Aspect Ratio Calculator
1920 x 1080 simplifies to 16:9, and 1080 x 1350 simplifies to 4:5. This aspect ratio calculator reduces any width and height pair to the simplest ratio, shows the decimal ratio, and estimates a matching resized width or height that keeps the same shape. It is useful when you are exporting video, resizing images for social posts, checking whether a screen matches a published spec, or figuring out why one format crops while another letterboxes. Enter the original dimensions first, then add one target dimension if you want a proportional resize.
Quick answer
The ratio compares width to height. Two files with the same ratio have the same shape, even if their resolutions are very different.
What this tells you
- •The ratio compares width to height. Two files with the same ratio have the same shape, even if their resolutions are very different.
- •The simplified ratio is found by dividing width and height by their greatest common divisor.
- •The decimal ratio is width divided by height, which is handy when software lists ratios as decimals instead of `16:9` style labels.
- •If you enter a target width, the calculator finds the matching height. If you enter a target height, it finds the matching width.
- •Resized values are rounded to whole pixels because exported images and videos need integer dimensions.
How to Use
- 1Enter the original width and height in pixels or any other matching unit. The ratio only works if both dimensions use the same unit.
- 2Click Calculate to see the simplified ratio and decimal ratio for the original size.
- 3If you want to resize proportionally, enter either a target width or a target height. Leave the other target field blank.
- 4Use the resized result as a starting dimension for exports, thumbnails, canvas setup, CSS mockups, or screen comparisons.
- 5If the output is not what you expected, double-check orientation. `1080 x 1920` is `9:16`, while `1920 x 1080` is `16:9`.
How It Works
Formula
Simplified ratio = width / gcd(width, height) : height / gcd(width, height)
Decimal ratio = width / height
Scaled height = target width x original height / original width
Scaled width = target height x original width / original heightThe calculator first reduces the original dimensions with the greatest common divisor so the ratio is shown in lowest terms. Then, if you provide one target dimension, it scales the missing side by the same proportion. For example, `1920 x 1080` reduces to `16:9` because both values divide by 120. If you resize that frame to 1280 pixels wide, the matching height is `1280 x 1080 / 1920 = 720`.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Simplify a Full HD frame
Both dimensions divide by 120, so `1920 / 120 = 16` and `1080 / 120 = 9`. This is the standard widescreen ratio used for YouTube, presentations, and most HD video.
Check a portrait social image
The greatest common divisor is 270. Dividing both values by 270 gives `4:5`, which is a common portrait layout for social feed images because it uses more vertical space than a square without becoming a full story format.
Resize 4K video down to 1280 pixels wide
The source frame is also `16:9`, so scaling down by width keeps that shape. The calculator multiplies `1280 x 2160 / 3840` and rounds the result to 720 pixels high.
Resize a phone photo to 1080 pixels wide
This image simplifies to `3:4`, so keeping the ratio means the new height must stay one third larger than the width. `1080 x 4032 / 3024` equals 1440 exactly, which makes it a clean resize for portrait sharing.
Scale by height when width is unknown
The calculator uses `500 x 1600 / 900`, which equals about 888.8889. It rounds to 889 pixels because image and video exports need whole-number dimensions.
Common Aspect Ratios
These common ratios help when you are matching export presets, social templates, or display specs.
| Ratio | Typical use |
|---|---|
| 1:1 | Square profile images and feed posts |
| 4:5 | Portrait social feed images |
| 3:2 | Many DSLR photo exports and prints |
| 16:9 | HD and 4K video, laptops, slides |
| 16:10 | Some monitors and productivity laptops |
| 21:9 | Ultrawide displays and cinematic crops |
| 9:16 | Stories, reels, and vertical video |
A matching ratio does not guarantee a matching resolution. It only guarantees the same shape.
Why Aspect Ratio Matters More Than Raw Pixel Count
Aspect ratio answers a shape question, not a quality question. A `1600 x 900` file and a `3840 x 2160` file are both `16:9`, so they fit the same frame without stretching, but the 4K version carries far more pixels and detail. This is why software can accept both files in the same layout while still showing one as much sharper than the other.
Ratio also explains common crop and letterbox problems. If you place a `4:5` image inside a `16:9` frame, something has to give. You either crop part of the image, add empty space, or distort it. Checking the ratio before export helps you pick the right canvas size first instead of fixing the mismatch later.
Common mistakes
- Confusing `16:10` with `16:9`. They look similar, but `1920 x 1200` is not the same shape as `1920 x 1080`.
- Entering both target width and target height even though only one is needed for a proportional resize.
- Mixing portrait and landscape orientation. Swapping the same numbers changes `16:9` into `9:16`.
- Expecting rounded resize results to preserve the ratio perfectly at every decimal place. Whole-pixel rounding can create a tiny decimal difference.
- Assuming two assets with the same ratio will have the same clarity, file size, or print quality. Ratio only describes proportion.
Embed this calculator on your site
Drop this single line where you want the calculator to appear. It is responsive, mobile-friendly, resizes automatically, and is free to use with attribution.
<script src="https://calctide.com/embed.js" data-tool="aspect-ratio-calculator" async></script>Preview the embed at /embed/aspect-ratio-calculator/.