let the calculator work out the other, so you don't do any math. For a 16:9 clip you want 1280 px wide, the height is 1280 × 9 ÷ 16 = 720 px. That's why the resize mode asks for the ratio first, then one dimension: it returns the partner value that holds the proportion exactly, so nothing gets squashed.
Common aspect ratios and where they're used
| Ratio | Example size | Used for |
|---|---|---|
| 16:9 | 1920×1080 | TVs, monitors, YouTube |
| 4:3 | 1024×768 | Older screens, slides |
| 3:2 | 1080×720 | DSLR photos, prints |
| 21:9 | 2560×1080 | Ultrawide monitors, cinema |
| 1:1 | 1080×1080 | Square social posts |
| 9:16 | 1080×1920 | Vertical video, stories |
Tap any preset above and you'll load it instantly. For a plain number ratio that isn't about pixels, our ratio calculator simplifies and solves proportions the same way.
Frequently asked questions
Does aspect ratio change image quality?
The ratio sets the shape; it doesn't touch the sharpness. Quality comes from the pixel count and the source. Resizing to a smaller size within the same ratio keeps the look; forcing a different ratio is what distorts the picture.
Divide the width by the height, then reduce that fraction to its simplest whole-number form. For 1920 by 1080, both divide by 120, so it simplifies to 16:9. The quick way's to find the greatest common factor of the two pixel values and divide each by it. This aspect ratio calculator does that for you, so you don't reduce by hand and you'll get a clean ratio in one go.
16:9 means that for every 16 units of width there are 9 units of height, so any matching size works. Common 16:9 resolutions are 1280×720, 1920×1080, 2560×1440, and 3840×2160. To get a height from a width, multiply the width by 9 and divide by 16; a width of 1920 gives 1920 × 9 ÷ 16 = 1080, so you're never locked to one size.
Lock the original ratio, then scale one side and let the other follow. If a photo is 16:9 and you want it 1280 px wide, the height is 1280 × 9 ÷ 16 = 720 px. Keeping width and height in the same proportion means it doesn't look stretched or squashed. The resize mode above does this in one step, either way round.
Aspect ratio is the proportional relationship between width and height, written as two numbers with a colon, like 4:3 or 16:9. It's about shape, not size, so a tiny thumbnail and a huge billboard share the same ratio if they're the same proportions. It matters for screens, photos, and video, where the wrong one leaves black bars or crops the picture.
16:9 is the standard for modern TVs, monitors, laptops, and online video like YouTube. Older screens and many documents used 4:3. Photography leans on 3:2 and 4:3, ultrawide monitors use 21:9, and social posts often want 1:1 or 9:16 for vertical. The presets above cover the ones you'll meet most.
Enter the width and height in pixels in the find-the-ratio mode and the tool divides both by their greatest common factor to give the simplest ratio, plus the decimal value and the closest standard ratio. So 1366×768 reduces to 683:384, which sits very close to 16:9, the ratio that screen's built to approximate. It's close to 16:9, though it isn't exact.
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Resizing media?
Set your ratio above, or browse the full math hub.