Screen size is measured diagonally from corner to corner, in inches

Computer screens are measured by the distance from the bottom-left corner to the top-right corner (or any opposite corners) in a straight line. This diagonal measurement is always given in inches, even on monitors sold outside the United States. A 24-inch monitor means the diagonal distance across the screen is 24 inches—not the width, not the height, and not the area of the screen itself.

The reason manufacturers use diagonal measurement is historical: it's the number that sounds largest and is easiest to compare across different screen shapes. A 27-inch monitor with a wider aspect ratio (like 21:9) will have a different width and height than a 27-inch monitor with a standard 16:9 shape, but the diagonal is always 27 inches on both.

If you're buying a replacement monitor or trying to figure out what size you already own, the diagonal measurement is what you'll find in the product name, the manual, and the specifications.

Key Takeaways

  • Screen size is the diagonal measurement from one corner to the opposite corner, measured in inches.
  • You can measure your own screen with a ruler or measuring tape, starting from the bottom-left corner and going to the top-right corner.
  • The screen size does not include the bezel (the plastic or metal frame around the display), only the visible picture area.
  • Your monitor's size is printed on the box, in the manual, in Windows settings, or on the manufacturer's website if you search by model number.
  • Diagonal measurement is used because it's consistent across different aspect ratios and easier to compare between monitors.

How to measure with a ruler or tape measure

Place a ruler or measuring tape at the bottom-left corner of your screen where the picture area begins (not the plastic frame). Stretch it diagonally to the top-right corner of the screen. Make sure you're measuring only the display itself, not the bezel. Write down the number where the tape ends.

If your screen is curved or mounted at an angle, measure as if the screen were flat and facing you straight on. The measurement should be in a straight line from corner to corner, not following the curve of the screen. If you don't have a measuring tape long enough, you can use a piece of string, mark where it reaches, and then measure the string against a ruler.

Your measurement might not be a whole number. A screen that measures 23.8 inches diagonally is often sold and labeled as a 24-inch monitor because manufacturers round to the nearest inch.

Finding the screen size in your computer settings

On Windows, right-click on your desktop and select Display settings. Scroll down to the "Display" section. The size won't be listed directly, but you'll see the resolution (like 1920 × 1080). Write down your resolution and your monitor's model number (usually printed on the back or bottom of the monitor or on the stand).

Search the model number on the manufacturer's website or on a search engine. The product page will list the screen size in the specifications. This is faster and more accurate than measuring, especially if your monitor is mounted high on a wall or in an awkward position.

On Mac, click the Apple menu, select About This Mac, then click Displays. The screen size is listed there. If you're using an external monitor on a Mac, the same Windows method applies—find the model number and search for the specifications online.

Understanding aspect ratio and how it affects what you see

Two monitors can have the same diagonal size but different shapes. A 27-inch monitor with a 16:9 aspect ratio (standard widescreen) will be wider and shorter than a 27-inch monitor with a 4:3 aspect ratio (older, more square shape). The diagonal is still 27 inches on both, but the width and height are different.

Aspect ratio describes the relationship between width and height. Common ratios are 16:9 (most modern monitors), 16:10 (some laptops and older monitors), 21:9 (ultrawide), and 4:3 (older monitors and some specialized displays). If you need to know the exact width and height of your screen, you'll need both the diagonal size and the aspect ratio.

You can find the aspect ratio in your display settings or in the monitor's manual. Knowing the aspect ratio helps when you're shopping for a replacement or trying to fit a monitor into a specific space.

What the bezel is and why you shouldn't measure it

The bezel is the frame—usually plastic or metal—that surrounds the actual display. When you measure screen size, you measure only the lit picture area, not the bezel. Modern monitors have very thin bezels, but older monitors had thick frames that added an inch or more on each side.

If you measure from the outside edge of the bezel to the outside edge on the opposite side, you'll get a number that's larger than the actual screen size. This is why it's important to start your measurement at the corner where the picture actually begins, not at the outer edge of the plastic frame.

The bezel size matters if you're fitting a monitor into a tight space or mounting multiple monitors side by side, but it doesn't affect the screen size rating.

Screen size versus resolution: they're not the same thing

Screen size (measured in inches) and resolution (measured in pixels, like 1920 × 1080) are two separate things. A 24-inch monitor and a 27-inch monitor can both have 1920 × 1080 resolution. The larger monitor will have the same number of pixels spread across a bigger area, so text and images will appear larger and less sharp.

Resolution tells you how many dots of color the screen can display. Screen size tells you the physical dimensions. Together, they determine how sharp the image looks—a measure called pixel density or pixels per inch (PPI). A 27-inch monitor with 4K resolution (3840 × 2160) will look sharper than a 27-inch monitor with 1080p resolution (1920 × 1080) because the pixels are packed more densely.

When you're shopping for a monitor, check both the size and the resolution to understand what you're getting. A larger screen with lower resolution might look blurry, while a smaller screen with high resolution might have text too small to read comfortably.

Common monitor sizes and what they're used for

Most desktop monitors range from 21 inches to 32 inches. A 24-inch monitor is the standard size for office work and general use. A 27-inch monitor gives you more screen space without being so large that you have to move your head to see all of it. Monitors larger than 32 inches are usually called TVs or specialized displays because they're too big to view comfortably from a normal desk distance.

Laptop screens are typically 13 to 17 inches. Tablets range from 7 to 13 inches. Phones are usually 5 to 7 inches. These smaller sizes are measured the same way—diagonally—but they're rarely called out in everyday conversation the way monitor sizes are.

If you're replacing a monitor, matching the size you're used to is usually the best choice. If you're upgrading, moving from 24 inches to 27 inches is a noticeable improvement without being overwhelming. Anything larger than 32 inches for desktop use requires you to sit farther away or accept that you'll be turning your head to see the edges.

Frequently Asked Questions

Why do manufacturers measure diagonally instead of width or height?

Diagonal measurement makes the number sound larger and is consistent across different aspect ratios. A 27-inch monitor is 27 inches diagonally whether it's 16:9, 16:10, or 21:9. If manufacturers used width instead, the same physical monitor would have different numbers depending on its shape, making comparison confusing.

Can I measure my phone or tablet screen the same way?

Yes. Phone and tablet screens are measured diagonally just like monitors. A 6-inch phone has a 6-inch diagonal screen. You can measure it the same way with a ruler, or search your device model online to find the official size in the specifications.

What if my monitor size doesn't match what I measured?

Your measurement might be off by a quarter inch or so, which is normal. Manufacturers round to the nearest inch, so a monitor that measures 23.8 inches is sold as 24 inches. If your measurement is significantly different, make sure you're measuring from the edge of the picture area, not the edge of the bezel, and that you're going from corner to corner in a straight line.

Does screen size affect how much desk space I need?

Yes. A larger diagonal size usually means a wider and taller monitor, so it takes up more desk space. However, the exact footprint depends on the aspect ratio and the stand design. Check the width and depth measurements in the product specifications if you're concerned about fit. Ultrawide monitors (21:9) are wider than standard monitors of the same diagonal size.

Is there a standard viewing distance for different screen sizes?

A common rule is to sit about 1.5 to 2 times the width of the screen away from it. For a 24-inch monitor, that's roughly 24 to 36 inches. For a 27-inch monitor, roughly 27 to 40 inches. If you sit too close, you'll have to move your eyes and head more. If you sit too far, text and details become hard to read. Your comfort is the best guide.