How to Calculate Image Aspect Ratio

Step 01

Reduce width and height to the simplest ratio.

Write the pixel width and height in the same orientation shown by the file. Divide both numbers by their greatest common divisor to obtain the smallest whole-number ratio.

A 1920 × 1080 image has a greatest common divisor of 120. Dividing both dimensions by 120 gives 16 × 9, so the aspect ratio is 16:9.

Step 02

Calculate the missing dimension.

To preserve the frame at a new width, multiply the original height by the new width and divide by the original width. To preserve it at a new height, multiply the original width by the new height and divide by the original height.

• New height = original height × new width ÷ original width.
• New width = original width × new height ÷ original height.
• Use the full calculation before rounding to a whole pixel.
• Check whether the destination has an exact required size as well as a required ratio.

Step 03

Worked example: scale 1920 × 1080 to 1280 pixels wide.

The matching height is 1080 × 1280 ÷ 1920 = 720 pixels. The resulting 1280 × 720 image remains 16:9 and does not require cropping.

If the destination instead requires a 1200 × 1200 square, proportional resizing alone cannot fill the frame. The image must be cropped, placed inside a padded square or distorted; distortion is normally the least useful choice.

Step 04

Separate resizing, cropping and compression.

Resizing changes pixel dimensions. Cropping removes part of the frame and may change the aspect ratio. Padding adds space around the image to reach another ratio. Compression changes how the file is encoded and can reduce file size without changing the frame.

Choose the operation from the destination requirement rather than treating these terms as interchangeable. Keep an original file before destructive cropping or repeated lossy compression.

Step 05

Check orientation and rounding.

• Keep width before height consistently; 16:9 and 9:16 describe different orientations.
• Use actual pixel dimensions after any automatic camera-orientation correction.
• Allow a one-pixel rounding difference when a target dimension does not divide exactly.
• Verify the exported file rather than relying only on values entered before processing.