News flash—we didn't become designers to do math.
We became designers because we want to make nice things and help people.
Turns out, it's hard to be creative when you're constantly interrupted with math problems. Shocker.
Let's say we're designing an app for the latest mobile device. We're designing a video player on a vertically-oriented display.
We know the video is 16:9.
The device width is 375.
We want the video to take up the entire width of a vertical display. What should the height of the video be?
There's no way in hell we're trying to use math—so we stick with what we know. We draw a rectangle that's 160 by 90. We lock the proportions of the rectangle. We resize the rectangle to 375, and we see what happens.
We do this again and again every time we need a new size for something. 4:3—12:5—8:10…
This time we're going to use Sizey.
375 and 16:9
We have our answer—210.9
And because half-pixels are as bad as having to share Nutella, we’ll call it a solid 211.
Your 16:9 video is now perfectly 375 by 211.