Resolutioner

Resolutioner is a simple utility app for calculating resolutions, aspect ratios, and saving resolutions for future reference.
The resolution calculator helps you to calculate a resolution by filling in the other side, according to your set ratio. Editing the set ratio is as simple as a tap, too. When choosing your scale, you can either put in a standard fraction like 16:9, or put in a resolution you want to scale up or down while keeping the same aspect.
And when you've got a resolution you want to save, just tap Add Preset to save it with a custom name. You can edit your presets at any time.
Resolutioner supports iPhone, iPad, and Mac. All of your saved presets sync over iCloud, too.