Measure diagonally from corner to corner, in inches
A monitor's size is always stated as the diagonal distance from one corner of the screen to the opposite corner, measured in inches. This is the only measurement that matters when you are comparing monitors or replacing one. The diagonal runs from the top-left to the bottom-right (or top-right to the bottom-left — both give the same result).
Use a measuring tape or ruler and place it at the inside edge of the screen in one corner. Pull it straight across to the opposite corner, staying on the glass or display surface itself. Do not measure the plastic bezel or frame around the screen — measure only the lit area where the image appears. Write down the number where the tape ends.
If your tape measure shows 21.5 inches, your monitor is a 21.5-inch screen. If it shows 27 inches, it is a 27-inch monitor. Most monitors fall into standard sizes: 19, 21.5, 24, 27, 32, 34, or 38 inches. Your measurement should land close to one of these.
Key Takeaways
- Measure only the lit display area from corner to corner diagonally, not the plastic frame or bezel around it.
- The diagonal measurement in inches is the only size specification that matters for monitor comparison and replacement.
- Most monitors come in standard sizes: 24 and 27 inches are the most common for desktop work.
- If you cannot physically measure, check the monitor's manual, the back of the unit, or the manufacturer's website using the model number.
Why the diagonal measurement is the standard
Monitor size has always been stated diagonally because it is the one measurement that stays consistent across different aspect ratios. A 24-inch monitor with a 16:9 aspect ratio (wider, shorter) has a different width and height than a 24-inch monitor with a 16:10 aspect ratio (slightly taller), but both have the same diagonal.
Manufacturers use the diagonal because it is straightforward to compare across their entire product line. When you shop for a replacement monitor or upgrade, the diagonal size tells you when ready whether the new screen will be noticeably larger or smaller than what you have now. Width and height alone would be confusing because they change depending on the shape of the screen.
Measure the width and height if you need them for your desk
The diagonal tells you the monitor's size, but the width and height tell you whether it will fit in your space. Measure the width from the left edge of the bezel to the right edge, and the height from the top of the bezel to the bottom. These measurements include the plastic frame, because that is the actual space the monitor takes up on your desk.
Write down both numbers. A 27-inch monitor is typically about 24 inches wide and 14 inches tall, but this varies by model. Some monitors have thicker bezels or stands that add extra width or depth. If you are replacing a monitor in a tight space, these measurements matter more than the diagonal.
You should also measure the depth — the distance from the front of the screen to the back of the stand. This tells you how much space the monitor needs behind your desk. Most stands add 6 to 8 inches of depth, though wall-mounted monitors need almost none.
Check the model number if you cannot measure
If you do not have a measuring tape or cannot reach the screen easily, find the model number on the back of the monitor or on the bottom of the stand. It usually looks like a string of letters and numbers, such as "LG 27UP550" or "Dell S2721DGF".
Search that model number on the manufacturer's website or on a retailer site like Amazon or Best Buy. The product page will list the screen size, width, height, and depth in the specifications section. This is faster than measuring and gives you the exact dimensions the manufacturer designed the monitor to have.
If you cannot find the model number, look for a sticker or label on the power cable, the back panel, or inside the battery compartment of any wireless peripherals that came with the monitor. Manufacturers always print the model somewhere visible.
Understand aspect ratio and resolution separately
Monitor size, aspect ratio, and resolution are three different things, and they do not determine each other. A 27-inch monitor can be 16:9 (standard widescreen), 16:10 (slightly taller), or 21:9 (ultrawide). The diagonal stays 27 inches no matter which shape it is.
Resolution — the number of pixels on the screen, like 1920×1080 or 2560×1440 — also does not depend on size. You can have a 24-inch monitor with 1920×1080 resolution or a 24-inch monitor with 2560×1440 resolution. The second one will show more detail because the pixels are packed tighter, but the screen is still 24 inches diagonally.
When you measure the diagonal, you are finding the size. The aspect ratio and resolution are separate specifications you can find on the product page or manual.
Account for curved monitors and unusual shapes
Curved monitors still measure diagonally from corner to corner on the display surface, the same way flat monitors do. The curve does not change how you measure — you still run the tape from one corner to the opposite corner across the glass.
Ultrawide monitors (21:9 aspect ratio) are longer left to right but shorter top to bottom than standard monitors of the same diagonal size. A 34-inch ultrawide is 34 inches diagonally, but it is much wider than a 34-inch standard monitor. If you are replacing a standard monitor with an ultrawide, measure the width of your desk space carefully, because the diagonal alone does not tell you how much horizontal room you need.
Frequently Asked Questions
Do I measure the screen with or without the bezel?
Measure only the lit display area — the part that shows the image. Do not include the plastic frame or bezel around it. The bezel does not display anything, so it is not part of the monitor's size. The diagonal measurement should go from the inside edge of the screen in one corner to the inside edge in the opposite corner.
What if my measurement does not match a standard size?
Most monitors are manufactured in standard sizes, so your measurement should land within half an inch of a common size like 24, 27, or 32 inches. If it is off by more than that, you may have measured the bezel by mistake. Measure again, starting from where the glass begins, not where the plastic frame starts.
Is the diagonal measurement the same as the screen resolution?
No. Diagonal size and resolution are separate. A 27-inch monitor can have 1920×1080 resolution or 2560×1440 resolution — the diagonal stays 27 inches either way. Resolution tells you how many pixels fit on the screen; size tells you the physical dimensions.
Can I use my phone to measure the monitor?
A measuring app on your phone can work if the app uses your phone's camera to estimate distance. However, a physical tape measure or ruler is more accurate. If you use a phone app, place your phone flat against the screen and measure from corner to corner the same way you would with a tape measure.
Why do monitor sizes seem smaller than they are?
A 27-inch monitor feels smaller than a 27-inch television because monitors sit closer to your eyes. The diagonal measurement is the same, but the viewing distance makes the monitor appear smaller. This is normal and does not mean your measurement is wrong.