Aspect ratio is the proportional relationship between an image's width and height, expressed as width:height. A 1920×1080 image has a 16:9 aspect ratio (divide both dimensions by 120). Common aspect ratios: 16:9 (widescreen video, YouTube, monitors), 4:3 (traditional TV, early digital cameras), 1:1 (square, Instagram), 3:2 (35mm film, most DSLR cameras), and 9:16 (vertical video, Instagram Stories, TikTok).
Aspect ratio matters when displaying images in fixed-size containers. If an image's natural aspect ratio doesn't match the display container, you must choose between: letterboxing/pillarboxing (adding black bars), cropping (removing parts of the image), or stretching (distorting proportions). Most display systems default to maintaining aspect ratio to avoid distortion.
When using FileCurve's Resize Image tool, you can lock or unlock the aspect ratio. Locking it ensures the image isn't distorted when you change one dimension. Unlocking allows you to crop to an exact dimension — useful for social media where images must fit specific ratios. The Crop Image tool lets you select a specific aspect ratio crop area.