Odd Aspect Ratios, Explained

The Short Answer

1366×768 is treated as 16:9, but it isn't exactly 16:9. Its exact ratio is 683:384, which is 0.05% wider. Screen resolutions are chosen for engineering reasons, not so that they divide neatly: an exact 16:9 screen with 768 rows would need to be 1,365.33 pixels wide. Half a pixel can't exist, so manufacturers made it 1366. That tiny rounding is why a calculator reduces 1366×768 to 683:384 instead of 16:9.

The calculator is right, and so is the label on the box. 683:384 is 0.05% away from 16:9, and nobody can see that.

Common Resolutions and Their Real Ratios

Two patterns explain most of the table: rounding to whole pixels, and marketing names that were never meant to be exact.

Where 1366×768 and 1360×768 Came From

Both come from the older 1024×768 standard. Widening it to 16:9 while keeping the same 768 rows let manufacturers reuse existing panel and signal designs. The exact width, 1365.33, had to be rounded.

• 1366 is the nearest whole number that is at least 16:9.
• 1360 was used on some hardware because it divides by 8, which graphics hardware handles more simply. It also keeps each 8-bit channel just under 1 MB of memory.

Other resolutions have similar histories. A resolution is a compromise between the ratio you want and the numbers the hardware likes.

"21:9" Is a Label, Not a Ratio

Ultrawide monitors are all sold as 21:9, but none of the common sizes is exactly that. 2560×1080 is 64:27, 3440×1440 is 43:18, and 3840×1600 is 12:5. Written over 9, they come to roughly 21.3, 21.5 and 21.6.

The name echoes cinema widescreen. Most films shot in "scope" are 2.39:1, which is closer to 3440×1440 (2.389:1) than to a true 21:9 (2.333:1).

If a calculator shows 64:27 for your 2560×1080 monitor, it isn't broken. The aspect ratio calculator here shows the exact ratio, the closest standard ratio, and a note when a size falls in the range sold as 21:9.

16:10 Shows Up as 8:5

1920×1200, 1440×900 and 2560×1600 are all 16:10. A calculator that simplifies fully shows 8:5, because 16:10 still shares a factor of 2. Both describe the same shape.

This matters when someone asks "is 1920×1200 16:9?" The answer is no: 16:10 is taller than 16:9.

When the Difference Actually Matters

What you notice in practice is black bars, which appear when the content and the screen have different shapes. Some worked examples:

• 16:9 video on a 1366×768 screen: scaled to 1366 wide, the video is 768.4 pixels tall. That's under one pixel of difference, so there are no visible bars.
• 16:9 video on a 1920×1200 screen: the video is 1920×1080, leaving a 60-pixel bar at the top and bottom.
• 16:9 video on a 2560×1080 ultrawide: the video is 1920 wide, leaving a 320-pixel bar on each side.
• A 2.39:1 film on a 2560×1080 ultrawide: the film is about 1071 pixels tall, leaving bars of about 4 pixels. That's why ultrawides are loved for movies.
• The same film on a 1920×1080 TV: the film is about 803 pixels tall, leaving bars of about 138 pixels at the top and bottom.

For images, such as a wallpaper or a banner with exact slots, ignore the ratio and use the exact pixel size. A wallpaper made at 1920×1080 and shown on a 1366×768 screen is scaled down cleanly, because the shapes match to within 0.05%. The same wallpaper on a 1920×1200 screen gets cropped or stretched. The image resizer and image cropper handle both jobs.

Working It Out Yourself

To simplify any resolution, divide both numbers by their greatest common divisor:

• 1920 and 1080 share 120, so the ratio is 16:9.
• 1366 and 768 share only 2, so the ratio is 683:384.
• 2560 and 1080 share 40, so the ratio is 64:27.

When the result has large, odd numbers like 683:384, divide the width by the height instead. That gives 1.779, very close to 16:9's 1.778, so the screen is effectively 16:9. The calculator does both steps and names the closest standard ratio.