Measure diagonally from corner to corner with a tape measure

Screen size is always the diagonal measurement in inches, taken from the bottom-left corner to the top-right corner (or any opposite corners). This is the standard the industry uses, and it is what manufacturers list on the box and in the specs. You do not measure the width or height alone.

Use a cloth tape measure or a metal ruler — a soft tape measure is easier to position without slipping. Place one end at the inner corner of the screen (where the black bezel or frame starts) and stretch it to the opposite corner. Write down the number where the tape meets the other corner. If you get 21.5 inches, your screen is a 21.5-inch monitor. Round to the nearest half-inch if the measurement falls between two sizes.

Measure three times if the tape keeps shifting. The most common mistake is measuring the outer edge of the plastic frame instead of the actual display area. You want the glass or panel itself, not the border around it.

Key Takeaways

  • Screen size is always measured diagonally in inches, from one corner of the display to the opposite corner, not the width or height.
  • Measure the display panel itself, not the plastic frame or bezel around it, and round to the nearest half-inch if your measurement falls between standard sizes.
  • If you cannot physically measure, check the monitor's manual, the back of the device for a label, or the manufacturer's website using your monitor model number.
  • Laptop screens are measured the same way as desktop monitors, but the measurement includes only the display area, not the keyboard or bottom border.
  • Common desktop monitor sizes are 21.5, 24, 27, 32, and 34 inches; laptop screens typically range from 13 to 17 inches.

Find the model number and look it up online

If you cannot or do not want to measure, the easiest route is to find your monitor's model number and search for the specs online. The model number is usually printed on a sticker on the back of the monitor, at the bottom, or sometimes on the bottom edge of the frame itself. It looks like a string of letters and numbers — for example, "Dell S2721DGF" or "LG 27UP550".

Once you have the model number, go to the manufacturer's website (Dell, LG, ASUS, HP, or whoever made it) and search for that model in their product database. The screen size will be listed in the specifications section, usually near the top. You can also type the model number into Google along with the word "specs" and find the information on retailer sites like Amazon or Best Buy, which display full specifications for most monitors.

For laptops, the model number is often on the bottom of the device or inside the battery compartment. If you cannot find a sticker, open the Settings app (Windows) or System Preferences (Mac) and look for the display information — it usually shows the screen size or resolution, which you can cross-reference with your laptop's manual or the manufacturer's website.

Check the manual or original packaging

The monitor's manual or the original box will always list the screen size on the first page or on the back of the box. If you still have the packaging, the size is printed prominently on the front and back. If you have the manual but not the box, the specifications page (usually near the beginning) will show the diagonal measurement in inches.

Many people keep the manual in a drawer or file, or it may be stored digitally. If you have the model number, you can read the manual as a PDF from the manufacturer's support page. Search for "[Model Number] manual PDF" and you will usually find it within the first few results. The manual is often faster than measuring if you have it on hand.

Understand the difference between screen size and resolution

Screen size (measured in inches) and resolution (measured in pixels) are two separate things. A 24-inch monitor might have a resolution of 1920 × 1080 pixels, while another 24-inch monitor might have 2560 × 1440 pixels. Both are 24 inches diagonally, but the second one packs more pixels into the same space, making text and images sharper.

Do not confuse the two. If someone asks "what size is your screen," they are asking for the diagonal measurement in inches. If they ask "what is your resolution," they want the pixel count. You will see resolution listed as "1080p," "1440p," "4K," or as specific numbers like "1920 × 1080." Screen size is always a single number followed by "inches" — like "27 inches."

Common screen sizes for desktops and laptops

Desktop monitors come in standard sizes: 21.5, 24, 27, 32, and 34 inches are the most common. Ultrawide monitors (which are wider than they are tall) are usually 34 or 38 inches. Smaller specialty monitors for tight spaces are often 17 or 19 inches. Larger displays for video editing or gaming can reach 40 inches or more, though these are less common for everyday use.

Laptop screens typically range from 13 to 17 inches. A 13-inch laptop is compact and portable; a 15 or 16-inch laptop is the middle ground; a 17-inch laptop is larger and heavier but gives you more screen space. Tablets usually fall between 7 and 13 inches. Smartphones are measured in inches too, but they are usually listed as 5 to 7 inches.

Why manufacturers measure diagonally

The diagonal measurement became the standard because it is the most consistent way to describe a screen's overall size across different aspect ratios. A 16:9 widescreen monitor and a 4:3 older monitor could have the same diagonal measurement but very different widths and heights. By using the diagonal, manufacturers can give you one number that fairly represents how much screen space you have, regardless of the shape.

This standard has been used since the days of television and has carried over to computer monitors, laptops, and tablets. It is why every monitor box, manual, and spec sheet lists size as a single diagonal measurement in inches. Understanding this helps you compare screens accurately and know what to expect when you buy a replacement or upgrade.

Frequently Asked Questions

Does screen size include the plastic frame around the display?

No. Measure only the glass or panel itself, from the inner edge where the display starts to the opposite inner corner. The plastic bezel or frame around the screen is not part of the measurement. This is why a 24-inch monitor does not have a 24-inch-wide case — the case is larger because it includes the frame.

What if my measurement does not match a standard size?

Round to the nearest half-inch. If you measure 23.8 inches, that is a 24-inch monitor. If you measure 27.2 inches, that is a 27-inch monitor. Manufacturers round their measurements too, so your tape measure will rarely land exactly on a whole number. Small variations are normal and do not mean your monitor is mislabeled.

Can I measure a laptop screen the same way as a desktop monitor?

Yes. Measure diagonally from corner to corner of the display panel only, not including the keyboard or the border below the screen. A 15-inch laptop has a 15-inch diagonal display, measured the same way as a 15-inch desktop monitor would be.

Is the screen size the same as the viewing area?

Mostly yes, but not exactly. The diagonal measurement includes the entire display panel, but a small portion around the very edge (usually less than half an inch) may not be usable for displaying content because of how the monitor is designed. For practical purposes, the screen size and viewing area are the same, and the difference is negligible.

Why do some monitors list size in centimeters instead of inches?

Some manufacturers, especially those selling in Europe or Asia, list screen size in centimeters. To convert, multiply inches by 2.54. A 24-inch monitor is about 61 centimeters. If you see a size in centimeters and want inches, divide by 2.54. Most retailers and spec sheets use inches, but checking the label on your monitor will tell you which unit is being used.