Making Measurements

November 1, 2017 • 8 min read

As I write this, the iPhone X is about to launch, and I’m surprised by how many phone reviewers (I’ve found 14) seem to conflate screen size to the diagonal length of the display.1

Here’s the crux of the issue. The iPhone X has a 19.5:9 aspect ratio. This means the screen is approximately twice as tall as it is wide. Previous iPhones have had 16:9 (iPhone 5 and later) or 3:2 aspect ratios (iPhone 4s and earlier).

Of course, no one is actually interested in the diagonal of a screen (it’s the area of the screen that matters), but if two screens have the same aspect ratio, then screen diagonal is a fine proxy.

However, if two screens have different aspect ratios, then the diagonal length of the display is misleading. Here’s an example to illustrate that, where I’ve drawn two shapes. On the left is a square with a diagonal of 1 unit. On the right is a rectangle with a diagonal of 1.5 units. The rectangle has a 50% longer diagonal. But by screen area, it’s 23% smaller than the square.

dimensions.pngScreen size != screen area.

I can continue elongating the diagonal while maintaining a constant screen area by making the rectangle skinnier. There is no limit to how much longer I can make the diagonal, while keeping screen area constant.

Not all tech reviewers have overlooked this fact. Phone Arena did the math, and the screen area of the 5.8 inch iPhone X is 2.6% smaller than the 5.5 inch iPhone Plus. Vlad Savov of The Verge has also been on top of this.

Ideally, as phones move to different shapes (with cutouts, notches, and curved edges), we can talk about screen area instead of diagonal length.2 Even screen area isn’t a perfect proxy of the screen size, though. Because at the end of the day, a bigger screen size is only useful if it lets you do or see more. For some use cases, like watching a movie, this might mean that an 19:9 ratio with smaller screen area than 3:2 ratio is actually better. Or consider on laptops, where vertical space is often at a premium, so 3:2 ratio is preferred to 16:9 ratio, all else being equal.

This is all to say that screen size cannot be summed up by one metric (as is often the case, things are more grey than they are black and white). But if I had to pick one metric, I certainly wouldn’t choose screen diagonal.3 Wouldn’t it be simpler if we just quoted screen area?

  1. Some reviewers, though, have gotten it right.

    Mark Spoonauer of Tom’s Guide:

    It’s worth noting that this 5.8-inch screen gives you less viewing area than the 5.5-inch iPhone 8 Plus, because the iPhone X’s screen has a narrower aspect ratio.

    Neil Cybart of Above Avalon:

    iPhone X has a little bit less screen real estate (in terms of area) than iPhone Plus. The 5.8-inch screen has a more vertical element than its iPhone Plus sibling.

  2. In fact, the “diagonal” reported for phones like the iPhone X and Galaxy Note 8 are not even their actual diagonal. Even though these screens have rounded corners, when measuring the diagonal, the companies are measuring as if the screen were an actual rectangle.

    From Apple’s iPhone X tech specs:

    The iPhone X display has rounded corners that follow a beautiful curved design, and these corners are within a standard rectangle. When measured as a standard rectangular shape, the screen is 5.85 inches diagonally (actual viewable area is less).

    What, then, are we actually measuring? How much can a corner be rounded off before we stop using it’s “rectangular” diagonal? What about measuring the rectangular diagonal of a circular display, because we can think of a circle as actually transcribed in a square, so that it actually has (to quote Apple) “corners within a standard rectangle.” This practice seem disingenuous, and another reason not to measure diagonal length of screens. 

  3. I can’t vouch for the validity of this fact, but I recall reading that television manufacturers were a big proponent of diagonal measuring, especially as TVs moved from 4:3 aspect ratio to 16:9 aspect ratio. A flat-screen LCD at 16:9 with the same screen area as a CRT display at 4:3 could suddenly be marketed as significantly “larger.”