About the Aspect Ratio & Crop Calculator
This aspect ratio calculator simplifies an image or video's width and height down to its ratio, such as 16:9 or 4:3, and works out the exact pixel dimensions and centred crop position needed to hit a target ratio for a specific platform. It is built for the planning step before opening an editor — knowing the exact numbers in advance rather than cropping by eye.
How to use the Aspect Ratio & Crop Calculator
- 1
Enter the original width and height in pixels.
- 2
Choose the target ratio from the list — common social media and video formats are included.
- 3
Read the simplified original ratio and the crop dimensions needed to hit the target.
- 4
Use the offset values to position the crop centred within the original image.
What people use it for
Preparing an image for Instagram or TikTok
Work out the exact crop needed for a square post, a 4:5 portrait post, or a 9:16 story or Reel before opening an editor.
Checking a video's aspect ratio before export
Confirm what ratio a video's current resolution actually is, useful when receiving raw footage from an unfamiliar source.
Planning a thumbnail or banner crop
Calculate the crop region needed to fit a photo into a YouTube thumbnail's 16:9 ratio or a website banner's ultrawide format.
How the ratio is simplified
The width and height are divided by their greatest common divisor, the largest number that evenly divides both — exactly the same arithmetic used to simplify a fraction. A 1920 by 1080 image shares a greatest common divisor of 120, and dividing both numbers by 120 gives 16 and 9, the familiar 16:9 widescreen ratio. This is an exact calculation, not an approximation, so an unusual resolution that doesn't match a standard name will simply show its own precise simplified ratio.
How the centred crop is chosen
Given a target ratio, the calculator finds the largest region of that shape that fits entirely inside the original image, then centres it — trimming evenly from whichever dimension is proportionally longer. Cropping a wide 16:9 image to a square keeps the full height and trims equally from each side; cropping it to a tall 9:16 keeps the full width and trims from top and bottom. This mirrors exactly how a dedicated image cropping tool positions an automatic centred crop.
Tips
- If the main subject isn't centred in the original image, use the crop dimensions as a size guide but reposition the crop manually in an editor.
- 9:16 and 16:9 are exact inverses of each other — useful when converting between a horizontal video and a vertical one.
- Export video and images at a resolution that is an exact multiple of the target ratio to avoid a one-pixel rounding gap after cropping.