This Windows application helps you to obtain a desired aspect ratio of an image without cutting it.
The application helps you calculate the new width and height. You can then use an image editor to set the new width and height. For example, in Photoshop you can fill the resulting width and height into Image -> Canvas size. Don't forget to set the meaasurement unit to pixels.
Fill in the ratio and the initial image width and height and get your resulting values from the right side.
You can download the app from the Releases section.