Measure the screen diagonally from corner to corner

Screen size is always measured diagonally—from the bottom-left corner to the top-right corner (or bottom-right to top-left). This single measurement, given in inches, is what manufacturers list as the screen size. A 24-inch monitor means the diagonal distance is 24 inches, not the width or height alone.

Use a measuring tape or ruler and place it at one corner of the actual display area—not the bezel (the plastic frame around the screen). Pull it straight across to the opposite corner. If you are measuring a laptop or tablet, measure from corner to corner of the lit display itself, not including any border or case.

Write down the number in inches. If your tape shows centimeters, divide by 2.54 to convert to inches. Most screens sold in North America are labeled in inches, so this is the standard you will see on spec sheets and product listings.

Key Takeaways

  • Screen size is always the diagonal measurement from one corner to the opposite corner, measured in inches.
  • Measure only the lit display area, not the plastic bezel or frame around it.
  • If you need width and height instead, use the aspect ratio (like 16:9) to calculate them from the diagonal.
  • Manufacturers round measurements, so a screen labeled 24 inches may actually measure 23.8 or 24.1 inches.
  • For laptops and tablets, the screen size is separate from the device size—a 15-inch laptop has a 15-inch screen, not a 15-inch case.

Why diagonal measurement matters

The diagonal is the only measurement that stays consistent across different screen shapes. A 16:9 widescreen monitor and a 4:3 older monitor could have the same diagonal but very different widths and heights. By using the diagonal, manufacturers can compare screens fairly regardless of their proportions.

This also means you cannot judge how much desk space a monitor will take up by its size alone. A 27-inch ultrawide (21:9 aspect ratio) will be much wider than a 27-inch standard (16:9) monitor, even though both are called 27 inches. If you need to know whether a screen fits your desk, measure or calculate the actual width and height separately.

How to find width and height if you need them

If you know the diagonal and the aspect ratio, you can calculate width and height using basic math. The aspect ratio is usually printed on the box or listed in the specs—common ones are 16:9 (most modern monitors), 16:10 (some laptops), and 4:3 (older screens).

For a 16:9 screen: multiply the diagonal by 0.87 to get the width, and multiply by 0.49 to get the height. For a 16:10 screen: multiply by 0.86 for width and 0.54 for height. For a 4:3 screen: multiply by 0.8 for width and 0.6 for height. These numbers are approximations, but they are close enough for planning desk space.

Alternatively, look up the exact model number on the manufacturer's website. Spec sheets always list the full dimensions in width × height × depth, usually in both inches and millimeters.

Measuring a screen that is already installed

If the monitor is mounted on a wall or built into a device, you still measure the same way—diagonally across the lit display area. For a TV mounted on a wall, measure from the bottom-left corner of the picture to the top-right corner of the picture, not including any black bars or bezels.

If the screen is curved (like some gaming monitors), measure along the curve from corner to corner. The tape will follow the bend naturally. The measurement will be slightly longer than if the screen were flat, but this is normal and manufacturers account for it in their labeling.

For a phone or tablet, measure the screen itself—the part that lights up. Do not include the frame or any notch at the top. Most phones list their screen size in the technical specs, so you can verify your measurement against the official number.

Common mistakes when measuring screens

Including the bezel: The plastic or metal frame around the screen does not count. Measure only the lit display area. This is the most common error and will give you a number larger than the actual screen size.

Measuring width or height instead of diagonal: A 24-inch monitor is 24 inches diagonally, not 24 inches wide. If you measure just the width, you will get a much smaller number and think you have the wrong screen.

Forgetting to account for rounding: Manufacturers round to the nearest inch or half-inch. A screen labeled 24 inches might actually measure 23.8 inches. This does not mean your measurement is wrong—it means the label is rounded for marketing.

Measuring a screen in its case or stand: For laptops, tablets, and all-in-one computers, the screen size is separate from the device size. A 15-inch laptop has a 15-inch screen, but the whole device (including bezels and case) may be 16 or 17 inches wide.

What to do if you cannot measure physically

If you do not have access to a tape measure or the screen is too high or awkward to reach, look up the model number. Every monitor, TV, laptop, and tablet has a model number printed on the back or bottom. Search that number plus "specs" or "specifications" on the manufacturer's website, and you will find the exact diagonal measurement listed.

For older devices where the model number is worn off, take a photo of the back or any visible labels and do a reverse image search. You can also describe the device (brand, approximate age, any visible text) in a search, and forums or product databases often have the specs archived.

Frequently Asked Questions

Is screen size measured in inches everywhere or just in North America?

Screen size is measured in inches worldwide, even in countries that use the metric system. A 24-inch monitor is called 24 inches in Europe, Asia, and Australia. Manufacturers list the metric equivalent (usually in centimeters) on spec sheets, but the primary label is always inches.

Why do some screens say 23.8 inches when they are labeled as 24 inches?

Manufacturers round the diagonal measurement for marketing purposes. A screen that measures 23.8 inches is labeled and sold as 24 inches because it is close enough. This is standard practice and does not mean the screen is defective or mislabeled.

Can I measure a screen on a phone or tablet the same way as a monitor?

Yes. Measure diagonally from corner to corner of the lit display area only. Do not include the frame, bezels, or any notch. The measurement should match the screen size listed in the device specs.

What if my screen is curved or has rounded corners?

Measure along the curve or around the rounded corners with your tape. The measurement will be slightly longer than a flat screen of the same size, but manufacturers account for this when they label the screen. Your measurement should still match the listed size within a tenth of an inch.

Does the resolution affect the screen size measurement?

No. A 1080p monitor and a 4K monitor can both be 27 inches—the diagonal is the same, but the resolution (how many pixels fit on the screen) is different. Screen size and resolution are separate measurements.