Measure diagonally from corner to corner

Screen size is always the diagonal measurement in inches, taken from the bottom-left corner to the top-right corner (or top-left to bottom-right — both give the same result). This is the number you see in product listings: a "24-inch monitor" means 24 inches measured corner to corner, not the width or height alone.

Use a measuring tape or ruler and place one end at the inside corner of the screen frame. Pull it straight across to the opposite corner, keeping the tape taut and flat against the glass. Write down the number where the tape ends. If you get 23.6 inches, that rounds to the 24-inch label you see in stores — manufacturers round to the nearest whole number.

Do not measure the plastic bezel (the frame around the screen). Measure only the actual display area where the image appears. If you include the bezel, your number will be larger than the advertised size.

Key Takeaways

  • Screen size is always measured diagonally from one corner to the opposite corner, in inches.
  • Measure only the glass or display surface itself, not the plastic frame or bezel around it.
  • The width and height of a screen depend on its aspect ratio, so diagonal measurement is the standard way to compare sizes.
  • If you need the width or height for a desk or wall mount, calculate it using the aspect ratio after you know the diagonal.

Why diagonal measurement matters

Screens come in different shapes. A 24-inch monitor might be a standard 16:9 widescreen, an older 4:3 square shape, or an ultrawide 21:9 format. All three could have the same diagonal measurement but very different widths and heights. Diagonal is the only measurement that stays consistent across all these shapes, which is why it became the industry standard.

If you only knew the width, you could not compare two screens fairly — a 20-inch-wide 4:3 monitor is actually smaller overall than a 20-inch-wide 16:9 monitor. The diagonal tells you the true size regardless of shape.

Find the aspect ratio to calculate width and height

Once you have the diagonal, you can work out the width and height if you know the aspect ratio. The aspect ratio is the proportion of width to height, written as two numbers like 16:9 or 4:3. You can find this in your monitor's manual, on the manufacturer's website, or by looking at the product page where you bought it.

For a 24-inch monitor with a 16:9 aspect ratio, the math works like this: the diagonal is 24 inches. Using the Pythagorean theorem (width² + height² = diagonal²) and the ratio 16:9, the width comes out to about 20.9 inches and the height about 11.8 inches. Most people do not need to do this calculation — you can search "24 inch 16:9 monitor dimensions" and find the exact measurements — but it is useful if you are checking whether a monitor will fit a specific space.

Measure the full footprint for desk space

The screen diagonal does not tell you how much desk space the monitor needs. For that, measure the width of the monitor from the left edge of the bezel to the right edge, and the depth from the front of the screen to the back of the stand. These measurements matter if you have a small desk or are fitting multiple monitors side by side.

Write down both numbers. The width is usually the more important one for desk planning — most monitor stands do not take up much depth. If you are mounting the monitor on an arm or wall bracket, you only need to know that the mounting pattern (usually a 75mm or 100mm square hole on the back) matches your bracket.

Check the screen resolution separately

Screen size and resolution are not the same thing. A 24-inch monitor might have a resolution of 1920×1080 pixels or 2560×1440 pixels — both are 24 inches diagonally, but the second one packs more pixels into the same space, making text and images sharper. Resolution is listed separately in the monitor's specifications and does not change how you measure the physical screen.

If you are shopping for a replacement monitor and want the same sharpness as your current one, you need both the diagonal size and the resolution. A 27-inch monitor at 1920×1080 will look blurry compared to a 24-inch monitor at the same resolution, because the pixels are spread over a larger area.

Measure a laptop or tablet screen the same way

Laptops and tablets use the same diagonal measurement system. Measure from corner to corner of the actual display, not including the plastic border or keyboard. A 15-inch laptop means the screen is 15 inches diagonally. A 10-inch tablet is 10 inches corner to corner.

For laptops, the diagonal measurement is usually printed on the product page or in the manual. If you need to measure it yourself, use the same technique: place a measuring tape at one corner of the glass and pull it to the opposite corner, keeping it flat and tight.

Frequently Asked Questions

Should I measure the curved part of a curved monitor?

No. Even though the screen curves, you still measure the straight-line diagonal from corner to corner. The curve does not change the diagonal measurement — it only affects how the image appears to your eye. Curved monitors are labeled the same way as flat ones.

What if my monitor has a notch or cutout at the top?

Measure the diagonal of the full rectangular area where the image appears, including the space behind the notch. The notch is a design feature, not a reduction in screen size. The diagonal measurement stays the same as it would be for a monitor without a notch.

Do I measure in inches or centimeters?

Screen sizes are always listed in inches. If your measuring tape shows centimeters, divide by 2.54 to convert to inches. A 61-centimeter diagonal is about 24 inches.

Why is my measured size slightly different from what the box says?

Manufacturers round to the nearest whole number. A monitor that measures 23.6 inches is sold as a 24-inch monitor. Rounding differences of 0.2 to 0.4 inches are normal and do not mean your monitor is the wrong size.

Can I use my phone's measuring app instead of a tape measure?

Phone measuring apps work reasonably well for screens, but a physical tape measure is more accurate. If you use an app, hold your phone steady and parallel to the screen, and double-check the result with a tape measure if the measurement matters for a purchase or mounting decision.