Calculate the missing dimension to keep an image proportional, or upload a photo to detect its exact aspect ratio.
Keeping an image proportional when resizing is essential to avoid a stretched or squashed result, but manually calculating the correct matching dimension involves a small bit of ratio math that's easy to get wrong under time pressure. Given an original size and a target width, the matching height is: new height = original height × (new width ÷ original width) — simple in principle, but tedious to redo by hand every time a new size requirement comes up.
This tool handles that calculation directly: enter your image's original dimensions, then enter either a target width or height, and the missing value is calculated automatically to preserve the exact same proportions. The simplified ratio (like 16:9 or 4:3) is also shown, which is useful context when a platform specifies its size requirements in ratio form rather than exact pixels. A second mode lets you skip the manual entry entirely — upload the actual image, and its exact pixel dimensions and aspect ratio are detected automatically.
It's used by designers and developers who need to resize an image for a specific slot in a layout without distorting it, video editors matching thumbnail dimensions to a target ratio, and anyone submitting a photo to a platform with strict aspect ratio requirements. The image detection mode runs locally by reading the file's dimensions directly in your browser, with nothing uploaded to a server.