Sometimes the most simple tools are the best and this is a great example of one. The aspect ratio calculator allows you to enter specific file dimensions and get the ratio of those dimensions.
From there you can enter the dimensions of an image you want to create and the tool then returns the values that you require. This can be useful if you wish to make an image that is 1200px wide and uses a HD ratio, which would require a height of 675px.
https://andrew.hedges.name/experiments/aspect_ratio/
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