Screen size is measured diagonally from corner to corner, in inches
Computer screens are measured by the distance from one corner to the opposite corner—not the width or height alone. A 24-inch monitor, for example, means the diagonal measurement is 24 inches. This diagonal measurement is the standard used by manufacturers, so it's what you'll see on the box and in product listings.
The reason manufacturers use diagonal measurement is historical: it's how television screens were measured first, and the practice carried over to computer monitors. It also happens to be the hardest measurement to fake, since it's a single number that can't be manipulated by stretching the aspect ratio.
If you're shopping for a replacement monitor or trying to figure out what you already own, knowing how to measure correctly will save you from buying the wrong size or thinking your screen is bigger or smaller than it actually is.
Key Takeaways
- Measure diagonally from the bottom-left corner to the top-right corner (or vice versa) using a tape measure or ruler, in inches.
- Measure only the visible screen area—do not include the plastic bezel or frame around the edges.
- If you know the width and height in inches, you can calculate the diagonal using the Pythagorean theorem: diagonal = √(width² + height²).
- Your monitor's actual size is printed on the box it came in, in the manual, or in the device settings on your computer.
- Common desktop monitor sizes are 21.5, 24, 27, 32, and 34 inches; laptop screens are typically 13, 14, 15, or 17 inches.
Using a tape measure or ruler
The most straightforward way to measure your screen is with a tape measure or a ruler long enough to reach corner to corner. Place one end at the bottom-left corner of the visible screen (where the image actually starts, not the plastic frame) and stretch it to the top-right corner. Write down the number where the tape or ruler ends.
Be precise about where the screen begins and ends. The plastic or metal border around the screen—called the bezel—does not count. You're measuring only the lit area where the image appears. If you're measuring a laptop, measure only the glass or plastic display panel, not the keyboard or bottom edge of the computer.
If your tape measure or ruler isn't long enough, you can measure the width and height separately and use math to find the diagonal (see the next section). Measuring diagonally is faster if you have the right tool, but either method works.
Calculating diagonal size from width and height
If you can't measure diagonally easily, measure the width and height of the visible screen separately, then use the Pythagorean theorem to find the diagonal. The formula is: diagonal = √(width² + height²). This means you multiply the width by itself, multiply the height by itself, add those two numbers together, and then find the square root of the result.
Here's a concrete example: if your screen is 20 inches wide and 11.25 inches tall, the math works like this: (20 × 20) + (11.25 × 11.25) = 400 + 126.56 = 526.56. The square root of 526.56 is approximately 22.9 inches. So your screen is roughly a 23-inch monitor.
You can use any calculator app on your phone or computer to find the square root—just type the number and look for the √ button. Many online calculators also let you enter width and height directly and will calculate the diagonal for you.
Finding the size in your computer's settings
Your operating system usually knows your monitor's size and can tell you. On Windows, right-click on the desktop, select Display settings, scroll down, and look for a section labeled Display or Advanced display settings. The monitor size may be listed there, though not all Windows computers display this information.
On a Mac, click the Apple menu, go to System Settings (or System Preferences on older versions), then select Displays. The resolution and sometimes the size will appear, though Macs often show resolution rather than physical size.
For a laptop, check the original box or the manual that came with it—the screen size is always printed there. You can also search your laptop model number online; the specifications will include the screen size.
Checking the monitor box, manual, or manufacturer label
The easiest way to find your screen size is to look at what you already have. If you still have the original box your monitor came in, the size is printed on the front and back. If you have the manual or documentation, it's listed in the specifications section.
Many monitors also have a label on the back or bottom that includes the model number and size. If you can see the back of your monitor, look for a sticker with text like "24-inch" or "27-inch" printed on it. Laptop screens often have the size printed on the bezel (the frame around the screen) or on the bottom edge of the lid.
If you have neither the box nor the manual, search the monitor's model number online. The model number is usually on the back of the monitor or in your computer's display settings. Entering that number into a search engine will pull up the manufacturer's product page, where the screen size is always listed.
Understanding aspect ratio and resolution
Screen size alone doesn't tell you the full picture. Two 24-inch monitors can look different if they have different aspect ratios—the relationship between width and height. Most desktop monitors are 16:9 (widescreen), which means for every 16 units of width, there are 9 units of height. Some older or specialized monitors are 4:3 or 16:10.
Resolution—the number of pixels displayed—is separate from physical size. A 24-inch monitor might have 1920 × 1080 pixels (Full HD) or 2560 × 1440 pixels (2K). Higher resolution means sharper text and images, but it doesn't change the physical size of the screen. A 24-inch monitor is 24 inches diagonally no matter what resolution it displays.
When you're shopping for a replacement monitor, knowing both the size and the aspect ratio helps you find something that will fit your desk and match your current setup. If your current monitor is 24 inches and 16:9, a replacement 24-inch 16:9 monitor will be nearly identical in physical dimensions.
Common screen sizes for desktops and laptops
Desktop monitors come in a range of standard sizes. The most common are 21.5 inches, 24 inches, 27 inches, 32 inches, and 34 inches. Smaller monitors (under 22 inches) are less common now but still available. Larger monitors (40+ inches) exist but are usually used for specialized work like video editing or design.
Laptop screens are typically smaller: 13 inches, 14 inches, 15 inches, and 17 inches are the standard sizes. A 13-inch laptop screen is noticeably smaller than a 24-inch desktop monitor, so if you're used to a desktop, a laptop may feel cramped until you adjust.
Ultrawide monitors—which are wider than standard 16:9 screens—come in sizes like 29 inches, 34 inches, and 38 inches. These are measured diagonally just like regular monitors, but they give you much more horizontal space because of their unusual aspect ratio (usually 21:9 or 32:9).
Frequently Asked Questions
Do I measure the screen with or without the plastic frame?
Measure only the visible screen area—the part that lights up and displays the image. Do not include the plastic or metal bezel (frame) around the edges. If you include the bezel, your measurement will be larger than the actual screen size.
What if my screen is curved or has rounded corners?
Measure from the outermost corners of the lit area, even if the corners are rounded. The diagonal measurement standard assumes you're measuring from corner to corner, so measure the actual distance even if the screen curves slightly at the edges.
Can I measure in centimeters instead of inches?
Yes, but monitor sizes are always listed in inches by manufacturers. If you measure in centimeters, divide by 2.54 to convert to inches. A 61-centimeter diagonal measurement is roughly 24 inches (61 ÷ 2.54 = 24).
Is the screen size the same as the monitor size?
Yes. The screen size and monitor size are the same thing—they both refer to the diagonal measurement of the display. The monitor is the entire device, but its size is defined by the screen inside it.
Why do manufacturers use diagonal measurement instead of width or height?
Diagonal measurement is a single number that can't be manipulated by changing the aspect ratio. A manufacturer can't make a screen look bigger by stretching it wider without changing the diagonal. It's also the standard that's been used since television screens were first measured, so it stuck.