How to Measure Screen Size (Phone, Monitor, TV)

The industry definition of screen size is the diagonal length of the visible display area. That is why a “65-inch TV” refers to the diagonal across the screen, not the width. This applies whether you are asking how to measure screen size for a phone, monitor, or TV.

To get an accurate number, measure from one corner of the lit display to the opposite corner. Do not include the bezel, frame, or stand.

You only need a tape measure or ruler. Turn on the screen so you can see the exact visible area, then measure the diagonal across the display. If you are measuring a TV mounted on a wall, it helps to have a second person hold the tape.

If you are wondering how to measure TV size, the method is identical: measure the diagonal of the visible screen only. This answers questions like how do you measure the size of a TV or how to measure TV screen size correctly.

After you get the diagonal, you can estimate the width and height using the TV aspect ratio, usually 16:9. This is helpful for fit checks on stands and cabinets.

For monitors, the same diagonal method applies. The only difference is that aspect ratios vary more: 16:9, 21:9, and 32:9 are all common. So if you are asking how to measure monitor screen size, measure the diagonal, then confirm the aspect ratio for width and height.

Curved monitors should still be measured along a straight diagonal. Do not wrap the tape along the curve, because that will overstate the size.

Phone screen sizes are measured the same way, corner to corner on the lit display. Because phones use tall aspect ratios, the diagonal alone does not tell you the width. That is why two phones with the same diagonal can feel different in hand.

Once you know the diagonal and aspect ratio, you can calculate the width and height with simple geometry. This is the same method used by a screen size calculator or size comparison tool.

For example, a 16:9 screen has a width-to-height ratio of 16:9. Divide the diagonal into those proportions and you can compute the real width and height for fit checks.

Before comparing two devices, run this checklist to avoid measurement drift.

The most common mistake is measuring the outside edges of the device. This inflates the number and creates confusion when you compare to official sizes. Another mistake is measuring width only; diagonal is the industry standard.

If you want speed, a screen size calculator can convert diagonal and aspect ratio into width and height in seconds. You can also compare two screens side by side to see the real-world difference.

That is especially useful when you compare a 65-inch TV to a 75-inch TV, or a 27-inch monitor to a 32-inch monitor. The diagonal numbers look close, but the area jump is larger than most people expect.