Lightroom - Using Preset & Custom Aspect Ratios

To change your image aspect ratio, head over to the Crop Overlay tool using the icon or using the short-code R. Lightroom has a number of factory-presets at your disposal that conform to the most frequently used crop settings. Using the pull-down menu illustrated, select a crop, then drag and position the crop on your image using the drag handles.

If you want to use a custom crop, then select 'Enter Custom' from the pull-down menu and follow the prompts. You can also set a custom crop using by selecting the padlock. This toggles the constrained portions; i.e. the width and height of the image. So, with the padlock unlocked you can set whatever aspect ratio you want visually. 

TOP TIP - If you want to toggle between a landscape and portrait aspect ratio, just use the shortcode T. For instance, if you have an aspect ratio of 4x3, this will toggle that to 3x4.